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author | Grant <grant@jesanna.com> | 2019-02-08 15:46:14 -0800 |
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committer | Jan Kotas <jkotas@microsoft.com> | 2019-02-08 15:46:14 -0800 |
commit | 8e79024b436f348ea9f96487a6d54067e750f596 (patch) | |
tree | e65cbd1dbfb7517171a2a9fb01432ad6f2f5effd | |
parent | 4e5df11e42457b6201545e672a2f3d1eb18e47e7 (diff) | |
download | coreclr-8e79024b436f348ea9f96487a6d54067e750f596.tar.gz coreclr-8e79024b436f348ea9f96487a6d54067e750f596.tar.bz2 coreclr-8e79024b436f348ea9f96487a6d54067e750f596.zip |
BitOps.TrailingZeroCount has inconsistent software fallback (#22333)
Fix #22326
-rw-r--r-- | src/System.Private.CoreLib/shared/System/BitOps.cs | 57 |
1 files changed, 41 insertions, 16 deletions
diff --git a/src/System.Private.CoreLib/shared/System/BitOps.cs b/src/System.Private.CoreLib/shared/System/BitOps.cs index 39caa592af..99cb19917f 100644 --- a/src/System.Private.CoreLib/shared/System/BitOps.cs +++ b/src/System.Private.CoreLib/shared/System/BitOps.cs @@ -8,31 +8,56 @@ using System.Runtime.Intrinsics.X86; using Internal.Runtime.CompilerServices; +// Some routines inspired by the Stanford Bit Twiddling Hacks by Sean Eron Anderson: +// http://graphics.stanford.edu/~seander/bithacks.html + namespace System { internal static class BitOps { + // C# no-alloc optimization that directly wraps the data section of the dll (similar to string constants) + // https://github.com/dotnet/roslyn/pull/24621 + + private static ReadOnlySpan<byte> s_TrailingZeroCountDeBruijn => new byte[32] + { + 00, 01, 28, 02, 29, 14, 24, 03, + 30, 22, 20, 15, 25, 17, 04, 08, + 31, 27, 13, 23, 21, 19, 16, 07, + 26, 12, 18, 06, 11, 05, 10, 09 + }; + + /// <summary> + /// Count the number of trailing zero bits in an integer value. + /// Similar in behavior to the x86 instruction TZCNT. + /// </summary> + /// <param name="value">The value.</param> + [MethodImpl(MethodImplOptions.AggressiveInlining)] + public static int TrailingZeroCount(int value) + => TrailingZeroCount((uint)value); + + /// <summary> + /// Count the number of trailing zero bits in an integer value. + /// Similar in behavior to the x86 instruction TZCNT. + /// </summary> + /// <param name="value">The value.</param> [MethodImpl(MethodImplOptions.AggressiveInlining)] - public static int TrailingZeroCount(int matches) + public static int TrailingZeroCount(uint value) { if (Bmi1.IsSupported) { - return (int)Bmi1.TrailingZeroCount((uint)matches); + // Note that TZCNT contract specifies 0->32 + return (int)Bmi1.TrailingZeroCount(value); } - else // Software fallback - { - // https://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightMultLookup - // uint.MaxValue >> 27 is always in range [0 - 31] so we use Unsafe.AddByteOffset to avoid bounds check - return Unsafe.AddByteOffset( - ref MemoryMarshal.GetReference(TrailingCountMultiplyDeBruijn), - ((uint)((matches & -matches) * 0x077CB531U)) >> 27); - } - } - private static ReadOnlySpan<byte> TrailingCountMultiplyDeBruijn => new byte[32] - { - 0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8, - 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9 - }; + // Main code has behavior 0->0, so special-case in order to match intrinsic path 0->32 + if (value == 0u) + return 32; + + // uint.MaxValue >> 27 is always in range [0 - 31] so we use Unsafe.AddByteOffset to avoid bounds check + return Unsafe.AddByteOffset( + ref MemoryMarshal.GetReference(s_TrailingZeroCountDeBruijn), + // Using deBruijn sequence, k=2, n=5 (2^5=32) : 0b_0000_0111_0111_1100_1011_0101_0011_0001u + ((uint)((value & -value) * 0x077CB531u)) >> 27); + } } } |