Add partitionAllocGetSize() for determining actual size of allocation

Source/wtf/PartitionAlloc.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 547 matching lines @@
 
     ptr = partitionCookieFreePointerAdjust(ptr);
     ASSERT(partitionPointerIsValid(ptr));
     PartitionPage* page = partitionPointerToPage(ptr);
     spinLockLock(&root->lock);
     partitionFreeWithPage(ptr, page);
     spinLockUnlock(&root->lock);
 #endif
 }
 
+ALWAYS_INLINE size_t partitionAllocGetSize(PartitionRootGeneric* root, void* ptr)
+{
+#if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
+    ASSERT_NOT_REACHED();

[Chris Evans, 2014/02/18 22:19:59]

I think this definitely will be reached. We want the client to call this API
unconditionally, right?

Hmm, I think the way to resolve this that doesn't add cost (branching, etc.) to
the common case would be to add a new ALWAYS_INLINE API:

partitionAllocSupportsGetSize(). It'd just return a hardwired true / false,
depending on MEMORY_TOOL_REPLACES_ALLOCATOR, allowing the compiler to optimize
fully.

+#else
+    ASSERT(root->initialized);
+    if (UNLIKELY(!ptr))
+        return 0;

[Chris Evans, 2014/02/18 22:19:59]

For simplicity, maybe try not supporting passing in NULL? I don't think the
Vector client will need to do that, for example.

+    ptr = partitionCookieFreePointerAdjust(ptr);
+    ASSERT(partitionPointerIsValid(ptr));
+    PartitionPage* page = partitionPointerToPage(ptr);
+    spinLockLock(&root->lock);

[Chris Evans, 2014/02/18 22:19:59]

No need for locking, as covered separately. But definitely add a small comment
to make it clear that the lack of locking is willful.

+    size_t size = page->bucket->slotSize;
+    spinLockUnlock(&root->lock);
+    return partitionCookieSizeAdjustSubtract(size);
+#endif
+}
+
 // N (or more accurately, N - sizeof(void*)) represents the largest size in
 // bytes that will be handled by a SizeSpecificPartitionAllocator.
 // Attempts to partitionAlloc() more than this amount will fail.
 template <size_t N>
 class SizeSpecificPartitionAllocator {
 public:
     static const size_t kMaxAllocation = N - kAllocationGranularity;
     static const size_t kNumBuckets = N / kAllocationGranularity;
     void init() { partitionAllocInit(&m_partitionRoot, kNumBuckets, kMaxAllocation); }
     bool shutdown() { return partitionAllocShutdown(&m_partitionRoot); }
@@ skipping 19 matching lines @@
 using WTF::PartitionRoot;
 using WTF::partitionAllocInit;
 using WTF::partitionAllocShutdown;
 using WTF::partitionAlloc;
 using WTF::partitionFree;
 using WTF::partitionAllocGeneric;
 using WTF::partitionFreeGeneric;
 using WTF::partitionReallocGeneric;
 
 #endif // WTF_PartitionAlloc_h