[compiler-rt] f7de708 - scudo: Simplify getClassIdBySize() logic. NFCI.

[Peter Collingbourne via llvm-commits]

Author: Peter Collingbourne
Date: 2020-02-04T09:32:27-08:00
New Revision: f7de7084f4a2c346b3db87c26fa519db0781a09a

URL: https://github.com/llvm/llvm-project/commit/f7de7084f4a2c346b3db87c26fa519db0781a09a
DIFF: https://github.com/llvm/llvm-project/commit/f7de7084f4a2c346b3db87c26fa519db0781a09a.diff

LOG: scudo: Simplify getClassIdBySize() logic. NFCI.

By subtracting 1 from Size at the beginning we can simplify the
subsequent calculations. This also saves 4 instructions on aarch64
and 9 instructions on x86_64, but seems to be perf neutral.

Differential Revision: https://reviews.llvm.org/D73936

Added: 
    

Modified: 
    compiler-rt/lib/scudo/standalone/size_class_map.h

Removed: 
    


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diff  --git a/compiler-rt/lib/scudo/standalone/size_class_map.h b/compiler-rt/lib/scudo/standalone/size_class_map.h
index f16fed73833f..3849feaf38dd 100644
--- a/compiler-rt/lib/scudo/standalone/size_class_map.h
+++ b/compiler-rt/lib/scudo/standalone/size_class_map.h
@@ -66,11 +66,11 @@ class SizeClassMap {
     DCHECK_LE(Size, MaxSize);
     if (Size <= MidSize)
       return (Size + MinSize - 1) >> MinSizeLog;
+    Size -= 1;
     const uptr L = getMostSignificantSetBitIndex(Size);
-    const uptr HBits = (Size >> (L - S)) & M;
-    const uptr LBits = Size & ((1UL << (L - S)) - 1);
-    const uptr L1 = L - MidSizeLog;
-    return MidClass + (L1 << S) + HBits + (LBits > 0);
+    const uptr LBits = (Size >> (L - S)) - (1 << S);
+    const uptr HBits = (L - MidSizeLog) << S;
+    return MidClass + 1 + HBits + LBits;
   }
 
   static u32 getMaxCachedHint(uptr Size) {