Track malloc size with a header for SHOW_MEMORY

Internal.h

@@ -41,45 +41,47 @@ operator<=>(const std::span<const uint8_t> &lhs,
 // GCOVR_EXCL_START
 
 #if SHOW_MEMORY
-#ifdef __APPLE__
-#include <malloc/malloc.h>
-#else
-#include <malloc.h>
-#endif
 inline int64_t mallocBytes = 0;
 inline int64_t peakMallocBytes = 0;
+constexpr auto kIntMallocHeaderSize = 16;
 #endif
 
-// malloc that aborts on OOM and thus always returns a non-null pointer
+// malloc that aborts on OOM and thus always returns a non-null pointer. Must be
+// paired with `safe_free`.
 __attribute__((always_inline)) inline void *safe_malloc(size_t s) {
+#if SHOW_MEMORY
+  mallocBytes += s;
+  if (mallocBytes > peakMallocBytes) {
+    peakMallocBytes = mallocBytes;
+  }
+  void *p = malloc(s + kIntMallocHeaderSize);
+  if (p == nullptr) {
+    abort();
+  }
+  memcpy(p, &s, sizeof(s));
+  return (char *)p + kIntMallocHeaderSize;
+#else
   void *p = malloc(s);
   if (p == nullptr) {
     abort();
   }
-#if SHOW_MEMORY
-#ifdef __APPLE__
-  mallocBytes += s;
-#else
-  mallocBytes += malloc_usable_size(p);
-#endif
-  if (mallocBytes > peakMallocBytes) {
-    peakMallocBytes = mallocBytes;
-  }
-#endif
   return p;
+#endif
 }
 
+// Must be paired with `safe_malloc`.
+//
 // There's nothing safer about this than free. Only called safe_free for
 // symmetry with safe_malloc.
 __attribute__((always_inline)) inline void safe_free(void *p) {
 #if SHOW_MEMORY
-#ifdef __APPLE__
-  mallocBytes -= malloc_size(p);
+  size_t s;
+  memcpy(&s, (char *)p - kIntMallocHeaderSize, sizeof(s));
+  mallocBytes -= s;
+  free((char *)p - kIntMallocHeaderSize);
 #else
-  mallocBytes -= malloc_usable_size(p);
-#endif
-#endif
   free(p);
+#endif
 }
 
 // ==================== BEGIN ARENA IMPL ====================
@@ -164,7 +166,7 @@ inline Arena::Arena(int initialSize) : impl(nullptr) {
 inline void onDestroy(Arena::ArenaImpl *impl) {
   while (impl) {
     auto *prev = impl->prev;
-    free(impl);
+    safe_free(impl);
     impl = prev;
   }
 }