[PATCH] D42916: [sanitizer] SizeClassMap minor improvement/correctness changes

[Kostya Kortchinsky via Phabricator via llvm-commits]

cryptoad created this revision.
cryptoad added a reviewer: alekseyshl.
Herald added subscribers: Sanitizers, delcypher, kubamracek.

In `ClassID`, make sure we use an unsigned as based for the `lbits` shift.
The previous code resulted in spurious sign extensions like for x64:

  add     esi, 0FFFFFFFFh
  movsxd  rcx, esi
  and     rcx, r15

The code with the `U` added is:

  add     esi, 0FFFFFFFFh
  and     rsi, r15

And for `MaxCachedHint`, use a 32-bit division instead of `64-bit`, which is
faster (https://lemire.me/blog/2017/11/16/fast-exact-integer-divisions-using-floating-point-operations/)
and already used in other parts of the code (64-bit `GetChunkIdx`, 32-bit
`GetMetaData` enforce 32-bit divisions)

Not major performance gains by any mean, but they don't hurt.


Repository:
  rCRT Compiler Runtime

https://reviews.llvm.org/D42916

Files:
  lib/sanitizer_common/sanitizer_allocator_size_class_map.h


Index: lib/sanitizer_common/sanitizer_allocator_size_class_map.h
===================================================================
--- lib/sanitizer_common/sanitizer_allocator_size_class_map.h
+++ lib/sanitizer_common/sanitizer_allocator_size_class_map.h
@@ -163,7 +163,7 @@
       return (size + kMinSize - 1) >> kMinSizeLog;
     uptr l = MostSignificantSetBitIndex(size);
     uptr hbits = (size >> (l - S)) & M;
-    uptr lbits = size & ((1 << (l - S)) - 1);
+    uptr lbits = size & ((1U << (l - S)) - 1);
     uptr l1 = l - kMidSizeLog;
     return kMidClass + (l1 << S) + hbits + (lbits > 0);
   }
@@ -176,7 +176,8 @@
       return 16;
     if (UNLIKELY(class_id == 0))
       return 0;
-    uptr n = (1UL << kMaxBytesCachedLog) / Size(class_id);
+    const uptr n =
+        (1U << kMaxBytesCachedLog) / static_cast<u32>(Size(class_id));
     return Max<uptr>(1, Min(kMaxNumCachedHint, n));
   }
 


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