diff --git a/ThunkLibs/Generator/CMakeLists.txt b/ThunkLibs/Generator/CMakeLists.txt
index 55f27216b..8214c558f 100644
--- a/ThunkLibs/Generator/CMakeLists.txt
+++ b/ThunkLibs/Generator/CMakeLists.txt
@@ -1,13 +1,6 @@
 find_package(Clang REQUIRED CONFIG)
 find_package(OpenSSL REQUIRED COMPONENTS Crypto)
 
-add_library(thunkgenlib analysis.cpp gen.cpp)
-target_include_directories(thunkgenlib INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
-target_include_directories(thunkgenlib SYSTEM PUBLIC ${CLANG_INCLUDE_DIRS})
-target_link_libraries(thunkgenlib PUBLIC clang-cpp LLVM)
-target_link_libraries(thunkgenlib PRIVATE OpenSSL::Crypto)
-target_link_libraries(thunkgenlib PRIVATE fmt::fmt)
-
 # Query clang's global resource directory for system include directories
 if (NOT CLANG_RESOURCE_DIR)
   find_program(CLANG_EXEC_PATH clang REQUIRED)
@@ -18,6 +11,13 @@ if (NOT CLANG_RESOURCE_DIR)
     OUTPUT_STRIP_TRAILING_WHITESPACE)
 endif()
 
+add_library(thunkgenlib analysis.cpp data_layout.cpp gen.cpp)
+target_include_directories(thunkgenlib INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
+target_include_directories(thunkgenlib SYSTEM PUBLIC ${CLANG_INCLUDE_DIRS})
+target_link_libraries(thunkgenlib PUBLIC clang-cpp LLVM)
+target_link_libraries(thunkgenlib PRIVATE OpenSSL::Crypto)
+target_link_libraries(thunkgenlib PRIVATE fmt::fmt)
+target_compile_definitions(thunkgenlib INTERFACE -DCLANG_RESOURCE_DIR="${CLANG_RESOURCE_DIR}")
+
 add_executable(thunkgen main.cpp)
 target_link_libraries(thunkgen PRIVATE thunkgenlib)
-target_compile_definitions(thunkgen PRIVATE -DCLANG_RESOURCE_DIR="${CLANG_RESOURCE_DIR}")
diff --git a/ThunkLibs/Generator/analysis.cpp b/ThunkLibs/Generator/analysis.cpp
index 39f02577c..6e395479e 100644
--- a/ThunkLibs/Generator/analysis.cpp
+++ b/ThunkLibs/Generator/analysis.cpp
@@ -128,7 +128,10 @@ void AnalysisAction::ExecuteAction() {
 
     try {
         ParseInterface(context);
-        EmitOutput(context);
+        if (StrictModeEnabled(context)) {
+            CoverReferencedTypes(context);
+        }
+        OnAnalysisComplete(context);
     } catch (ClangDiagnosticAsException& exception) {
         exception.Report(context.getDiagnostics());
     }
@@ -149,9 +152,25 @@ FindClassTemplateDeclByName(clang::DeclContext& decl_context, std::string_view s
     }
 }
 
+static ParameterAnnotations GetParameterAnnotations(clang::ASTContext& context, clang::CXXRecordDecl* decl) {
+    if (!decl->hasDefinition()) {
+        return {};
+    }
+
+    ErrorReporter report_error { context };
+    ParameterAnnotations ret;
+
+    for (const clang::CXXBaseSpecifier& base : decl->bases()) {
+        throw report_error(base.getSourceRange().getBegin(), "Unknown parameter annotation");
+    }
+
+    return ret;
+}
+
 void AnalysisAction::ParseInterface(clang::ASTContext& context) {
     ErrorReporter report_error { context };
 
+    // TODO: Assert fex_gen_type is not declared at non-global namespaces
     if (auto template_decl = FindClassTemplateDeclByName(*context.getTranslationUnitDecl(), "fex_gen_type")) {
         for (auto* decl : template_decl->specializations()) {
             const auto& template_args = decl->getTemplateArgs();
@@ -161,8 +180,42 @@ void AnalysisAction::ParseInterface(clang::ASTContext& context) {
             //       named types (e.g. GLuint/GLenum) are represented by
             //       different Type instances. The canonical type they refer
             //       to is unique, however.
-            auto type = context.getCanonicalType(template_args[0].getAsType()).getTypePtr();
-            funcptr_types.insert(type);
+            clang::QualType type = context.getCanonicalType(template_args[0].getAsType());
+            type = type->getLocallyUnqualifiedSingleStepDesugaredType();
+
+            if (type->isFunctionPointerType() || type->isFunctionType()) {
+                funcptr_types.insert(type.getTypePtr());
+            } else {
+                [[maybe_unused]] auto [it, inserted] = types.emplace(context.getCanonicalType(type.getTypePtr()), RepackedType { });
+                assert(inserted);
+            }
+        }
+    }
+
+    // Process function parameter annotations
+    std::unordered_map<const clang::FunctionDecl*, std::unordered_map<unsigned, ParameterAnnotations>> param_annotations;
+    for (auto& decl_context : decl_contexts) {
+        if (auto template_decl = FindClassTemplateDeclByName(*decl_context, "fex_gen_param")) {
+            for (auto* decl : template_decl->specializations()) {
+                const auto& template_args = decl->getTemplateArgs();
+                assert(template_args.size() == 3);
+
+                auto function = llvm::dyn_cast<clang::FunctionDecl>(template_args[0].getAsDecl());
+                auto param_idx = template_args[1].getAsIntegral().getZExtValue();
+                clang::QualType type = context.getCanonicalType(template_args[2].getAsType());
+                type = type->getLocallyUnqualifiedSingleStepDesugaredType();
+
+                if (param_idx >= function->getNumParams()) {
+                    throw report_error(decl->getTypeAsWritten()->getTypeLoc().getAs<clang::TemplateSpecializationTypeLoc>().getArgLoc(1).getLocation(), "Out-of-bounds parameter index passed to fex_gen_param");
+                }
+
+                if (!type->isVoidType() && !context.hasSameType(type, function->getParamDecl(param_idx)->getType())) {
+                    throw report_error(decl->getTypeAsWritten()->getTypeLoc().getAs<clang::TemplateSpecializationTypeLoc>().getArgLoc(2).getLocation(), "Type passed to fex_gen_param doesn't match the function signature")
+                              .addNote(report_error(function->getParamDecl(param_idx)->getTypeSourceInfo()->getTypeLoc().getBeginLoc(), "Expected this type instead"));
+                }
+
+                param_annotations[function][param_idx] = GetParameterAnnotations(context, decl);
+            }
         }
     }
 
@@ -220,94 +273,200 @@ void AnalysisAction::ParseInterface(clang::ASTContext& context) {
 
                 const auto template_arg_loc = decl->getTypeAsWritten()->getTypeLoc().castAs<clang::TemplateSpecializationTypeLoc>().getArgLoc(0).getLocation();
 
-                auto emitted_function = llvm::dyn_cast<clang::FunctionDecl>(template_args[0].getAsDecl());
-                assert(emitted_function && "Argument is not a function");
-                auto return_type = emitted_function->getReturnType();
+                if (auto emitted_function = llvm::dyn_cast<clang::FunctionDecl>(template_args[0].getAsDecl())) {
+                    auto return_type = emitted_function->getReturnType();
 
-                const auto annotations = GetAnnotations(context, decl);
-                if (return_type->isFunctionPointerType() && !annotations.returns_guest_pointer) {
-                    throw report_error( template_arg_loc,
-                                        "Function pointer return types require explicit annotation\n");
-                }
-
-                // TODO: Use the types as written in the signature instead?
-                ThunkedFunction data;
-                data.function_name = emitted_function->getName().str();
-                data.return_type = return_type;
-                data.is_variadic = emitted_function->isVariadic();
-
-                data.decl = emitted_function;
-
-                data.custom_host_impl = annotations.custom_host_impl;
-
-                for (std::size_t param_idx = 0; param_idx < emitted_function->param_size(); ++param_idx) {
-                    auto* param = emitted_function->getParamDecl(param_idx);
-                    data.param_types.push_back(param->getType());
-
-                    if (param->getType()->isFunctionPointerType()) {
-                        auto funcptr = param->getFunctionType()->getAs<clang::FunctionProtoType>();
-                        ThunkedCallback callback;
-                        callback.return_type = funcptr->getReturnType();
-                        for (auto& cb_param : funcptr->getParamTypes()) {
-                            callback.param_types.push_back(cb_param);
-                        }
-                        callback.is_stub = annotations.callback_strategy == CallbackStrategy::Stub;
-                        callback.is_guest = annotations.callback_strategy == CallbackStrategy::Guest;
-                        callback.is_variadic = funcptr->isVariadic();
-
-                        if (callback.is_guest && !data.custom_host_impl) {
-                            throw report_error(template_arg_loc, "callback_guest can only be used with custom_host_impl");
-                        }
-
-                        data.callbacks.emplace(param_idx, callback);
-                        if (!callback.is_stub && !callback.is_guest) {
-                            funcptr_types.insert(context.getCanonicalType(funcptr));
-                        }
-
-                        if (data.callbacks.size() != 1) {
-                            throw report_error(template_arg_loc, "Support for more than one callback is untested");
-                        }
-                        if (funcptr->isVariadic() && !callback.is_stub) {
-                            throw report_error(template_arg_loc, "Variadic callbacks are not supported");
-                        }
-                    }
-                }
-
-                thunked_api.push_back(ThunkedAPIFunction { (const FunctionParams&)data, data.function_name, data.return_type,
-                                                            namespace_info.host_loader.empty() ? "dlsym_default" : namespace_info.host_loader,
-                                                            data.is_variadic || annotations.custom_guest_entrypoint,
-                                                            data.is_variadic,
-                                                            std::nullopt });
-                if (namespace_info.generate_guest_symtable) {
-                    thunked_api.back().symtable_namespace = namespace_idx;
-                }
-
-                if (data.is_variadic) {
-                    if (!annotations.uniform_va_type) {
-                        throw report_error(decl->getBeginLoc(), "Variadic functions must be annotated with parameter type using uniform_va_type");
+                    const auto annotations = GetAnnotations(context, decl);
+                    if (return_type->isFunctionPointerType() && !annotations.returns_guest_pointer) {
+                        throw report_error( template_arg_loc,
+                                            "Function pointer return types require explicit annotation\n");
                     }
 
-                    // Convert variadic argument list into a count + pointer pair
-                    data.param_types.push_back(context.getSizeType());
-                    data.param_types.push_back(context.getPointerType(*annotations.uniform_va_type));
-                }
+                    // TODO: Use the types as written in the signature instead?
+                    ThunkedFunction data;
+                    data.function_name = emitted_function->getName().str();
+                    data.return_type = return_type;
+                    data.is_variadic = emitted_function->isVariadic();
 
-                if (data.is_variadic) {
-                    // This function is thunked through an "_internal" symbol since its signature
-                    // is different from the one in the native host/guest libraries.
-                    data.function_name = data.function_name + "_internal";
-                    if (data.custom_host_impl) {
-                        throw report_error(decl->getBeginLoc(), "Custom host impl requested but this is implied by the function signature already");
+                    data.decl = emitted_function;
+
+                    data.custom_host_impl = annotations.custom_host_impl;
+
+                    data.param_annotations = param_annotations[emitted_function];
+
+                    const int retval_index = -1;
+                    for (int param_idx = retval_index; param_idx < (int)emitted_function->param_size(); ++param_idx) {
+                        auto param_type = param_idx == retval_index ? emitted_function->getReturnType() : emitted_function->getParamDecl(param_idx)->getType();
+                        auto param_loc = param_idx == retval_index ? emitted_function->getReturnTypeSourceRange().getBegin() : emitted_function->getParamDecl(param_idx)->getBeginLoc();
+
+                        if (param_idx != retval_index) {
+                            data.param_types.push_back(param_type);
+                        } else if (param_type->isVoidType()) {
+                            continue;
+                        }
+
+                        auto check_struct_type = [&](const clang::Type* type) {
+                            if (type->isIncompleteType()) {
+                                if (!StrictModeEnabled(context)) {
+                                    return;
+                                }
+                                throw report_error(type->getAsTagDecl()->getBeginLoc(), "Unannotated pointer with incomplete struct type; consider using an opaque_type annotation")
+                                      .addNote(report_error(emitted_function->getNameInfo().getLoc(), "in function", clang::DiagnosticsEngine::Note))
+                                      .addNote(report_error(template_arg_loc, "used in annotation here", clang::DiagnosticsEngine::Note));
+                            }
+
+                            for (auto* member : type->getAsStructureType()->getDecl()->fields()) {
+                                  /*if (!member->getType()->isPointerType())*/ {
+                                      // TODO: Perform more elaborate validation for non-pointers to ensure ABI compatibility
+                                      continue;
+                                  }
+
+                                  throw report_error(member->getBeginLoc(), "Unannotated pointer member")
+                                        .addNote(report_error(param_loc, "in struct type", clang::DiagnosticsEngine::Note))
+                                        .addNote(report_error(template_arg_loc, "used in annotation here", clang::DiagnosticsEngine::Note));
+                            }
+                        };
+
+                        if (param_type->isFunctionPointerType()) {
+                            if (param_idx == retval_index) {
+                                // TODO: We already rely on this in a few places...
+//                                throw report_error(template_arg_loc, "Support for returning function pointers is not implemented");
+                                continue;
+                            }
+                            auto funcptr = emitted_function->getParamDecl(param_idx)->getFunctionType()->getAs<clang::FunctionProtoType>();
+                            ThunkedCallback callback;
+                            callback.return_type = funcptr->getReturnType();
+                            for (auto& cb_param : funcptr->getParamTypes()) {
+                                callback.param_types.push_back(cb_param);
+                            }
+                            callback.is_stub = annotations.callback_strategy == CallbackStrategy::Stub;
+                            callback.is_guest = annotations.callback_strategy == CallbackStrategy::Guest;
+                            callback.is_variadic = funcptr->isVariadic();
+
+                            if (callback.is_guest && !data.custom_host_impl) {
+                                throw report_error(template_arg_loc, "callback_guest can only be used with custom_host_impl");
+                            }
+
+                            data.callbacks.emplace(param_idx, callback);
+                            if (!callback.is_stub && !callback.is_guest && !data.custom_host_impl) {
+                                funcptr_types.insert(context.getCanonicalType(funcptr));
+                            }
+
+                            if (data.callbacks.size() != 1) {
+                                throw report_error(template_arg_loc, "Support for more than one callback is untested");
+                            }
+                            if (funcptr->isVariadic() && !callback.is_stub) {
+                                throw report_error(template_arg_loc, "Variadic callbacks are not supported");
+                            }
+                        } else if (param_type->isBuiltinType()) {
+                            // NOTE: Intentionally not using getCanonicalType here since that would turn e.g. size_t into platform-specific types
+                            // TODO: Still, we may want to de-duplicate some of these...
+                            types.emplace(param_type.getTypePtr(), RepackedType { });
+                        } else if (param_type->isEnumeralType()) {
+                            types.emplace(context.getCanonicalType(param_type.getTypePtr()), RepackedType { });
+                        } else if ( param_type->isStructureType()) {
+                            check_struct_type(param_type.getTypePtr());
+                            types.emplace(context.getCanonicalType(param_type.getTypePtr()), RepackedType { });
+                        } else if (param_type->isPointerType()) {
+                            auto pointee_type = param_type->getPointeeType()->getLocallyUnqualifiedSingleStepDesugaredType();
+                            if ( pointee_type->isStructureType()) {
+                                check_struct_type(pointee_type.getTypePtr());
+                                types.emplace(context.getCanonicalType(pointee_type.getTypePtr()), RepackedType { });
+                            } else if (false /* TODO: Can't check if this is unsupported until data layout analysis is complete */) {
+                                throw report_error(param_loc, "Unsupported parameter type")
+                                              .addNote(report_error(emitted_function->getNameInfo().getLoc(), "in function", clang::DiagnosticsEngine::Note))
+                                              .addNote(report_error(template_arg_loc, "used in definition here", clang::DiagnosticsEngine::Note));
+                            }
+                        } else {
+                            // TODO: For non-pointer parameters, perform more elaborate validation to ensure ABI compatibility
+                        }
                     }
-                    data.custom_host_impl = true;
+
+                    thunked_api.push_back(ThunkedAPIFunction { (const FunctionParams&)data, data.function_name, data.return_type,
+                                                                namespace_info.host_loader.empty() ? "dlsym_default" : namespace_info.host_loader,
+                                                                data.is_variadic || annotations.custom_guest_entrypoint,
+                                                                data.is_variadic,
+                                                                std::nullopt });
+                    if (namespace_info.generate_guest_symtable) {
+                        thunked_api.back().symtable_namespace = namespace_idx;
+                    }
+
+                    if (data.is_variadic) {
+                        if (!annotations.uniform_va_type) {
+                            throw report_error(decl->getBeginLoc(), "Variadic functions must be annotated with parameter type using uniform_va_type");
+                        }
+
+                        // Convert variadic argument list into a count + pointer pair
+                        data.param_types.push_back(context.getSizeType());
+                        data.param_types.push_back(context.getPointerType(*annotations.uniform_va_type));
+                        types.emplace(context.getSizeType()->getTypePtr(), RepackedType { });
+                        if (!annotations.uniform_va_type.value()->isVoidPointerType()) {
+                            types.emplace(annotations.uniform_va_type->getTypePtr(), RepackedType { });
+                        }
+                    }
+
+                    if (data.is_variadic) {
+                        // This function is thunked through an "_internal" symbol since its signature
+                        // is different from the one in the native host/guest libraries.
+                        data.function_name = data.function_name + "_internal";
+                        if (data.custom_host_impl) {
+                            throw report_error(decl->getBeginLoc(), "Custom host impl requested but this is implied by the function signature already");
+                        }
+                        data.custom_host_impl = true;
+                    }
+
+                    // For indirect calls, register the function signature as a function pointer type
+                    if (namespace_info.indirect_guest_calls) {
+                        funcptr_types.insert(context.getCanonicalType(emitted_function->getFunctionType()));
+                    }
+
+                    thunks.push_back(std::move(data));
+                }
+            }
+        }
+    }
+}
+
+void AnalysisAction::CoverReferencedTypes(clang::ASTContext& context) {
+    // Repeat until no more children are appended
+    for (bool changed = true; std::exchange(changed, false);) {
+        for ( auto next_type_it = types.begin(), type_it = next_type_it;
+              type_it != types.end();
+              type_it = next_type_it) {
+            ++next_type_it;
+            const auto& [type, type_repack_info] = *type_it;
+            if (!type->isStructureType()) {
+                continue;
+            }
+
+            for (auto* member : type->getAsStructureType()->getDecl()->fields()) {
+                auto member_type = member->getType().getTypePtr();
+                while (member_type->isArrayType()) {
+                    member_type = member_type->getArrayElementTypeNoTypeQual();
+                }
+                while (member_type->isPointerType()) {
+                    member_type = member_type->getPointeeType().getTypePtr();
                 }
 
-                // For indirect calls, register the function signature as a function pointer type
-                if (namespace_info.indirect_guest_calls) {
-                    funcptr_types.insert(context.getCanonicalType(emitted_function->getFunctionType()));
+                if (!member_type->isBuiltinType()) {
+                    member_type = context.getCanonicalType(member_type);
+                }
+                if (member_type->isUnionType() && !types.contains(member_type)) {
+                    throw std::runtime_error(fmt::format("\"{}\" has unannotated member \"{}\" of union type \"{}\"",
+                                                         clang::QualType { type, 0 }.getAsString(),
+                                                         member->getNameAsString(),
+                                                         clang::QualType { member_type, 0 }.getAsString()));
                 }
 
-                thunks.push_back(std::move(data));
+                if (!member_type->isStructureType() && !(member_type->isBuiltinType() && !member_type->isVoidType()) && !member_type->isEnumeralType()) {
+                    continue;
+                }
+
+                auto [new_type_it, inserted] = types.emplace(member_type, RepackedType { });
+                if (inserted) {
+                    changed = true;
+                    next_type_it = new_type_it;
+                }
             }
         }
     }
diff --git a/ThunkLibs/Generator/analysis.h b/ThunkLibs/Generator/analysis.h
index 7aeaa9d77..3255444ce 100644
--- a/ThunkLibs/Generator/analysis.h
+++ b/ThunkLibs/Generator/analysis.h
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <clang/Basic/FileEntry.h>
 #include <clang/Frontend/FrontendAction.h>
 
 #include <memory>
@@ -21,6 +22,10 @@ struct ThunkedCallback : FunctionParams {
     bool is_variadic = false;
 };
 
+struct ParameterAnnotations {
+    bool operator==(const ParameterAnnotations&) const = default;
+};
+
 /**
  * Guest<->Host transition point.
  *
@@ -52,6 +57,10 @@ struct ThunkedFunction : FunctionParams {
     // Maps parameter index to ThunkedCallback
     std::unordered_map<unsigned, ThunkedCallback> callbacks;
 
+    // Maps parameter index to ParameterAnnotations
+    // TODO: Use index -1 for the return value?
+    std::unordered_map<unsigned, ParameterAnnotations> param_annotations;
+
     clang::FunctionDecl* decl;
 };
 
@@ -105,19 +114,76 @@ public:
 
     std::unique_ptr<clang::ASTConsumer> CreateASTConsumer(clang::CompilerInstance&, clang::StringRef /*file*/) override;
 
+    struct RepackedType {
+    };
+
 protected:
     // Build the internal API representation by processing fex_gen_config and other annotated entities
     void ParseInterface(clang::ASTContext&);
 
+    // Recursively extend the type set to include types of struct members
+    void CoverReferencedTypes(clang::ASTContext&);
+
     // Called from ExecuteAction() after parsing is complete
-    virtual void EmitOutput(clang::ASTContext&) {};
+    virtual void OnAnalysisComplete(clang::ASTContext&) {};
 
     std::vector<clang::DeclContext*> decl_contexts;
 
     std::vector<ThunkedFunction> thunks;
     std::vector<ThunkedAPIFunction> thunked_api;
+
     std::unordered_set<const clang::Type*> funcptr_types;
 
+    std::unordered_map<const clang::Type*, RepackedType> types;
     std::optional<unsigned> lib_version;
     std::vector<NamespaceInfo> namespaces;
+
+    RepackedType& LookupType(clang::ASTContext& context, const clang::Type* type) {
+      return types.at(context.getCanonicalType(type));
+    }
 };
+
+inline std::string get_type_name(const clang::ASTContext& context, const clang::Type* type) {
+    if (type->isBuiltinType()) {
+        // Skip canonicalization
+        return clang::QualType { type, 0 }.getAsString();
+    }
+
+    if (auto decl = type->getAsTagDecl()) {
+        // Replace unnamed types with a placeholder. This will fail to compile if referenced
+        // anywhere in generated code, but at least it will point to a useful location.
+        //
+        // A notable exception are C-style struct declarations like "typedef struct (unnamed) { ... } MyStruct;".
+        // A typedef name is associated with these for linking purposes, so
+        // getAsString() will produce a usable identifier.
+        // TODO: Consider turning this into a hard error instead of replacing the name
+        if (!decl->getDeclName() && !decl->getTypedefNameForAnonDecl()) {
+            auto loc = context.getSourceManager().getPresumedLoc(decl->getLocation());
+            std::string filename = loc.getFilename();
+            filename = std::move(filename).substr(filename.rfind("/"));
+            filename = std::move(filename).substr(1);
+            std::replace(filename.begin(), filename.end(), '.', '_');
+            return "unnamed_type_" + filename + "_" + std::to_string(loc.getLine());
+        }
+    }
+
+    auto type_name = clang::QualType { context.getCanonicalType(type), 0 }.getAsString();
+    if (type_name.starts_with("struct ")) {
+        type_name = type_name.substr(7);
+    }
+    if (type_name.starts_with("class ") || type_name.starts_with("union ")) {
+        type_name = type_name.substr(6);
+    }
+    if (type_name.starts_with("enum ")) {
+        type_name = type_name.substr(5);
+    }
+    return type_name;
+}
+
+// Analysis can't process interfaces of real libraries, yet. This function
+// defines a "strict mode" to use for tests, only. Real libraries will switch
+// to strict mode once analysis is more feature-complete.
+inline bool StrictModeEnabled(clang::ASTContext& context) {
+    auto filename = context.getSourceManager().getFileEntryForID(context.getSourceManager().getMainFileID())->getName();
+    return filename.endswith("libfex_thunk_test_interface.cpp") || filename.endswith("gen_input.cpp");
+}
diff --git a/ThunkLibs/Generator/data_layout.cpp b/ThunkLibs/Generator/data_layout.cpp
new file mode 100644
index 000000000..da05a6ee8
--- /dev/null
+++ b/ThunkLibs/Generator/data_layout.cpp
@@ -0,0 +1,318 @@
+#include "analysis.h"
+#include "data_layout.h"
+#include "interface.h"
+
+#include <fmt/format.h>
+
+#include <openssl/sha.h>
+
+constexpr bool enable_debug_output = false;
+
+// Visitor for gathering data layout information that can be passed across libclang invocations
+class AnalyzeDataLayoutAction : public AnalysisAction {
+    ABI& type_abi;
+
+    void OnAnalysisComplete(clang::ASTContext&) override;
+
+public:
+    AnalyzeDataLayoutAction(ABI&);
+};
+
+AnalyzeDataLayoutAction::AnalyzeDataLayoutAction(ABI& abi_) : type_abi(abi_) {
+}
+
+std::unordered_map<const clang::Type*, TypeInfo> ComputeDataLayout(const clang::ASTContext& context, const std::unordered_map<const clang::Type*, AnalysisAction::RepackedType>& types) {
+    std::unordered_map<const clang::Type*, TypeInfo> layout;
+
+    // First, add all types directly used in function signatures of the library API to the meta set
+    for (const auto& [type, type_repack_info] : types) {
+        if (type->isIncompleteType()) {
+            throw std::runtime_error("Cannot compute data layout of incomplete type \"" + clang::QualType { type, 0 }.getAsString() + "\". Did you forget any annotations?");
+        }
+
+        if (type->isStructureType()) {
+            StructInfo info;
+            info.size_bits = context.getTypeSize(type);
+            info.alignment_bits = context.getTypeAlign(type);
+
+            auto [_, inserted] = layout.insert(std::pair { context.getCanonicalType(type), info });
+            if (!inserted) {
+                throw std::runtime_error("Failed to gather type metadata: Type \"" + clang::QualType { type, 0 }.getAsString() + "\" already registered");
+            }
+        } else if (type->isBuiltinType() || type->isEnumeralType()) {
+            SimpleTypeInfo info;
+            info.size_bits = context.getTypeSize(type);
+            info.alignment_bits = context.getTypeAlign(type);
+
+            // NOTE: Non-enum types are intentionally not canonicalized since that would turn e.g. size_t into platform-specific types
+            auto [_, inserted] = layout.insert(std::pair { type->isEnumeralType() ? context.getCanonicalType(type) : type, info });
+            if (!inserted) {
+                throw std::runtime_error("Failed to gather type metadata: Type \"" + clang::QualType { type, 0 }.getAsString() + "\" already registered");
+            }
+        }
+    }
+
+    // Then, add information about members
+    for (const auto& [type, type_repack_info] : types) {
+        if (!type->isStructureType()) {
+            continue;
+        }
+
+        auto& info = *layout.at(context.getCanonicalType(type)).get_if_struct();
+
+        for (auto* field : type->getAsStructureType()->getDecl()->fields()) {
+            auto field_type = field->getType().getTypePtr();
+            std::optional<uint64_t> array_size;
+            if (auto array_type = llvm::dyn_cast<clang::ConstantArrayType>(field->getType())) {
+                array_size = array_type->getSize().getZExtValue();
+                field_type = array_type->getElementType().getTypePtr();
+                if (llvm::isa<clang::ConstantArrayType>(field_type)) {
+                    throw std::runtime_error("Unsupported multi-dimensional array member \"" + field->getNameAsString() + "\" in type \"" + clang::QualType { type, 0 }.getAsString() + "\"");
+                }
+            }
+
+            StructInfo::MemberInfo member_info {
+                .size_bits = context.getTypeSize(field->getType()), // Total size even for arrays
+                .offset_bits = context.getFieldOffset(field),
+                .type_name = get_type_name(context, field_type),
+                .member_name = field->getNameAsString(),
+                .array_size = array_size,
+            };
+
+            // TODO: Process types in dependency-order. Currently we skip this
+            //       check if we haven't processed the member type already,
+            //       which is only safe since this is a consistency check
+            if (field_type->isStructureType() && layout.contains(context.getCanonicalType(field_type))) {
+                // Assert for self-consistency
+                auto field_meta = layout.at(context.getCanonicalType(field_type));
+                (void)types.at(context.getCanonicalType(field_type));
+                if (auto field_info = field_meta.get_if_simple_or_struct()) {
+                    if (field_info->size_bits != member_info.size_bits / member_info.array_size.value_or(1)) {
+                        throw std::runtime_error("Inconsistent type size detected");
+                    }
+                }
+            }
+
+            // Add built-in types, even if referenced through a pointer
+            for (auto* inner_field_type = field_type; inner_field_type; inner_field_type = inner_field_type->getPointeeType().getTypePtrOrNull()) {
+                if (inner_field_type->isBuiltinType() || inner_field_type->isEnumeralType()) {
+                    // The analysis pass doesn't explicitly register built-in types, so add them manually here
+                    SimpleTypeInfo info {
+                        .size_bits = context.getTypeSize(inner_field_type),
+                        .alignment_bits = context.getTypeAlign(inner_field_type),
+                    };
+                    if (!inner_field_type->isBuiltinType()) {
+                        inner_field_type = context.getCanonicalType(inner_field_type);
+                    }
+                    [[maybe_unused]] auto [prev, inserted] = layout.insert(std::pair { inner_field_type, info });
+//                    if (!inserted && prev->second != TypeInfo { info }) {
+//                        // TODO: Throw error since consistency check failed
+//                    }
+                }
+            }
+
+            info.members.push_back(member_info);
+        }
+    }
+
+    if (enable_debug_output) {
+        for (const auto& [type, info] : layout) {
+            auto basic_info = info.get_if_simple_or_struct();
+            if (!basic_info) {
+                continue;
+            }
+
+            fprintf(stderr, "  Host entry %s: %lu (%lu)\n", clang::QualType { type, 0 }.getAsString().c_str(), basic_info->size_bits / 8, basic_info->alignment_bits / 8);
+
+            if (auto struct_info = info.get_if_struct()) {
+                for (const auto& member : struct_info->members) {
+                    fprintf(stderr, "    Offset %lu-%lu: %s %s%s\n", member.offset_bits / 8, (member.offset_bits + member.size_bits - 1) / 8, member.type_name.c_str(), member.member_name.c_str(), member.array_size ? fmt::format("[{}]", member.array_size.value()).c_str() : "");
+                }
+            }
+        }
+    }
+
+    return layout;
+}
+
+ABI GetStableLayout(const clang::ASTContext& context, const std::unordered_map<const clang::Type*, TypeInfo>& data_layout) {
+    ABI stable_layout;
+
+    for (auto [type, type_info] : data_layout) {
+        auto type_name = get_type_name(context, type);
+        auto [it, inserted] = stable_layout.insert(std::pair { type_name, type_info });
+        if (!inserted && it->second != type_info) {
+            throw std::runtime_error("Duplicate type information: Tried to re-register type \"" + type_name + "\"");
+        }
+    }
+
+    stable_layout.pointer_size = context.getTypeSize(context.getUIntPtrType()) / 8;
+
+    return stable_layout;
+}
+
+void AnalyzeDataLayoutAction::OnAnalysisComplete(clang::ASTContext& context) {
+    if (StrictModeEnabled(context)) {
+    type_abi = GetStableLayout(context, ComputeDataLayout(context, types));
+    }
+}
+
+TypeCompatibility DataLayoutCompareAction::GetTypeCompatibility(
+        const clang::ASTContext& context,
+        const clang::Type* type,
+        const std::unordered_map<const clang::Type*, TypeInfo> host_abi,
+        std::unordered_map<const clang::Type*, TypeCompatibility>& type_compat) {
+    assert(type->isCanonicalUnqualified() || type->isBuiltinType() || type->isEnumeralType());
+
+    {
+        // Reserve a slot to be filled later. The placeholder value is used
+        // to detect infinite recursions.
+        constexpr auto placeholder_compat = TypeCompatibility { 100 };
+        auto [existing_compat_it, is_new_type] = type_compat.emplace(type, placeholder_compat);
+        if (!is_new_type) {
+            if (existing_compat_it->second == placeholder_compat) {
+                throw std::runtime_error("Found recursive reference to type \"" + clang::QualType { type, 0 }.getAsString() + "\"");
+            }
+
+            return existing_compat_it->second;
+        }
+    }
+
+    auto type_name = get_type_name(context, type);
+    auto& guest_info = guest_abi.at(type_name);
+    auto& host_info = host_abi.at(type->isBuiltinType() ? type : context.getCanonicalType(type));
+
+    const bool is_32bit = (guest_abi.pointer_size == 4);
+
+    // Assume full compatibility, then downgrade as needed
+    auto compat = TypeCompatibility::Full;
+
+    if (guest_info != host_info) {
+        // Non-matching data layout... downgrade to Repackable
+        // TODO: Even for non-structs, this only works if the types are reasonably similar (e.g. uint32_t -> uint64_t)
+        compat = TypeCompatibility::Repackable;
+    }
+
+    auto guest_struct_info = guest_info.get_if_struct();
+    if (guest_struct_info && guest_struct_info->members.size() != host_info.get_if_struct()->members.size()) {
+        // Members are missing from either the guest or host layout
+        // NOTE: If the members are merely named differently, this will be caught in the else-if below
+        compat = TypeCompatibility::None;
+    } else if (guest_struct_info) {
+        std::vector<TypeCompatibility> member_compat;
+        for (std::size_t member_idx = 0; member_idx < guest_struct_info->members.size(); ++member_idx) {
+            // Look up the corresponding member in the host struct definition.
+            // The members may be listed in a different order, so we can't
+            // directly use member_idx for this
+            auto* host_member_field = [&]() -> clang::FieldDecl* {
+                auto struct_decl = type->getAsStructureType()->getDecl();
+                auto it = std::find_if(struct_decl->field_begin(), struct_decl->field_end(), [&](auto* field) {
+                    return field->getName() == guest_struct_info->members.at(member_idx).member_name;
+                });
+                if (it == struct_decl->field_end()) {
+                    return nullptr;
+                }
+                return *it;
+            }();
+            if (!host_member_field) {
+                // No corresponding host struct member
+                // TODO: Also detect host members that are missing from the guest struct
+                member_compat.push_back(TypeCompatibility::None);
+                break;
+            }
+
+            auto host_member_type = context.getCanonicalType(host_member_field->getType().getTypePtr());
+            if (auto array_type = llvm::dyn_cast<clang::ConstantArrayType>(host_member_type)) {
+                // Compare array element type only. The array size is already considered by the layout information of the containing struct.
+                host_member_type = context.getCanonicalType(array_type->getElementType().getTypePtr());
+            }
+
+            if (host_member_type->isPointerType()) {
+                // Automatic repacking of pointers to non-compatible types is only possible if:
+                // * Pointee is fully compatible, or
+                // * Pointer member is annotated
+                // TODO: Don't restrict this to structure types. it applies to pointers to builtin types too!
+                auto host_member_pointee_type = context.getCanonicalType(host_member_type->getPointeeType().getTypePtr());
+                if (host_member_pointee_type->isPointerType()) {
+                    // This is a nested pointer, e.g. void**
+
+                    if (is_32bit) {
+                        // Nested pointers can't be repacked on 32-bit
+                        member_compat.push_back(TypeCompatibility::None);
+                    } else {
+                        // Check the innermost type's compatibility on 64-bit
+                        auto pointee_pointee_type = host_member_pointee_type->getPointeeType().getTypePtr();
+                        // TODO: Not sure how to handle void here. Probably should require an annotation instead of "just working"
+                        auto pointee_pointee_compat = pointee_pointee_type->isVoidType() ? TypeCompatibility::Full : GetTypeCompatibility(context, pointee_pointee_type, host_abi, type_compat);
+                        if (pointee_pointee_compat == TypeCompatibility::Full) {
+                            member_compat.push_back(TypeCompatibility::Full);
+                        } else {
+                            member_compat.push_back(TypeCompatibility::None);
+                        }
+                    }
+                } else if (!host_member_pointee_type->isVoidType() && (host_member_pointee_type->isBuiltinType() || host_member_pointee_type->isEnumeralType())) {
+                    // TODO: What are good heuristics for this?
+                    // size_t should yield TypeCompatibility::Repackable
+                    // inconsistent types should probably default to TypeCompatibility::None
+                    // For now, just always assume compatible... (will degrade to Repackable below)
+                    member_compat.push_back(TypeCompatibility::Full);
+                } else if (!host_member_pointee_type->isVoidType() && (host_member_pointee_type->isStructureType() || types.contains(host_member_pointee_type))) {
+                    auto pointee_compat = GetTypeCompatibility(context, host_member_pointee_type, host_abi, type_compat);
+                    if (pointee_compat == TypeCompatibility::Full) {
+                        // Pointee is fully compatible, so automatic repacking only requires converting the pointers themselves
+                        member_compat.push_back(is_32bit ? TypeCompatibility::Repackable : TypeCompatibility::Full);
+                    } else {
+                        // If the pointee is incompatible (even if repackable), automatic repacking isn't possible
+                        member_compat.push_back(TypeCompatibility::None);
+                    }
+                } else if (!is_32bit && host_member_pointee_type->isVoidType()) {
+                    // TODO: Not sure how to handle void here. Probably should require an annotation instead of "just working"
+                    member_compat.push_back(TypeCompatibility::Full);
+                } else {
+                    member_compat.push_back(TypeCompatibility::None);
+                }
+                continue;
+            }
+
+            if (guest_abi.at(guest_struct_info->members[member_idx].type_name).get_if_struct()) {
+                auto host_type_info = host_abi.at(host_member_type);
+                member_compat.push_back(GetTypeCompatibility(context, host_member_type, host_abi, type_compat));
+            } else {
+                // Member was checked for size/alignment above already
+            }
+        }
+
+        if (std::all_of(member_compat.begin(), member_compat.end(), [](auto compat) { return compat == TypeCompatibility::Full; })) {
+            // TypeCompatibility::Full or ::Repackable
+        } else if (std::none_of(member_compat.begin(), member_compat.end(), [](auto compat) { return compat == TypeCompatibility::None; })) {
+            // Downgrade to Repackable
+            compat = TypeCompatibility::Repackable;
+        } else {
+            // Downgrade to None
+            compat = TypeCompatibility::None;
+        }
+    }
+
+    type_compat.at(type) = compat;
+    return compat;
+}
+
+DataLayoutCompareActionFactory::DataLayoutCompareActionFactory(const ABI& abi) : abi(abi) {
+
+}
+
+DataLayoutCompareActionFactory::~DataLayoutCompareActionFactory() = default;
+
+std::unique_ptr<clang::FrontendAction> DataLayoutCompareActionFactory::create() {
+    return std::make_unique<DataLayoutCompareAction>(abi);
+}
+
+AnalyzeDataLayoutActionFactory::AnalyzeDataLayoutActionFactory() : abi(std::make_unique<ABI>()) {
+
+}
+
+AnalyzeDataLayoutActionFactory::~AnalyzeDataLayoutActionFactory() = default;
+
+std::unique_ptr<clang::FrontendAction> AnalyzeDataLayoutActionFactory::create() {
+    return std::make_unique<AnalyzeDataLayoutAction>(*abi);
+}
diff --git a/ThunkLibs/Generator/data_layout.h b/ThunkLibs/Generator/data_layout.h
new file mode 100644
index 000000000..3d58357f4
--- /dev/null
+++ b/ThunkLibs/Generator/data_layout.h
@@ -0,0 +1,116 @@
+#pragma once
+
+#include "analysis.h"
+
+#include <clang/Frontend/FrontendAction.h>
+
+#include <cstdint>
+#include <optional>
+#include <string>
+#include <unordered_map>
+#include <variant>
+#include <vector>
+
+struct SimpleTypeInfo {
+    uint64_t size_bits;
+    uint64_t alignment_bits;
+
+    bool operator==(const SimpleTypeInfo& other) const {
+        return  size_bits == other.size_bits &&
+                alignment_bits == other.alignment_bits;
+    }
+};
+
+struct StructInfo : SimpleTypeInfo {
+    struct MemberInfo {
+        uint64_t size_bits; // size of this member. For arrays, total size of all elements
+        uint64_t offset_bits;
+        std::string type_name;
+        std::string member_name;
+        std::optional<uint64_t> array_size;
+
+        bool operator==(const MemberInfo& other) const {
+            return  size_bits == other.size_bits &&
+                    offset_bits == other.offset_bits &&
+                    type_name == other.type_name &&
+                    member_name == other.member_name &&
+                    array_size == other.array_size;
+        }
+    };
+
+    std::vector<MemberInfo> members;
+
+    bool operator==(const StructInfo& other) const {
+        return (const SimpleTypeInfo&)*this == (const SimpleTypeInfo&)other &&
+                std::equal(members.begin(), members.end(), other.members.begin(), other.members.end());
+    }
+};
+
+struct TypeInfo : std::variant<std::monostate, SimpleTypeInfo, StructInfo> {
+    using Parent = std::variant<std::monostate, SimpleTypeInfo, StructInfo>;
+
+    TypeInfo() = default;
+    TypeInfo(const SimpleTypeInfo& info) : Parent(info) {}
+    TypeInfo(const StructInfo& info) : Parent(info) {}
+
+    // Opaque declaration with no full definition.
+    // Pointers to these can still be passed along ABI boundaries assuming
+    // implementation details are only ever accessed on one side.
+    bool is_opaque() const {
+        return std::holds_alternative<std::monostate>(*this);
+    }
+
+    const StructInfo* get_if_struct() const {
+        return std::get_if<StructInfo>(this);
+    }
+
+    StructInfo* get_if_struct() {
+        return std::get_if<StructInfo>(this);
+    }
+
+    const SimpleTypeInfo* get_if_simple_or_struct() const {
+        auto as_struct = std::get_if<StructInfo>(this);
+        if (as_struct) {
+            return as_struct;
+        }
+        return std::get_if<SimpleTypeInfo>(this);
+    }
+};
+
+struct FuncPtrInfo {
+    std::array<uint8_t, 32> sha256;
+    std::string result;
+    std::vector<std::string> args;
+};
+
+struct ABI : std::unordered_map<std::string, TypeInfo> {
+    int pointer_size; // in bytes
+};
+
+std::unordered_map<const clang::Type*, TypeInfo>
+ComputeDataLayout(const clang::ASTContext& context, const std::unordered_map<const clang::Type*, AnalysisAction::RepackedType>& types);
+
+// Convert the output of ComputeDataLayout to a format that isn't tied to a libclang session.
+// As a consequence, type information is indexed by type name instead of clang::Type.
+ABI GetStableLayout(const clang::ASTContext& context, const std::unordered_map<const clang::Type*, TypeInfo>& data_layout);
+
+enum class TypeCompatibility {
+    Full,       // Type has matching data layout across architectures
+    Repackable, // Type has different data layout but can be repacked automatically
+    None,       // Type has different data layout and cannot be repacked automatically
+};
+
+class DataLayoutCompareAction : public AnalysisAction {
+public:
+    DataLayoutCompareAction(const ABI& guest_abi) : guest_abi(guest_abi) {
+    }
+
+    TypeCompatibility GetTypeCompatibility(
+            const clang::ASTContext&,
+            const clang::Type*,
+            const std::unordered_map<const clang::Type*, TypeInfo> host_abi,
+            std::unordered_map<const clang::Type*, TypeCompatibility>& type_compat);
+
+private:
+    const ABI& guest_abi;
+};
diff --git a/ThunkLibs/Generator/gen.cpp b/ThunkLibs/Generator/gen.cpp
index bef050da2..5db5142a1 100644
--- a/ThunkLibs/Generator/gen.cpp
+++ b/ThunkLibs/Generator/gen.cpp
@@ -1,6 +1,7 @@
 #include "analysis.h"
+#include "data_layout.h"
+#include "diagnostics.h"
 #include "interface.h"
-
 #include <clang/Frontend/CompilerInstance.h>
 
 #include <fstream>
@@ -8,27 +9,28 @@
 #include <iostream>
 #include <string_view>
 #include <unordered_map>
+#include <variant>
 
 #include <fmt/format.h>
 #include <fmt/ostream.h>
 
 #include <openssl/sha.h>
 
-class GenerateThunkLibsAction : public AnalysisAction {
+class GenerateThunkLibsAction : public DataLayoutCompareAction {
 public:
-    GenerateThunkLibsAction(const std::string& libname, const OutputFilenames&);
+    GenerateThunkLibsAction(const std::string& libname, const OutputFilenames&, const ABI& abi);
 
 private:
     // Generate helper code for thunk libraries and write them to the output file
-    void EmitOutput(clang::ASTContext&) override;
+    void OnAnalysisComplete(clang::ASTContext&) override;
 
     const std::string& libfilename;
     std::string libname; // sanitized filename, usable as part of emitted function names
     const OutputFilenames& output_filenames;
 };
 
-GenerateThunkLibsAction::GenerateThunkLibsAction(const std::string& libname_, const OutputFilenames& output_filenames_)
-    : libfilename(libname_), libname(libname_), output_filenames(output_filenames_) {
+GenerateThunkLibsAction::GenerateThunkLibsAction(const std::string& libname_, const OutputFilenames& output_filenames_, const ABI& abi)
+    : DataLayoutCompareAction(abi), libfilename(libname_), libname(libname_), output_filenames(output_filenames_) {
     for (auto& c : libname) {
         if (c == '-') {
             c = '_';
@@ -47,7 +49,21 @@ static std::string format_function_args(const FunctionParams& params, Fn&& forma
     return ret;
 };
 
-void GenerateThunkLibsAction::EmitOutput(clang::ASTContext& context) {
+void GenerateThunkLibsAction::OnAnalysisComplete(clang::ASTContext& context) {
+    ErrorReporter report_error { context };
+
+    // Compute data layout differences between host and guest
+    auto type_compat = [&]() {
+        std::unordered_map<const clang::Type*, TypeCompatibility> ret;
+        if (StrictModeEnabled(context)) {
+        const auto host_abi = ComputeDataLayout(context, types);
+        for (const auto& [type, type_repack_info] : types) {
+            GetTypeCompatibility(context, type, host_abi, ret);
+        }
+        }
+        return ret;
+    }();
+
     static auto format_decl = [](clang::QualType type, const std::string_view& name) {
         clang::QualType innermostPointee = type;
         while (innermostPointee->isPointerType()) {
@@ -270,6 +286,24 @@ void GenerateThunkLibsAction::EmitOutput(clang::ASTContext& context) {
                 file << ") -> " << thunk.return_type.getAsString() << ";\n";
             }
 
+            // Check data layout compatibility of parameter types
+            // TODO: Also check non-struct/non-pointer types
+            // TODO: Also check return type
+            for (size_t param_idx = 0; StrictModeEnabled(context) && param_idx != thunk.param_types.size(); ++param_idx) {
+                const auto& param_type = thunk.param_types[param_idx];
+                if (!param_type->isPointerType() || !param_type->getPointeeType()->isStructureType()) {
+                    continue;
+                }
+                auto type = param_type->getPointeeType();
+                if (type_compat.at(context.getCanonicalType(type.getTypePtr())) == TypeCompatibility::None) {
+                    // TODO: Factor in "assume_compatible_layout" annotations here
+                    //       That annotation should cause the type to be treated as TypeCompatibility::Full
+                    {
+                        throw report_error(thunk.decl->getLocation(), "Unsupported parameter type %0").AddTaggedVal(param_type);
+                    }
+                }
+            }
+
             // Packed argument structs used in fexfn_unpack_*
             auto GeneratePackedArgs = [&](const auto &function_name, const auto &thunk) -> std::string {
                 std::string struct_name = "fexfn_packed_args_" + libname + "_" + function_name;
@@ -295,6 +329,31 @@ void GenerateThunkLibsAction::EmitOutput(clang::ASTContext& context) {
 
             file << "static void fexfn_unpack_" << libname << "_" << function_name << "(" << struct_name << "* args) {\n";
             file << (thunk.return_type->isVoidType() ? "  " : "  args->rv = ") << function_to_call << "(";
+
+            for (unsigned param_idx = 0; StrictModeEnabled(context) && param_idx != thunk.param_types.size(); ++param_idx) {
+                if (thunk.callbacks.contains(param_idx) && thunk.callbacks.at(param_idx).is_stub) {
+                    continue;
+                }
+
+                auto& param_type = thunk.param_types[param_idx];
+
+                std::optional<TypeCompatibility> pointee_compat;
+                if (param_type->isPointerType()) {
+                    // Get TypeCompatibility from existing entry, or register TypeCompatibility::None if no entry exists
+                    // TODO: Currently needs TypeCompatibility::Full workaround...
+                    pointee_compat = type_compat.emplace(context.getCanonicalType(param_type->getPointeeType().getTypePtr()), TypeCompatibility::Full).first->second;
+                }
+
+                if (!param_type->isPointerType() || pointee_compat == TypeCompatibility::Full ||
+                    param_type->getPointeeType()->isBuiltinType() /* TODO: handle size_t. Actually, properly check for data layout compatibility */) {
+                    // Fully compatible
+                } else if (pointee_compat == TypeCompatibility::Repackable) {
+                    throw report_error(thunk.decl->getLocation(), "Pointer parameter %1 of function %0 requires automatic repacking, which is not implemented yet").AddString(function_name).AddTaggedVal(param_type);
+                } else {
+                    throw report_error(thunk.decl->getLocation(), "Cannot generate unpacking function for function %0 with unannotated pointer parameter %1").AddString(function_name).AddTaggedVal(param_type);
+                }
+            }
+
             {
                 auto format_param = [&](std::size_t idx) {
                     auto cb = thunk.callbacks.find(idx);
@@ -371,7 +430,7 @@ bool GenerateThunkLibsActionFactory::runInvocation(
   Compiler.setInvocation(std::move(Invocation));
   Compiler.setFileManager(Files);
 
-  GenerateThunkLibsAction Action(libname, output_filenames);
+  GenerateThunkLibsAction Action(libname, output_filenames, abi);
 
   Compiler.createDiagnostics(DiagConsumer, false);
   if (!Compiler.hasDiagnostics())
diff --git a/ThunkLibs/Generator/interface.h b/ThunkLibs/Generator/interface.h
index 62883670a..86768d34f 100644
--- a/ThunkLibs/Generator/interface.h
+++ b/ThunkLibs/Generator/interface.h
@@ -8,10 +8,38 @@ struct OutputFilenames {
     std::string guest;
 };
 
+class AnalyzeDataLayoutActionFactory : public clang::tooling::FrontendActionFactory {
+    std::unique_ptr<struct ABI> abi;
+
+public:
+    AnalyzeDataLayoutActionFactory();
+    ~AnalyzeDataLayoutActionFactory();
+
+    std::unique_ptr<clang::FrontendAction> create() override;
+
+    const ABI& GetDataLayout() {
+        return *abi;
+    }
+
+    std::unique_ptr<ABI> TakeDataLayout() {
+        return std::move(abi);
+    }
+};
+
+class DataLayoutCompareActionFactory : public clang::tooling::FrontendActionFactory {
+    const ABI& abi;
+
+public:
+    DataLayoutCompareActionFactory(const ABI&);
+    ~DataLayoutCompareActionFactory();
+
+    std::unique_ptr<clang::FrontendAction> create() override;
+};
+
 class GenerateThunkLibsActionFactory : public clang::tooling::ToolAction {
 public:
-    GenerateThunkLibsActionFactory(std::string_view libname_, OutputFilenames output_filenames_)
-        : libname(std::move(libname_)), output_filenames(std::move(output_filenames_)) {
+    GenerateThunkLibsActionFactory(std::string_view libname_, OutputFilenames output_filenames_, const ABI& abi_)
+        : libname(std::move(libname_)), output_filenames(std::move(output_filenames_)), abi(abi_) {
     }
 
     bool runInvocation(
@@ -22,4 +50,5 @@ public:
 private:
     std::string libname;
     OutputFilenames output_filenames;
+    const ABI& abi;
 };
diff --git a/ThunkLibs/Generator/main.cpp b/ThunkLibs/Generator/main.cpp
index ecc32bd68..726e5cbfa 100644
--- a/ThunkLibs/Generator/main.cpp
+++ b/ThunkLibs/Generator/main.cpp
@@ -14,10 +14,10 @@ void print_usage(const char* program_name) {
     std::cerr << "Usage: " << program_name << " <filename> <libname> <gen_target> <output_filename> -- <clang_flags>\n";
 }
 
-int main(int argc, char* argv[]) {
+int main(int argc, char* const argv[]) {
     llvm::sys::PrintStackTraceOnErrorSignal(argv[0]);
 
-    if (argc < 6) {
+    if (argc < 5) {
         print_usage(argv[0]);
         return EXIT_FAILURE;
     }
@@ -32,12 +32,12 @@ int main(int argc, char* argv[]) {
     }
 
     // Process arguments before the "--" separator
-    if (argc != 5) {
+    if (argc != 5 && argc != 6) {
         print_usage(argv[0]);
         return EXIT_FAILURE;
     }
 
-    char** arg = argv + 1;
+    char* const* arg = argv + 1;
     const auto filename = *arg++;
     const std::string libname = *arg++;
     const std::string target_abi = *arg++;
@@ -62,5 +62,29 @@ int main(int argc, char* argv[]) {
         };
         Tool.appendArgumentsAdjuster(set_resource_directory);
     }
-    return Tool.run(std::make_unique<GenerateThunkLibsActionFactory>(std::move(libname), std::move(output_filenames)).get());
+
+    ClangTool GuestTool = Tool;
+
+    {
+        const bool is_32bit_guest = (argv[5] == std::string_view { "-for-32bit-guest" });
+        auto append_guest_args = [is_32bit_guest](const clang::tooling::CommandLineArguments &Args, clang::StringRef) {
+            clang::tooling::CommandLineArguments AdjustedArgs = Args;
+            const char* platform = is_32bit_guest ? "i686" : "x86_64";
+            if (is_32bit_guest) {
+                AdjustedArgs.push_back("-m32");
+                AdjustedArgs.push_back("-DIS_32BIT_THUNK");
+            }
+            AdjustedArgs.push_back(std::string { "--target=" } + platform + "-linux-unknown");
+            AdjustedArgs.push_back("-isystem");
+            AdjustedArgs.push_back(std::string { "/usr/" } + platform + "-linux-gnu/include/");
+            return AdjustedArgs;
+        };
+        GuestTool.appendArgumentsAdjuster(append_guest_args);
+    }
+
+    auto data_layout_analysis_factory = std::make_unique<AnalyzeDataLayoutActionFactory>();
+    GuestTool.run(data_layout_analysis_factory.get());
+    auto& data_layout = data_layout_analysis_factory->GetDataLayout();
+
+    return Tool.run(std::make_unique<GenerateThunkLibsActionFactory>(std::move(libname), std::move(output_filenames), data_layout).get());
 }
diff --git a/ThunkLibs/GuestLibs/CMakeLists.txt b/ThunkLibs/GuestLibs/CMakeLists.txt
index 69223e8a4..dbe718b49 100644
--- a/ThunkLibs/GuestLibs/CMakeLists.txt
+++ b/ThunkLibs/GuestLibs/CMakeLists.txt
@@ -52,6 +52,9 @@ function(generate NAME SOURCE_FILE)
   add_library(${NAME}-guest-deps INTERFACE)
   target_include_directories(${NAME}-guest-deps INTERFACE "${CMAKE_CURRENT_SOURCE_DIR}/../include")
   target_compile_definitions(${NAME}-guest-deps INTERFACE GUEST_THUNK_LIBRARY)
+  if (BITNESS EQUAL 32)
+    target_compile_definitions(${NAME}-guest-deps INTERFACE IS_32BIT_THUNK)
+  endif ()
   # Shorthand for the include directories added after calling this function.
   # This is not evaluated directly, hence directories added after return are still picked up
   set(prop "$<TARGET_PROPERTY:${NAME}-guest-deps,INTERFACE_INCLUDE_DIRECTORIES>")
@@ -64,16 +67,18 @@ function(generate NAME SOURCE_FILE)
   file(MAKE_DIRECTORY "${OUTFOLDER}")
 
   if (BITNESS EQUAL 32)
-    set (BITNESS_FLAGS "-m32" "--target=i686-linux-unknown" "-isystem" "/usr/i686-linux-gnu/include/")
+    set(BITNESS_FLAGS "-for-32bit-guest")
+    set(BITNESS_FLAGS2 "-m32" "--target=i686-linux-unknown" "-isystem" "/usr/i686-linux-gnu/include/")
   else()
-    set (BITNESS_FLAGS "--target=x86_64-linux-unknown" "-isystem" "/usr/x86_64-linux-gnu/include/")
+    set(BITNESS_FLAGS "")
+    set(BITNESS_FLAGS2 "--target=x86_64-linux-unknown" "-isystem" "/usr/x86_64-linux-gnu/include/")
   endif()
 
   add_custom_command(
     OUTPUT "${OUTFILE}"
     DEPENDS "${GENERATOR_EXE}"
     DEPENDS "${SOURCE_FILE}"
-    COMMAND "${GENERATOR_EXE}" "${SOURCE_FILE}" "${NAME}" "-guest" "${OUTFILE}" -- -std=c++17 ${BITNESS_FLAGS}
+    COMMAND "${GENERATOR_EXE}" "${SOURCE_FILE}" "${NAME}" "-guest" "${OUTFILE}" ${BITNESS_FLAGS} -- -std=c++20 ${BITNESS_FLAGS2}
       # Expand compile definitions to space-separated list of -D parameters
       "$<$<BOOL:${compile_prop}>:;-D$<JOIN:${compile_prop},;-D>>"
       # Expand include directories to space-separated list of -isystem parameters
diff --git a/ThunkLibs/HostLibs/CMakeLists.txt b/ThunkLibs/HostLibs/CMakeLists.txt
index 799bd53df..f02572750 100644
--- a/ThunkLibs/HostLibs/CMakeLists.txt
+++ b/ThunkLibs/HostLibs/CMakeLists.txt
@@ -2,7 +2,7 @@ cmake_minimum_required(VERSION 3.14)
 project(host-thunks)
 include(${FEX_PROJECT_SOURCE_DIR}/CMakeFiles/version_to_variables.cmake)
 
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 20)
 set (HOSTLIBS_DATA_DIRECTORY "${CMAKE_INSTALL_PREFIX}/lib/fex-emu" CACHE PATH "global data directory")
 option(ENABLE_CLANG_THUNKS "Enable building thunks with clang" FALSE)
 
@@ -20,6 +20,9 @@ function(generate NAME SOURCE_FILE GUEST_BITNESS)
   # Interface target for the user to add include directories
   add_library(${NAME}-${GUEST_BITNESS}-deps INTERFACE)
   target_include_directories(${NAME}-${GUEST_BITNESS}-deps INTERFACE "${CMAKE_CURRENT_SOURCE_DIR}/../include")
+  if (GUEST_BITNESS EQUAL 32)
+    target_compile_definitions(${NAME}-${GUEST_BITNESS}-deps INTERFACE IS_32BIT_THUNK)
+  endif ()
   # Shorthand for the include directories added after calling this function.
   # This is not evaluated directly, hence directories added after return are still picked up
   set(prop "$<TARGET_PROPERTY:${NAME}-${GUEST_BITNESS}-deps,INTERFACE_INCLUDE_DIRECTORIES>")
@@ -40,11 +43,16 @@ function(generate NAME SOURCE_FILE GUEST_BITNESS)
 
   file(MAKE_DIRECTORY "${OUTFOLDER}")
 
+  set (BITNESS_FLAGS "")
+  if (GUEST_BITNESS EQUAL 32)
+    set (BITNESS_FLAGS "-for-32bit-guest")
+  endif()
+
   add_custom_command(
     OUTPUT "${OUTFILE}"
     DEPENDS "${SOURCE_FILE}"
     DEPENDS thunkgen
-    COMMAND thunkgen "${SOURCE_FILE}" "${NAME}" "-host" "${OUTFILE}" -- -std=c++17
+    COMMAND thunkgen "${SOURCE_FILE}" "${NAME}" "-host" "${OUTFILE}" ${BITNESS_FLAGS} -- -std=c++20
       # Expand compile definitions to space-separated list of -D parameters
       "$<$<BOOL:${compile_prop}>:;-D$<JOIN:${compile_prop},;-D>>"
       # Expand include directories to space-separated list of -isystem parameters
@@ -91,7 +99,7 @@ function(add_host_lib NAME GUEST_BITNESS)
   endif()
 endfunction()
 
-set (BITNESS_LIST "32;64")
+set (BITNESS_LIST "64")
 foreach(GUEST_BITNESS IN LISTS BITNESS_LIST)
   #add_host_lib(fex_malloc_symbols ${GUEST_BITNESS})
 
@@ -189,7 +197,10 @@ foreach(GUEST_BITNESS IN LISTS BITNESS_LIST)
   target_include_directories(libdrm-${GUEST_BITNESS}-deps INTERFACE /usr/include/drm/)
   target_include_directories(libdrm-${GUEST_BITNESS}-deps INTERFACE /usr/include/libdrm/)
   add_host_lib(drm ${GUEST_BITNESS})
+endforeach()
 
+set (BITNESS_LIST "32;64")
+foreach(GUEST_BITNESS IN LISTS BITNESS_LIST)
   generate(libfex_thunk_test ${CMAKE_CURRENT_SOURCE_DIR}/../libfex_thunk_test/libfex_thunk_test_interface.cpp ${GUEST_BITNESS})
   add_host_lib(fex_thunk_test ${GUEST_BITNESS})
 endforeach()
diff --git a/unittests/ThunkLibs/CMakeLists.txt b/unittests/ThunkLibs/CMakeLists.txt
index ccde2918f..8618eaa66 100644
--- a/unittests/ThunkLibs/CMakeLists.txt
+++ b/unittests/ThunkLibs/CMakeLists.txt
@@ -1,5 +1,6 @@
-add_executable(thunkgentest generator.cpp)
+add_executable(thunkgentest generator.cpp abi.cpp)
 target_link_libraries(thunkgentest PRIVATE Catch2::Catch2WithMain)
+target_link_libraries(thunkgentest PRIVATE fmt::fmt)
 target_link_libraries(thunkgentest PRIVATE thunkgenlib)
 catch_discover_tests(thunkgentest TEST_SUFFIX ".ThunkGen")
 
diff --git a/unittests/ThunkLibs/abi.cpp b/unittests/ThunkLibs/abi.cpp
new file mode 100644
index 000000000..ca9b193c5
--- /dev/null
+++ b/unittests/ThunkLibs/abi.cpp
@@ -0,0 +1,643 @@
+#include <clang/Frontend/CompilerInstance.h>
+#include <catch2/catch.hpp>
+
+#include <data_layout.h>
+#include <interface.h>
+#include "common.h"
+
+#include <fmt/format.h>
+
+#include <string_view>
+
+// run_tool will leak memory when the ToolAction throws an exception, so
+// disable AddressSanitizer's leak detection
+const char* __asan_default_options() {
+    return "detect_leaks=0";
+}
+
+inline std::ostream& operator<<(std::ostream& os, TypeCompatibility compat) {
+    if (compat == TypeCompatibility::Full) {
+        os << "Compatible";
+    } else if (compat == TypeCompatibility::Repackable) {
+        os << "Repackable";
+    } else if (compat == TypeCompatibility::None) {
+        os << "Incompatible";
+    } else {
+        os << "(INVALID)";
+    }
+    return os;
+}
+
+class DataLayoutCompareActionForTest;
+
+namespace {
+
+struct Fixture {
+    /**
+     * Parses annotations from the input source and generates data layout descriptions from it.
+     *
+     * Input code with common definitions (types, functions, ...) should be specified in "prelude".
+     * It will be prepended to "code" before processing and also to the generator output.
+     */
+    std::unique_ptr<DataLayoutCompareActionForTest> compute_data_layout(std::string_view prelude, std::string_view code, GuestABI);
+};
+
+}
+
+class DataLayoutCompareActionForTest : public DataLayoutCompareAction {
+    std::unordered_map<const clang::Type*, TypeCompatibility> type_compat_cache;
+
+    // Persistent reference taken to enable accessing the ASTContext after CompilerInstance::ExecuteAction returns
+    llvm::IntrusiveRefCntPtr<clang::ASTContext> ast_context;
+    std::shared_ptr<clang::Preprocessor> preprocessor;
+
+public:
+    DataLayoutCompareActionForTest(std::unique_ptr<ABI> guest_layout) : DataLayoutCompareAction(*guest_layout), guest_layout(std::move(guest_layout)) {
+    }
+
+    void ExecuteAction() override {
+        AnalysisAction::ExecuteAction();
+
+        ast_context = &getCompilerInstance().getASTContext();
+        preprocessor = getCompilerInstance().getPreprocessorPtr();
+        host_layout = ComputeDataLayout(*ast_context, types);
+    }
+
+    std::unique_ptr<ABI> guest_layout;
+    std::unordered_map<const clang::Type*, TypeInfo> host_layout;
+
+    TypeCompatibility GetTypeCompatibility(std::string_view type_name) {
+        for (const auto& [type, _] : host_layout) {
+            if (clang::QualType { type, 0 }.getAsString() == type_name) {
+                return DataLayoutCompareAction::GetTypeCompatibility(*ast_context, type, host_layout, type_compat_cache);
+            }
+        }
+
+        throw std::runtime_error("No data layout information recorded for type \"" + std::string { type_name } + "\"");
+    }
+};
+
+/**
+ * Same as clang::FrontendActionFactory but takes an external FrontendAction
+ * reference instead of constructing an internal one. Since the FrontendAction
+ * lifetime may extend past this ToolAction, state captured by the
+ * FrontendAction can be accessed after the ToolAction returns.
+ */
+class ThunkTestToolAction : public clang::tooling::ToolAction {
+public:
+    clang::FrontendAction& ScopedToolAction;
+
+public:
+    ThunkTestToolAction(clang::FrontendAction& action) : ScopedToolAction(action) {
+    }
+    ~ThunkTestToolAction() = default;
+
+    // Same as FrontendActionFactory but keeps ScopedToolAction alive when returning
+    bool runInvocation( std::shared_ptr<clang::CompilerInvocation> invocation, clang::FileManager *files,
+                        std::shared_ptr<clang::PCHContainerOperations> pch,
+                        clang::DiagnosticConsumer *diag_consumer) override {
+
+        auto diagnostics = clang::CompilerInstance::createDiagnostics(&invocation->getDiagnosticOpts(), diag_consumer, false);
+
+        clang::CompilerInstance Compiler(std::move(pch));
+        Compiler.setInvocation(std::move(invocation));
+        Compiler.setFileManager(files);
+        Compiler.createDiagnostics(diag_consumer, false);
+        if (!Compiler.hasDiagnostics())
+          return false;
+        Compiler.createSourceManager(*files);
+
+        const bool Success = Compiler.ExecuteAction(ScopedToolAction);
+
+        files->clearStatCache();
+        return Success;
+    }
+};
+
+std::unique_ptr<DataLayoutCompareActionForTest> Fixture::compute_data_layout(std::string_view prelude, std::string_view code, GuestABI guest_abi) {
+    const std::string full_code = std::string { prelude } + std::string { code };
+
+    // Compute guest data layout
+    auto data_layout_analysis_factory = std::make_unique<AnalyzeDataLayoutActionFactory>();
+    run_tool(*data_layout_analysis_factory, full_code, false, guest_abi);
+
+    // Compute host data layout
+    auto ScopedToolAction = std::make_unique<DataLayoutCompareActionForTest>(data_layout_analysis_factory->TakeDataLayout());
+    run_tool(std::make_unique<ThunkTestToolAction>(*ScopedToolAction), full_code, false, std::nullopt);
+
+    return ScopedToolAction;
+}
+
+static std::string FormatDataLayout(const std::unordered_map<const clang::Type*, TypeInfo>& layout) {
+     std::string ret;
+
+    for (const auto& [type, info] : layout) {
+        auto basic_info = info.get_if_simple_or_struct();
+        if (!basic_info) {
+            continue;
+        }
+
+        ret += fmt::format("  Host entry {}: {} ({})\n", clang::QualType { type, 0 }.getAsString().c_str(), basic_info->size_bits / 8, basic_info->alignment_bits / 8);
+
+        if (auto struct_info = info.get_if_struct()) {
+            for (const auto& member : struct_info->members) {
+                ret += fmt::format("    Offset {}-{}: {} {}{}\n", member.offset_bits / 8, (member.offset_bits + member.size_bits - 1) / 8, member.type_name.c_str(), member.member_name.c_str(), member.array_size ? fmt::format("[{}]", member.array_size.value()).c_str() : "");
+            }
+        }
+    }
+
+    return ret;
+}
+
+TEST_CASE_METHOD(Fixture, "DataLayout") {
+    auto guest_abi = GENERATE(GuestABI::X86_32, GuestABI::X86_64);
+    INFO(guest_abi);
+
+    SECTION("Trivial") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n",
+            "struct A { int a; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        REQUIRE(action->guest_layout->contains("A"));
+
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::Full);
+    }
+
+    SECTION("Builtin types") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n",
+            "struct A { char a; short b; int c; float d; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        REQUIRE(action->guest_layout->contains("A"));
+
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::Full);
+        CHECK(action->GetTypeCompatibility("char") == TypeCompatibility::Full);
+        CHECK(action->GetTypeCompatibility("short") == TypeCompatibility::Full);
+        CHECK(action->GetTypeCompatibility("int") == TypeCompatibility::Full);
+        CHECK(action->GetTypeCompatibility("float") == TypeCompatibility::Full);
+    }
+
+    SECTION("Padding after int16_t") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct A { int16_t a; int32_t b; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::Full);
+    }
+
+    SECTION("Array of int16_t") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct A { int16_t a[64]; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::Full);
+    }
+
+    const auto compat_full64_repackable32 = (guest_abi == GuestABI::X86_32 ? TypeCompatibility::Repackable : TypeCompatibility::Full);
+
+    SECTION("Type with platform-dependent size (size_t)") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdlib>\n",
+            "struct A { size_t a; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->GetTypeCompatibility("struct A") == compat_full64_repackable32);
+    }
+
+    SECTION("int64_t has stricter alignment requirements on 64-bit platforms") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct A { int64_t a; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->guest_layout->at("A").get_if_struct()->alignment_bits == (guest_abi == GuestABI::X86_32 ? 32 : 64));
+        CHECK(action->GetTypeCompatibility("struct A") == compat_full64_repackable32);
+    }
+
+    SECTION("Array of int64_t") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct A { int64_t a[64]; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->GetTypeCompatibility("struct A") == compat_full64_repackable32);
+    }
+
+    SECTION("int64_t with explicit alignment specification") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct alignas(8) A { int64_t a; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->guest_layout->at("A").get_if_struct()->alignment_bits == 64);
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::Full);
+    }
+
+    SECTION("int64_t alignment requirements propagate to parent struct") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct A { int32_t a; int32_t b; int64_t c; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->guest_layout->at("A").get_if_struct()->alignment_bits == (guest_abi == GuestABI::X86_32 ? 32 : 64));
+        CHECK(action->GetTypeCompatibility("struct A") == compat_full64_repackable32);
+    }
+
+    SECTION("Padding before int64_t member") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct A { int32_t a; int64_t b; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->guest_layout->at("A").get_if_struct()->members[1].offset_bits == (guest_abi == GuestABI::X86_32 ? 32 : 64));
+
+        CHECK(action->GetTypeCompatibility("struct A") == compat_full64_repackable32);
+    }
+
+    SECTION("Padding at end of struct due to int64_t alignment (like VkMemoryHeap)") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct A { int64_t a; int32_t b; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->guest_layout->at("A").get_if_struct()->size_bits == (guest_abi == GuestABI::X86_32 ? 96 : 128));
+
+        CHECK(action->GetTypeCompatibility("struct A") == compat_full64_repackable32);
+    }
+
+    SECTION("Different struct definition between guest and host; different member order") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "#ifdef HOST\n"
+            "struct A { int32_t a; int32_t b; };\n"
+            "#else\n"
+            "struct A { int32_t b; int32_t a; };\n"
+            "#endif\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->guest_layout->at("A").get_if_struct()->members.at(0).member_name == "b");
+        CHECK(action->guest_layout->at("A").get_if_struct()->members.at(1).member_name == "a");
+
+        REQUIRE(!action->host_layout.empty());
+        CHECK(action->host_layout.begin()->second.get_if_struct()->members.at(0).member_name == "a");
+        CHECK(action->host_layout.begin()->second.get_if_struct()->members.at(1).member_name == "b");
+
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::Repackable);
+    }
+
+    SECTION("Different struct definition between guest and host; different member size") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "#ifdef HOST\n"
+            "struct A { int32_t a; int32_t b; };\n"
+            "#else\n"
+            "struct A { int32_t a; int64_t b; };\n"
+            "#endif\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->guest_layout->at("A").get_if_struct()->members.at(0).size_bits == 32);
+        CHECK(action->guest_layout->at("A").get_if_struct()->members.at(1).size_bits == 64);
+
+        REQUIRE(!action->host_layout.empty());
+        CHECK(action->host_layout.begin()->second.get_if_struct()->members.at(0).size_bits == 32);
+        CHECK(action->host_layout.begin()->second.get_if_struct()->members.at(1).size_bits == 32);
+
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::Repackable);
+    }
+
+    SECTION("Different struct definition between guest and host; completely different members") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "#ifdef HOST\n"
+            "struct A { int32_t a; int32_t b; };\n"
+            "#else\n"
+            "struct A { int32_t c; int32_t d; };\n"
+            "#endif\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::None);
+    }
+
+    SECTION("Different struct definition between guest and host; member missing from guest") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "#ifdef HOST\n"
+            "struct A { int32_t a; int32_t b; };\n"
+            "#else\n"
+            "struct A { int32_t a; };\n"
+            "#endif\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::None);
+    }
+
+    SECTION("Different struct definition between guest and host; member missing from host") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "#ifdef HOST\n"
+            "struct A { int32_t a; };\n"
+            "#else\n"
+            "struct A { int32_t a; int32_t b; };\n"
+            "#endif\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::None);
+    }
+
+    SECTION("Nesting structs of consistent data layout") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct C { int32_t a; int16_t b; };\n"
+            "struct B { C a; int16_t b; };\n"
+            "struct A { int32_t a; B b; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        REQUIRE(action->guest_layout->contains("B"));
+        REQUIRE(action->guest_layout->contains("C"));
+        CHECK(action->guest_layout->at("A").get_if_struct()->members.at(0).size_bits == 32);
+
+        CHECK(action->GetTypeCompatibility("struct C") == TypeCompatibility::Full);
+        CHECK(action->GetTypeCompatibility("struct B") == TypeCompatibility::Full);
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::Full);
+    }
+
+    SECTION("Nesting repackable structs by embedding") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "#ifdef HOST\n"
+            "struct C { int32_t a; int32_t b; };\n"
+            "#else\n"
+            "struct C { int32_t b; int32_t a; };\n"
+            "#endif\n"
+            "struct B { C a; int16_t b; };\n"
+            "struct A { int32_t a; B b; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        REQUIRE(action->guest_layout->contains("B"));
+        REQUIRE(action->guest_layout->contains("C"));
+        CHECK(action->guest_layout->at("A").get_if_struct()->size_bits == 128);
+        CHECK(action->guest_layout->at("A").get_if_struct()->alignment_bits == 32);
+
+        CHECK(action->GetTypeCompatibility("struct C") == TypeCompatibility::Repackable);
+        CHECK(action->GetTypeCompatibility("struct B") == TypeCompatibility::Repackable);
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::Repackable);
+    }
+
+    SECTION("Embedded union type (like VkRenderingAttachmentInfo)") {
+        SECTION("without annotation") {
+            CHECK_THROWS_WITH(compute_data_layout(
+                "#include <thunks_common.h>\n"
+                "#include <cstdint>\n",
+                "union B { int32_t a; uint32_t b; };\n"
+                "struct A { B a; };\n"
+                "template<> struct fex_gen_type<A> {};\n", guest_abi),
+                Catch::Contains("unannotated member") && Catch::Contains("union type"));
+        }
+    }
+}
+
+TEST_CASE_METHOD(Fixture, "DataLayoutPointers") {
+    auto guest_abi = GENERATE(GuestABI::X86_32, GuestABI::X86_64);
+    INFO(guest_abi);
+
+    const auto compat_full64_repackable32 = (guest_abi == GuestABI::X86_32 ? TypeCompatibility::Repackable : TypeCompatibility::Full);
+
+    SECTION("Pointer to data with consistent layout") {
+        std::string type = GENERATE("char", "short", "int", "float", "struct B { int a; }");
+        INFO(type);
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct A { " + type + "* a; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        // The pointer itself needs repacking on 32-bit. On 64-bit, no repacking is needed at all.
+        CHECK(action->GetTypeCompatibility("struct A") == compat_full64_repackable32);
+        if (!type.starts_with("struct B")) {
+            CHECK(action->GetTypeCompatibility(type) == TypeCompatibility::Full);
+        }
+    }
+
+    SECTION("Pointer to struct with consistent layout") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct B { int32_t a; };\n"
+            "struct A { B* a; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("B"));
+        CHECK(action->GetTypeCompatibility("struct B") == TypeCompatibility::Full);
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->GetTypeCompatibility("struct A") == compat_full64_repackable32);
+    }
+
+    SECTION("Unannotated pointer to incomplete type") {
+        CHECK_THROWS_WITH(compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct B;\n"
+            "struct A { B* a; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi),
+            Catch::Contains("incomplete type"));
+    }
+
+    SECTION("Unannotated pointer to repackable type") {
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "#ifdef HOST\n"
+            "struct B { int32_t a; int32_t b; };\n"
+            "#else\n"
+            "struct B { int32_t a; int64_t b; };\n"
+            "#endif\n"
+            "struct A { B* a; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::None);
+    }
+
+    SECTION("Nesting repackable structs by pointers") {
+        SECTION("Innermost struct is compatible") {
+            auto action = compute_data_layout(
+                "#include <thunks_common.h>\n"
+                "#include <cstdint>\n",
+                "struct C { int32_t a; int32_t b; };\n"
+                "struct B { C* a; int16_t b; };\n"
+                "struct A { int32_t a; B b; };\n"
+                "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+            INFO(FormatDataLayout(action->host_layout));
+
+            REQUIRE(action->guest_layout->contains("A"));
+            REQUIRE(action->guest_layout->contains("B"));
+            REQUIRE(action->guest_layout->contains("C"));
+
+            // 64-bit is fully compatible, but 32-bit needs to zero-extend the pointer itself
+            CHECK(action->GetTypeCompatibility("struct C") == TypeCompatibility::Full);
+            CHECK(action->GetTypeCompatibility("struct B") == compat_full64_repackable32);
+            CHECK(action->GetTypeCompatibility("struct A") == compat_full64_repackable32);
+        }
+
+        SECTION("Innermost struct is incompatible") {
+            auto action = compute_data_layout(
+                "#include <thunks_common.h>\n"
+                "#include <cstdint>\n",
+                "#ifdef HOST\n"
+                "struct C { int32_t a; int32_t b; };\n"
+                "#else\n"
+                "struct C { int32_t b; int32_t a; };\n"
+                "#endif\n"
+                "struct B { C* a; int16_t b; };\n"
+                "struct A { int32_t a; B b; };\n"
+                "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+            INFO(FormatDataLayout(action->host_layout));
+
+            REQUIRE(action->guest_layout->contains("A"));
+            REQUIRE(action->guest_layout->contains("B"));
+            REQUIRE(action->guest_layout->contains("C"));
+
+            CHECK(action->GetTypeCompatibility("struct C") == TypeCompatibility::Repackable);
+            CHECK(action->GetTypeCompatibility("struct B") == TypeCompatibility::None);
+            CHECK(action->GetTypeCompatibility("struct A") == TypeCompatibility::None);
+        }
+    }
+
+    SECTION("Unannotated pointer to union type") {
+        CHECK_THROWS_WITH(compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "union B { int32_t a; uint32_t b; };\n"
+            "struct A { B* a; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi),
+            Catch::Contains("unannotated member") && Catch::Contains("union type"));
+    }
+
+    SECTION("Self-referencing struct (like VkBaseOutStructure)") {
+        // Without annotation
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct A { A* a; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK_THROWS_WITH(action->GetTypeCompatibility("struct A"), Catch::Contains("recursive reference"));
+    }
+
+    SECTION("Circularly referencing structs") {
+        // Without annotation
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct B;\n"
+            "struct A { B* a; };\n"
+            "struct B { A* a; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        REQUIRE(action->guest_layout->contains("B"));
+        CHECK_THROWS_WITH(action->GetTypeCompatibility("struct A"), Catch::Contains("recursive reference"));
+        CHECK_THROWS_WITH(action->GetTypeCompatibility("struct B"), Catch::Contains("recursive reference"));
+    }
+
+    SECTION("Pointers to void") {
+        // Without annotation
+        auto action = compute_data_layout(
+            "#include <thunks_common.h>\n"
+            "#include <cstdint>\n",
+            "struct A { void* a; };\n"
+            "template<> struct fex_gen_type<A> {};\n", guest_abi);
+
+        INFO(FormatDataLayout(action->host_layout));
+
+        REQUIRE(action->guest_layout->contains("A"));
+        CHECK(action->GetTypeCompatibility("struct A") == (guest_abi == GuestABI::X86_32 ? TypeCompatibility::None : TypeCompatibility::Full));
+    }
+
+    // TODO: Double pointers to compatible data: struct B { int a ; }; struct A { B** b; };
+}
diff --git a/unittests/ThunkLibs/common.h b/unittests/ThunkLibs/common.h
index 47c83bd9b..6fe13022e 100644
--- a/unittests/ThunkLibs/common.h
+++ b/unittests/ThunkLibs/common.h
@@ -42,15 +42,46 @@ public:
     }
 };
 
+enum class GuestABI {
+    X86_32,
+    X86_64,
+};
+
+inline std::ostream& operator<<(std::ostream& os, GuestABI abi) {
+    if (abi == GuestABI::X86_32) {
+        os << "X86_32";
+    } else if (abi == GuestABI::X86_64) {
+        os << "X86_64";
+    }
+    return os;
+}
+
 /**
  * Run the given ToolAction on the input code.
  *
  * The "silent" parameter is used to suppress non-fatal diagnostics in tests that expect failure
  */
-inline void run_tool(clang::tooling::ToolAction& action, std::string_view code, bool silent = false) {
+inline void run_tool(clang::tooling::ToolAction& action, std::string_view code, bool silent = false, std::optional<GuestABI> guest_abi = std::nullopt) {
     const char* memory_filename = "gen_input.cpp";
     auto adjuster = clang::tooling::getClangStripDependencyFileAdjuster();
     std::vector<std::string> args = { "clang-tool", "-fsyntax-only", "-std=c++17", "-Werror", "-I.", memory_filename };
+    if (CLANG_RESOURCE_DIR[0] != 0) {
+        args.push_back("-resource-dir");
+        args.push_back(CLANG_RESOURCE_DIR);
+    }
+    if (guest_abi == GuestABI::X86_64) {
+        args.push_back("-target");
+        args.push_back("x86_64-linux-gnu");
+        args.push_back("-isystem");
+        args.push_back("/usr/x86_64-linux-gnu/include/");
+    } else if (guest_abi == GuestABI::X86_32) {
+        args.push_back("-target");
+        args.push_back("i686-linux-gnu");
+        args.push_back("-isystem");
+        args.push_back("/usr/i686-linux-gnu/include/");
+    } else {
+        args.push_back("-DHOST");
+    }
 
     // Corresponds to the content of GeneratorInterface.h
     const char* common_header_code = R"(namespace fexgen {
@@ -60,6 +91,12 @@ struct callback_annotation_base { bool prevent_multiple; };
 struct callback_stub : callback_annotation_base {};
 struct callback_guest : callback_annotation_base {};
 } // namespace fexgen
+
+template<typename>
+struct fex_gen_type;
+template<auto>
+struct fex_gen_config;
+
 )";
 
     llvm::IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem> overlay_fs(new llvm::vfs::OverlayFileSystem(llvm::vfs::getRealFileSystem()));
@@ -73,6 +110,9 @@ struct callback_guest : callback_annotation_base {};
 
     TestDiagnosticConsumer consumer(silent);
     invocation.setDiagnosticConsumer(&consumer);
+
+    // Process the actual ToolAction.
+    // NOTE: If the ToolAction throws an exception, clang will leak memory here.
     invocation.run();
 
     if (auto error = consumer.GetFirstError()) {
@@ -80,6 +120,6 @@ struct callback_guest : callback_annotation_base {};
     }
 }
 
-inline void run_tool(std::unique_ptr<clang::tooling::ToolAction> action, std::string_view code, bool silent = false) {
-    return run_tool(*action, code, silent);
+inline void run_tool(std::unique_ptr<clang::tooling::ToolAction> action, std::string_view code, bool silent = false, std::optional<GuestABI> guest_abi = std::nullopt) {
+    return run_tool(*action, code, silent, guest_abi);
 }
diff --git a/unittests/ThunkLibs/generator.cpp b/unittests/ThunkLibs/generator.cpp
index 4958d957e..7144062ae 100644
--- a/unittests/ThunkLibs/generator.cpp
+++ b/unittests/ThunkLibs/generator.cpp
@@ -73,7 +73,7 @@ struct Fixture {
      * It will be prepended to "code" before processing and also to the generator output.
      */
     SourceWithAST run_thunkgen_guest(std::string_view prelude, std::string_view code, bool silent = false);
-    SourceWithAST run_thunkgen_host(std::string_view prelude, std::string_view code, bool silent = false);
+    SourceWithAST run_thunkgen_host(std::string_view prelude, std::string_view code, GuestABI = GuestABI::X86_64, bool silent = false);
     GenOutput run_thunkgen(std::string_view prelude, std::string_view code, bool silent = false);
 
     const std::string libname = "libtest";
@@ -202,7 +202,14 @@ SourceWithAST::SourceWithAST(std::string_view input) : code(input) {
  */
 SourceWithAST Fixture::run_thunkgen_guest(std::string_view prelude, std::string_view code, bool silent) {
     const std::string full_code = std::string { prelude } + std::string { code };
-    run_tool(std::make_unique<GenerateThunkLibsActionFactory>(libname, output_filenames), full_code, silent);
+
+    // These tests don't deal with data layout differences, so just run data
+    // layout analysis with host configuration
+    auto data_layout_analysis_factory = std::make_unique<AnalyzeDataLayoutActionFactory>();
+    run_tool(*data_layout_analysis_factory, full_code, silent);
+    auto& data_layout = data_layout_analysis_factory->GetDataLayout();
+
+    run_tool(std::make_unique<GenerateThunkLibsActionFactory>(libname, output_filenames, data_layout), full_code, silent);
 
     std::string result =
         "#include <cstdint>\n"
@@ -229,9 +236,16 @@ SourceWithAST Fixture::run_thunkgen_guest(std::string_view prelude, std::string_
 /**
  * Generates host thunk library code from the given input
  */
-SourceWithAST Fixture::run_thunkgen_host(std::string_view prelude, std::string_view code, bool silent) {
+SourceWithAST Fixture::run_thunkgen_host(std::string_view prelude, std::string_view code, GuestABI guest_abi, bool silent) {
     const std::string full_code = std::string { prelude } + std::string { code };
-    run_tool(std::make_unique<GenerateThunkLibsActionFactory>(libname, output_filenames), full_code, silent);
+
+    // These tests don't deal with data layout differences, so just run data
+    // layout analysis with host configuration
+    auto data_layout_analysis_factory = std::make_unique<AnalyzeDataLayoutActionFactory>();
+    run_tool(*data_layout_analysis_factory, full_code, silent, guest_abi);
+    auto& data_layout = data_layout_analysis_factory->GetDataLayout();
+
+    run_tool(std::make_unique<GenerateThunkLibsActionFactory>(libname, output_filenames, data_layout), full_code, silent);
 
     std::string result =
         "#include <cstdint>\n"
@@ -268,17 +282,27 @@ SourceWithAST Fixture::run_thunkgen_host(std::string_view prelude, std::string_v
     auto& filename = output_filenames.host;
     {
         std::ifstream file(filename);
-        const auto current_size = result.size();
+        const auto prelude_size = result.size();
         const auto new_data_size = std::filesystem::file_size(filename);
         result.resize(result.size() + new_data_size);
-        file.read(result.data() + current_size, result.size());
+        file.read(result.data() + prelude_size, result.size());
+
+        // Force all functions to be non-static, since having to define them
+        // would add a lot of noise to simple tests.
+        while (true) {
+            auto pos = result.find("static ", prelude_size);
+            if (pos == std::string::npos) {
+                break;
+            }
+            result.replace(pos, 6, "      "); // Replace "static" with 6 spaces (avoiding reallocation)
+        }
     }
     return SourceWithAST { std::string { prelude } + result };
 }
 
 Fixture::GenOutput Fixture::run_thunkgen(std::string_view prelude, std::string_view code, bool silent) {
     return { run_thunkgen_guest(prelude, code, silent),
-             run_thunkgen_host(prelude, code, silent) };
+             run_thunkgen_host(prelude, code, GuestABI::X86_64, silent) };
 }
 
 TEST_CASE_METHOD(Fixture, "Trivial") {
@@ -511,3 +535,75 @@ TEST_CASE_METHOD(Fixture, "VariadicFunctionsWithoutAnnotation") {
         "template<auto> struct fex_gen_config {};\n"
         "template<> struct fex_gen_config<func> {};\n", true));
 }
+
+TEST_CASE_METHOD(Fixture, "StructRepacking") {
+    auto guest_abi = GENERATE(GuestABI::X86_32, GuestABI::X86_64);
+    INFO(guest_abi);
+
+    // All tests use the same function, but the prelude defining its parameter type "A" varies
+    const std::string code =
+        "#include <thunks_common.h>\n"
+        "void func(A*);\n"
+        "template<auto> struct fex_gen_config {};\n"
+        "template<> struct fex_gen_config<func> : fexgen::custom_host_impl {};\n";
+
+    SECTION("Pointer to struct with consistent data layout") {
+        CHECK_NOTHROW(run_thunkgen_host("struct A { int a; };\n", code, guest_abi));
+    }
+
+    SECTION("Pointer to struct with unannotated pointer member with inconsistent data layout") {
+        const auto prelude =
+            "#ifdef HOST\n"
+            "struct B { int a; };\n"
+            "#else\n"
+            "struct B { int b; };\n"
+            "#endif\n"
+            "struct A { B* a; };\n";
+
+        SECTION("Parameter unannotated") {
+            CHECK_THROWS(run_thunkgen_host(prelude, code, guest_abi, true));
+        }
+    }
+
+    SECTION("Pointer to struct with pointer member of opaque type") {
+        const auto prelude =
+            "struct B;\n"
+            "struct A { B* a; };\n";
+
+        // Unannotated
+        REQUIRE_THROWS_WITH(run_thunkgen_host(prelude, code, guest_abi), Catch::Contains("incomplete type"));
+    }
+}
+
+TEST_CASE_METHOD(Fixture, "VoidPointerParameter") {
+    auto guest_abi = GENERATE(GuestABI::X86_32, GuestABI::X86_64);
+    INFO(guest_abi);
+
+    SECTION("Unannotated") {
+        const char* code =
+            "#include <thunks_common.h>\n"
+            "void func(void*);\n"
+            "template<> struct fex_gen_config<func> {};\n";
+        if (guest_abi == GuestABI::X86_32) {
+            // TODO: Currently not considered an error
+//            CHECK_THROWS_WITH(run_thunkgen_host("", code, guest_abi, true), Catch::Contains("unsupported parameter type", Catch::CaseSensitive::No));
+        } else {
+            // Pointee data is assumed to be compatible on 64-bit
+            CHECK_NOTHROW(run_thunkgen_host("", code, guest_abi));
+        }
+    }
+
+    SECTION("Unannotated in struct") {
+        const char* prelude =
+            "struct A { void* a; };\n";
+        const char* code =
+            "#include <thunks_common.h>\n"
+            "void func(A*);\n"
+            "template<> struct fex_gen_config<func> {};\n";
+        if (guest_abi == GuestABI::X86_32) {
+            CHECK_THROWS_WITH(run_thunkgen_host(prelude, code, guest_abi, true), Catch::Contains("unsupported parameter type", Catch::CaseSensitive::No));
+        } else {
+            CHECK_NOTHROW(run_thunkgen_host(prelude, code, guest_abi));
+        }
+    }
+}