FEXCore: Fixes ever shrinking JIT code buffer

I accidentally replaced a couple usages of `AllocatedSize` with
`GetAllocatedSize()`. This resulted in a JIT buffer that ran out of
space would actually allocate a slightly smaller buffer each time, and
then it cascades downwards resulting in catastrophic performance.

Fix the use in `SharedCodeBufferManager.cpp` and `Core.cpp` which were
incorrect and renames the function to be more explicit.

FEXCore/Source/Interface/Core/CodeCache.cpp

@@ -314,7 +314,7 @@ bool CodeCache::SaveData(Core::InternalThreadState& Thread, int fd, const Execut
   std::ranges::copy(GIT_HASH, header.FEXVersion);
   header.NumBlocks = LookupCache.BlockList.size();
   header.NumCodePages = LookupCache.CodePages.size();
-  header.CodeBufferSize = FEXCore::AlignUp(CodeBuffer->GetAllocatedSize(), Utils::FEX_PAGE_SIZE);
+  header.CodeBufferSize = FEXCore::AlignUp(CodeBuffer->AllocatedSpaceUsed(), Utils::FEX_PAGE_SIZE);
   header.NumRelocations = Relocations.size();
   header.SerializedBaseAddress = SerializedBaseAddress;
   ::write(fd, &header, sizeof(header));
@@ -362,7 +362,7 @@ bool CodeCache::SaveData(Core::InternalThreadState& Thread, int fd, const Execut
 
   // Dump the host code (relocated for position-independent serialization)
   std::span CodeBufferData(reinterpret_cast<std::byte*>(CodeBuffer->GetBufferBase()),
-                           reinterpret_cast<std::byte*>(CodeBuffer->GetBufferBase()) + CodeBuffer->GetAllocatedSize());
+                           reinterpret_cast<std::byte*>(CodeBuffer->GetBufferBase()) + CodeBuffer->AllocatedSpaceUsed());
   if (!ApplyCodeRelocations(SerializedBaseAddress, CodeBufferData, Relocations, 0, true)) {
     LOGMAN_THROW_A_FMT(false, "Failed to apply code relocations");
     return false;
@@ -427,7 +427,7 @@ void CodeCache::Validate(const ExecutableFileSectionInfo& Section, fextl::set<ui
   while (CachedCode.size_bytes() > NewCodeBuffer->UsableSize()) {
     ValidationCTX->ClearCodeCache(ValidationThread.get());
     NewCodeBuffer = ValidationCTX->GetLatest();
-    LogMan::Msg::IFmt("Increased cache validation code buffer size to {} MiB", NewCodeBuffer->GetAllocatedSize() / 1024 / 1024);
+    LogMan::Msg::IFmt("Increased cache validation code buffer size to {} MiB", NewCodeBuffer->TotalAllocationSize() / 1024 / 1024);
   }
 
   std::span<std::byte> CodeBufferRangeRef =
@@ -448,10 +448,10 @@ void CodeCache::Validate(const ExecutableFileSectionInfo& Section, fextl::set<ui
   }));
   (void)ApplyCodeRelocations(Section.FileStartVA, CodeBufferRangeRef, NewRelocations, 0, false);
 
-  if (NewCodeBuffer->GetAllocatedSize() <= CodeBufferRangeRef.size()) {
+  if (NewCodeBuffer->AllocatedSpaceUsed() <= CodeBufferRangeRef.size()) {
     // Reference compilation produced fewer bytes than our cache, so validation is going to fail.
     // Make sure we don't output any garbage bytes though.
-    CodeBufferRangeRef = CodeBufferRangeRef.subspan(0, NewCodeBuffer->GetAllocatedSize());
+    CodeBufferRangeRef = CodeBufferRangeRef.subspan(0, NewCodeBuffer->AllocatedSpaceUsed());
   }
 
   auto [Mismatch, _] = std::mismatch(CodeBufferRangeRef.begin(), CodeBufferRangeRef.end(), CachedCode.begin());

FEXCore/Source/Interface/Core/Core.cpp

@@ -483,7 +483,7 @@ void ContextImpl::LockBeforeFork(FEXCore::Core::InternalThreadState* Thread) {
 
 void ContextImpl::OnCodeBufferAllocated(const fextl::shared_ptr<CPU::CodeBuffer>& Buffer) {
   if (Config.GlobalJITNaming()) {
-    Symbols.RegisterJITSpace(Buffer->GetBufferBase(), Buffer->GetAllocatedSize());
+    Symbols.RegisterJITSpace(Buffer->GetBufferBase(), Buffer->TotalAllocationSize());
   }
 
   {

FEXCore/Source/Interface/Core/SharedCodeBufferManager.cpp

@@ -88,7 +88,7 @@ fextl::shared_ptr<CodeBuffer> SharedCodeBufferManager::StartLargerCodeBuffer() {
     return GetLatest();
   }
 
-  auto NewCodeBufferSize = GetLatest()->GetAllocatedSize();
+  auto NewCodeBufferSize = GetLatest()->TotalAllocationSize();
   NewCodeBufferSize = std::min<size_t>(NewCodeBufferSize * 2, MAX_CODE_SIZE);
   return AllocateNew(NewCodeBufferSize);
 }

FEXCore/Source/Interface/Core/SharedCodeBufferManager.h

@@ -71,8 +71,13 @@ struct CodeBuffer {
     return AllocatedSize - FEXCore::Utils::FEX_PAGE_SIZE;
   }
 
+  // Returns the full size of the buffer, including the guard page.
+  size_t TotalAllocationSize() const {
+    return AllocatedSize;
+  }
+
   // Returns the num of bytes currently allocated from the allocator.
-  size_t GetAllocatedSize() const {
+  size_t AllocatedSpaceUsed() const {
     return CodeBufferOffset - Ptr;
   }