[PATCH] [sanitizer] Fix overflow in SizeClassAllocator64::GetChunkIdx().

[Sergey Matveev]

Hi kcc, glider,

The 32-bit offset overflowed when more than 4GB was allocated in a size
class. Also removed the misleading comment.

http://llvm-reviews.chandlerc.com/D797

Files:
  lib/sanitizer_common/sanitizer_allocator.h

Index: lib/sanitizer_common/sanitizer_allocator.h
===================================================================
--- lib/sanitizer_common/sanitizer_allocator.h
+++ lib/sanitizer_common/sanitizer_allocator.h
@@ -492,11 +492,7 @@
   }
 
   static uptr GetChunkIdx(uptr chunk, uptr size) {
-    u32 offset = chunk % kRegionSize;
-    // Here we divide by a non-constant. This is costly.
-    // We require that kRegionSize is at least 2^32 so that offset is 32-bit.
-    // We save 2x by using 32-bit div, but may need to use a 256-way switch.
-    return offset / (u32)size;
+    return (chunk % kRegionSize) / size;
   }
 
   NOINLINE Batch* PopulateFreeList(AllocatorStats *stat, AllocatorCache *c,
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