Introduce a CMake flag to disable fast-path context descriptor lookup based on short manglings (#39729)

Author: kubamracek

stdlib/CMakeLists.txt

@@ -138,6 +138,10 @@ option(SWIFT_STDLIB_PASSTHROUGH_METADATA_ALLOCATOR
        "Build stdlib without a custom implementation of MetadataAllocator, relying on malloc+free instead."
        FALSE)
 
+option(SWIFT_STDLIB_SHORT_MANGLING_LOOKUPS
+       "Build stdlib with fast-path context descriptor lookups based on well-known short manglings."
+       TRUE)
+
 option(SWIFT_STDLIB_HAS_COMMANDLINE
        "Build stdlib with the CommandLine enum and support for argv/argc."
        TRUE)

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -359,6 +359,10 @@ function(_add_target_variant_c_compile_flags)
     list(APPEND result "-DSWIFT_STDLIB_PASSTHROUGH_METADATA_ALLOCATOR")
   endif()
 
+  if(SWIFT_STDLIB_SHORT_MANGLING_LOOKUPS)
+    list(APPEND result "-DSWIFT_STDLIB_SHORT_MANGLING_LOOKUPS")
+  endif()
+
   if(SWIFT_STDLIB_SUPPORTS_BACKTRACE_REPORTING)
     list(APPEND result "-DSWIFT_STDLIB_SUPPORTS_BACKTRACE_REPORTING")
   endif()

stdlib/public/runtime/MetadataLookup.cpp

@@ -699,6 +699,7 @@ _findContextDescriptor(Demangle::NodePointer node,
       (const ContextDescriptor *)symbolicNode->getIndex());
   }
 
+#if SWIFT_STDLIB_SHORT_MANGLING_LOOKUPS
   // Fast-path lookup for standard library type references with short manglings.
   if (symbolicNode->getNumChildren() >= 2
       && symbolicNode->getChild(0)->getKind() == Node::Kind::Module
@@ -719,6 +720,7 @@ _findContextDescriptor(Demangle::NodePointer node,
 
 #include "swift/Demangling/StandardTypesMangling.def"
   }
+#endif
 
   const ContextDescriptor *foundContext = nullptr;
   auto &T = TypeMetadataRecords.get();

utils/build-presets.ini

@@ -2480,6 +2480,7 @@ extra-cmake-options=
     -DSWIFT_IMPLICIT_CONCURRENCY_IMPORT:BOOL=FALSE
     -DSWIFT_STDLIB_ENABLE_PRESPECIALIZATION:BOOL=FALSE
     -DSWIFT_STDLIB_PASSTHROUGH_METADATA_ALLOCATOR:BOOL=TRUE
+    -DSWIFT_STDLIB_SHORT_MANGLING_LOOKUPS:BOOL=FALSE
 
 [preset: stdlib_S_standalone_minimal_macho_x86_64,build]
 mixin-preset=