don't round up sizes for large allocation when sampling

This closes #723.

Since rounding up prior to sampling is introducing possibility of
arithmetic overflow, we're just not doing it.

It introduces some error (up to 4k), but since we're dealing with at
least 256k allocations, we're fine.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/tcmalloc.cc b/src/tcmalloc.cc
index 2f74fa5..578ea5c 100644
--- a/src/tcmalloc.cc
+++ b/src/tcmalloc.cc
@@ -1154,8 +1154,13 @@ inline void* do_malloc_pages(ThreadCache* heap, size_t size) {
   bool report_large;
 
   Length num_pages = tcmalloc::pages(size);
-  size = num_pages << kPageShift;
 
+  // NOTE: we're passing original size here as opposed to rounded-up
+  // size as we do in do_malloc_small. The difference is small here
+  // (at most 4k out of at least 256k). And not rounding up saves us
+  // from possibility of overflow, which rounding up could produce.
+  //
+  // See https://github.com/gperftools/gperftools/issues/723
   if (heap->SampleAllocation(size)) {
     result = DoSampledAllocation(size);