diff --git a/External/FEXCore/Source/Interface/Core/OpcodeDispatcher.cpp b/External/FEXCore/Source/Interface/Core/OpcodeDispatcher.cpp
index c9c939269..99a7109e1 100644
--- a/External/FEXCore/Source/Interface/Core/OpcodeDispatcher.cpp
+++ b/External/FEXCore/Source/Interface/Core/OpcodeDispatcher.cpp
@@ -126,10 +126,20 @@ void OpDispatchBuilder::ThunkOp(OpcodeArgs) {
   const uint8_t GPRSize = CTX->GetGPRSize();
   uint8_t *sha256 = (uint8_t *)(Op->PC + 2);
 
-  _Thunk(
-    _LoadContext(GPRSize, GPRClass, GPROffset(X86State::REG_RDI)),
-    *reinterpret_cast<SHA256Sum*>(sha256)
-  );
+  if (CTX->Config.Is64BitMode) {
+    // x86-64 ABI puts the function argument in RDI
+    _Thunk(
+      _LoadContext(GPRSize, GPRClass, GPROffset(X86State::REG_RDI)),
+      *reinterpret_cast<SHA256Sum*>(sha256)
+    );
+  }
+  else {
+    // x86 fastcall ABI puts the function argument in ECX
+    _Thunk(
+      _LoadContext(GPRSize, GPRClass, GPROffset(X86State::REG_RCX)),
+      *reinterpret_cast<SHA256Sum*>(sha256)
+    );
+  }
 
   auto Constant = _Constant(GPRSize);
   auto OldSP = _LoadContext(GPRSize, GPRClass, RSPOffset);
diff --git a/ThunkLibs/include/common/Guest.h b/ThunkLibs/include/common/Guest.h
index e0414761a..ef11d8595 100644
--- a/ThunkLibs/include/common/Guest.h
+++ b/ThunkLibs/include/common/Guest.h
@@ -4,18 +4,25 @@
 
 #include "PackedArguments.h"
 
+#if __SIZEOF_POINTER__ == 8
+#define THUNK_ABI
+#else
+#define THUNK_ABI [[gnu::fastcall]]
+#endif
+
 template<typename signature>
+THUNK_ABI
 const int (*fexthunks_invoke_callback)(void*);
 
 #ifndef _M_ARM_64
 #define MAKE_THUNK(lib, name, hash) \
-  extern "C" int fexthunks_##lib##_##name(void *args); \
+  extern "C" THUNK_ABI int fexthunks_##lib##_##name(void *args); \
   asm(".text\nfexthunks_" #lib "_" #name ":\n.byte 0xF, 0x3F\n.byte " hash );
 
 #define MAKE_CALLBACK_THUNK(name, signature, hash) \
-  extern "C" int fexthunks_##name(void *args); \
+  extern "C" THUNK_ABI int fexthunks_##name(void *args); \
   asm(".text\nfexthunks_" #name ":\n.byte 0xF, 0x3F\n.byte " hash ); \
-  template<> inline constexpr int (*fexthunks_invoke_callback<signature>)(void*) = fexthunks_##name;
+  template<> THUNK_ABI inline constexpr int (*fexthunks_invoke_callback<signature>)(void*) = fexthunks_##name;
 
 #else
 // We're compiling for IDE integration, so provide a dummy-implementation that just calls an undefined function.
@@ -112,14 +119,14 @@ inline Result CallHostFunction(Args... args) {
 // Convenience wrapper that returns the function pointer to a CallHostFunction
 // instantiation matching the function signature of `host_func`
 template<typename Result, typename...Args>
-static auto GetCallerForHostFunction(Result (*host_func)(Args...))
-    -> Result(*)(Args...) {
-  return &CallHostFunction<fexthunks_invoke_callback<Result(Args...)>, Result, Args...>;
+static auto GetCallerForHostFunction(THUNK_ABI Result (*host_func)(Args...))
+    -> THUNK_ABI Result(*)(Args...) {
+  return &CallHostFunction<fexthunks_invoke_callback<THUNK_ABI Result(Args...)>, Result, Args...>;
 }
 
 // Ensures the given host function can safely be called from guest code.
 template<typename Result, typename...Args>
-inline void MakeHostFunctionGuestCallable(Result (*host_func)(Args...)) {
+inline void MakeHostFunctionGuestCallable(THUNK_ABI Result (*host_func)(Args...)) {
   auto caller = (uintptr_t)GetCallerForHostFunction(host_func);
   LinkAddressToFunction((uintptr_t)host_func, (uintptr_t)caller);
 }