[heap] Verify that newly allocated MemoryChunks are pre-initialzed with 0.

Bug: chromium:829771
Change-Id: I78eab59fded3f41c93ecb3d5d8a30e1bddc4576e
Reviewed-on: https://chromium-review.googlesource.com/1039747Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52925}

src/heap/spaces.cc

@@ -883,6 +883,9 @@ MemoryChunk* MemoryAllocator::AllocateChunk(size_t reserve_area_size,
       size_executable_.Increment(reservation.size());
     }
 
+    VerifyCleared(base, CodePageGuardStartOffset());
+    VerifyCleared(base + CodePageAreaStartOffset(), commit_area_size);
+
     if (Heap::ShouldZapGarbage()) {
       ZapBlock(base, CodePageGuardStartOffset());
       ZapBlock(base + CodePageAreaStartOffset(), commit_area_size);
@@ -902,6 +905,8 @@ MemoryChunk* MemoryAllocator::AllocateChunk(size_t reserve_area_size,
 
     if (base == kNullAddress) return nullptr;
 
+    VerifyCleared(base, Page::kObjectStartOffset + commit_area_size);
+
     if (Heap::ShouldZapGarbage()) {
       ZapBlock(base, Page::kObjectStartOffset + commit_area_size);
     }
@@ -1216,6 +1221,14 @@ void MemoryAllocator::ZapBlock(Address start, size_t size) {
   }
 }
 
+void MemoryAllocator::VerifyCleared(Address start, size_t size) {
+#ifdef VERIFY_HEAP
+  for (size_t i = 0; i < size / kPointerSize; i++) {
+    CHECK_EQ(reinterpret_cast<uintptr_t*>(start)[i], kClearedFreeMemoryValue);
+  }
+#endif  // VERIFY_HEAP
+}
+
 size_t MemoryAllocator::CodePageGuardStartOffset() {
   // We are guarding code pages: the first OS page after the header
   // will be protected as non-writable.

src/heap/spaces.h

@@ -1393,6 +1393,9 @@ class V8_EXPORT_PRIVATE MemoryAllocator {
   // filling it up with a recognizable non-nullptr bit pattern.
   void ZapBlock(Address start, size_t size);
 
+  // Checks if the memory [start..(start+size)[ is initialized with 0.
+  void VerifyCleared(Address start, size_t size);
+
   V8_WARN_UNUSED_RESULT bool CommitExecutableMemory(VirtualMemory* vm,
                                                     Address start,
                                                     size_t commit_size,