[Runtime] Scribble on metadata allocations.

stdlib/public/runtime/EnvironmentVariables.def

@@ -41,4 +41,7 @@ VARIABLE(SWIFT_ENABLE_MANGLED_NAME_VERIFICATION, bool, false,
          "Enable verification that metadata can roundtrip through a mangled "
          "name each time metadata is instantiated.")
 
+VARIABLE(SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE, bool, false,
+         "Scribble on runtime allocations such as metadata allocations.")
+
 #undef VARIABLE

stdlib/public/runtime/Metadata.cpp

@@ -662,11 +662,6 @@ swift::swift_allocateGenericValueMetadata(const ValueTypeDescriptor *description
   auto bytes = (char*) cache.getAllocator().withTag(GenericValueMetadataTag)
     .Allocate(totalSize, alignof(void*));
 
-#ifndef NDEBUG
-  // Fill the metadata record with garbage.
-  memset(bytes, 0xAA, totalSize);
-#endif
-
   auto addressPoint = bytes + sizeof(ValueMetadata::HeaderType);
   auto metadata = reinterpret_cast<ValueMetadata *>(addressPoint);
 
@@ -5547,6 +5542,21 @@ static void recordBacktrace(void *allocation) {
   });
 }
 
+template <typename Pointee>
+static inline void memsetScribble(Pointee *bytes, size_t totalSize) {
+#ifndef NDEBUG
+  // When DEBUG is defined, always scribble.
+  memset(bytes, 0xAA, totalSize);
+#else
+  // When DEBUG is not defined, only scribble when the
+  // SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE environment variable is set.
+  if (SWIFT_UNLIKELY(
+          runtime::environment::SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE())) {
+    memset(bytes, 0xAA, totalSize);
+  }
+#endif
+}
+
 void *MetadataAllocator::Allocate(size_t size, size_t alignment) {
   assert(Tag != 0);
   assert(alignment <= alignof(void*));
@@ -5556,8 +5566,11 @@ void *MetadataAllocator::Allocate(size_t size, size_t alignment) {
   SWIFT_ONCE_F(getenvToken, checkAllocatorDebugEnvironmentVariable, nullptr);
 
   // If the size is larger than the maximum, just use malloc.
-  if (size > PoolRange::MaxPoolAllocationSize)
-    return malloc(size);
+  if (size > PoolRange::MaxPoolAllocationSize) {
+    void *allocation = malloc(size);
+    memsetScribble(allocation, size);
+    return allocation;
+  }
 
   // Allocate out of the pool.
   auto sizeWithHeader = size;
@@ -5601,7 +5614,8 @@ void *MetadataAllocator::Allocate(size_t size, size_t alignment) {
       // If that succeeded, we've successfully allocated.
       __msan_allocated_memory(allocation, sizeWithHeader);
       __asan_unpoison_memory_region(allocation, sizeWithHeader);
-
+      memsetScribble(allocation, sizeWithHeader);
+
       if (SWIFT_UNLIKELY(_swift_debug_metadataAllocationIterationEnabled)) {
         AllocationHeader *header = (AllocationHeader *)allocation;
         header->Size = size;

test/Runtime/environment_variables.swift

@@ -5,7 +5,7 @@
 // REQUIRES: executable_test
 // UNSUPPORTED: use_os_stdlib
 
-// RUN: env %env-SWIFT_DEBUG_HELP=YES %env-SWIFT_DEBUG_SOME_UNKNOWN_VARIABLE=42 %env-SWIFT_DEBUG_ENABLE_METADATA_ALLOCATION_ITERATION=YES %env-SWIFT_DEBUG_IMPLICIT_OBJC_ENTRYPOINT=abc %env-SWIFT_DETERMINISTIC_HASHING=whatever %env-SWIFT_ENABLE_MANGLED_NAME_VERIFICATION=YES %target-run %t/main 2>&1 | %FileCheck %s --dump-input fail
+// RUN: env %env-SWIFT_DEBUG_HELP=YES %env-SWIFT_DEBUG_SOME_UNKNOWN_VARIABLE=42 %env-SWIFT_DEBUG_ENABLE_METADATA_ALLOCATION_ITERATION=YES %env-SWIFT_DEBUG_IMPLICIT_OBJC_ENTRYPOINT=abc %env-SWIFT_DETERMINISTIC_HASHING=whatever %env-SWIFT_ENABLE_MANGLED_NAME_VERIFICATION=YES %env-SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE=YES %target-run %t/main 2>&1 | %FileCheck %s --dump-input fail
 
 // CHECK-DAG: {{Warning: unknown environment variable SWIFT_DEBUG_SOME_UNKNOWN_VARIABLE|Using getenv to read variables. Unknown SWIFT_DEBUG_ variables will not be flagged.}}
 // CHECK-DAG: Warning: cannot parse value SWIFT_DEBUG_IMPLICIT_OBJC_ENTRYPOINT=abc, defaulting to 2.
@@ -15,6 +15,7 @@
 // CHECK-DAG: uint8_t SWIFT_DEBUG_IMPLICIT_OBJC_ENTRYPOINT [default: 2] - Print warnings when using implicit @objc entrypoints. Set to desired reporting level, 0-3.
 // CHECK-DAG:    bool SWIFT_DETERMINISTIC_HASHING [default: false] - Disable randomized hash seeding.
 // CHECK-DAG:    bool SWIFT_ENABLE_MANGLED_NAME_VERIFICATION [default: false] - Enable verification that metadata can roundtrip through a mangled name each time metadata is instantiated.
+// CHECK-DAG:    bool SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE [default: false] - Scribble on runtime allocations such as metadata allocations.
 
 print("Hello, world")
 // CHECK: Hello, world

test/lit.cfg

@@ -1912,6 +1912,9 @@ if config.lldb_build_root != "":
 # variable.
 config.environment[TARGET_ENV_PREFIX + 'SWIFT_DETERMINISTIC_HASHING'] = '1'
 
+# Enable malloc scribble during tests by default.
+config.environment[TARGET_ENV_PREFIX + 'SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE'] = 'YES'
+
 # Run lsb_release on the target to be tested and return the results.
 def linux_get_lsb_release():
     lsb_release_path = '/usr/bin/lsb_release'