Introduce a CMake option to disable MetadataAllocator and use malloc+free instead, use it for freestandning preset (#39678)

Author: kubamracek

stdlib/CMakeLists.txt

@@ -123,6 +123,10 @@ option(SWIFT_STDLIB_OS_VERSIONING
        "Build stdlib with availability based on OS versions (Darwin only)."
        TRUE)
 
+option(SWIFT_STDLIB_PASSTHROUGH_METADATA_ALLOCATOR
+       "Build stdlib without a custom implementation of MetadataAllocator, relying on malloc+free instead."
+       FALSE)
+
 option(SWIFT_BUILD_TEST_SUPPORT_MODULES
        "Whether to build StdlibUnittest and other test support modules. Defaults to On when SWIFT_BUILD_SDK_OVERLAY is On, or when SWIFT_INCLUDE_TESTS is On."
        "${SWIFT_BUILD_TEST_SUPPORT_MODULES_default}")

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -355,6 +355,10 @@ function(_add_target_variant_c_compile_flags)
     list(APPEND result "-DSWIFT_RUNTIME_OS_VERSIONING")
   endif()
 
+  if(SWIFT_STDLIB_PASSTHROUGH_METADATA_ALLOCATOR)
+    list(APPEND result "-DSWIFT_STDLIB_PASSTHROUGH_METADATA_ALLOCATOR")
+  endif()
+
   if(SWIFT_STDLIB_SUPPORTS_BACKTRACE_REPORTING)
     list(APPEND result "-DSWIFT_STDLIB_SUPPORTS_BACKTRACE_REPORTING")
   endif()

stdlib/public/runtime/Metadata.cpp

@@ -5822,6 +5822,8 @@ void swift::checkMetadataDependencyCycle(const Metadata *startMetadata,
 /*** Allocator implementation **********************************************/
 /***************************************************************************/
 
+#if !SWIFT_STDLIB_PASSTHROUGH_METADATA_ALLOCATOR
+
 namespace {
   struct alignas(sizeof(uintptr_t) * 2) PoolRange {
     static constexpr uintptr_t PageSize = 16 * 1024;
@@ -6059,6 +6061,8 @@ void MetadataAllocator::Deallocate(const void *allocation, size_t size,
                                                  std::memory_order_relaxed);
 }
 
+#endif
+
 void *swift::allocateMetadata(size_t size, size_t alignment) {
   return MetadataAllocator(MetadataTag).Allocate(size, alignment);
 }

stdlib/public/runtime/MetadataCache.h

@@ -26,6 +26,8 @@
 
 namespace swift {
 
+#if !SWIFT_STDLIB_PASSTHROUGH_METADATA_ALLOCATOR
+
 class MetadataAllocator : public llvm::AllocatorBase<MetadataAllocator> {
 private:
   uint16_t Tag;
@@ -51,6 +53,26 @@ class MetadataAllocator : public llvm::AllocatorBase<MetadataAllocator> {
   }
 };
 
+#else
+
+class MetadataAllocator {
+public:
+  MetadataAllocator(uint16_t tag) {}
+  void *Allocate(size_t size, size_t alignment) {
+    if (alignment < sizeof(void*)) alignment = sizeof(void*);
+    void *ptr = nullptr;
+    if (posix_memalign(&ptr, alignment, size) != 0) {
+      return nullptr;
+    }
+    return ptr;
+  }
+  void Deallocate(const void *ptr, size_t size = 0, size_t Alignment = 0) {
+    return free(const_cast<void *>(ptr));
+  }
+};
+
+#endif
+
 template <uint16_t StaticTag>
 class TaggedMetadataAllocator : public MetadataAllocator {
 public:

utils/build-presets.ini

@@ -2478,6 +2478,7 @@ extra-cmake-options=
     -DSWIFT_ENABLE_DISPATCH:BOOL=FALSE
     -DSWIFT_IMPLICIT_CONCURRENCY_IMPORT:BOOL=FALSE
     -DSWIFT_STDLIB_ENABLE_PRESPECIALIZATION:BOOL=FALSE
+    -DSWIFT_STDLIB_PASSTHROUGH_METADATA_ALLOCATOR:BOOL=TRUE
 
 [preset: stdlib_S_standalone_minimal_macho_x86_64,build]
 mixin-preset=