1 /* 2 * Copyright (C) 2013 Google Inc. All rights reserved. 3 * 4 * Redistribution and use in source and binary forms, with or without 5 * modification, are permitted provided that the following conditions are 6 * met: 7 * 8 * * Redistributions of source code must retain the above copyright 9 * notice, this list of conditions and the following disclaimer. 10 * * Redistributions in binary form must reproduce the above 11 * copyright notice, this list of conditions and the following disclaimer 12 * in the documentation and/or other materials provided with the 13 * distribution. 14 * * Neither the name of Google Inc. nor the names of its 15 * contributors may be used to endorse or promote products derived from 16 * this software without specific prior written permission. 17 * 18 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 19 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 20 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR 21 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 22 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 23 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 24 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 25 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 26 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 27 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 28 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 29 */ 30 31 #ifndef WTF_BitwiseOperations_h 32 #define WTF_BitwiseOperations_h 33 34 // DESCRIPTION 35 // countLeadingZeros() is a bitwise operation that counts the number of leading 36 // zeros in a binary value, starting with the most significant bit. C does not 37 // have an operator to do this, but fortunately the various compilers have 38 // built-ins that map to fast underlying processor instructions. 39 40 #include "wtf/CPU.h" 41 #include "wtf/Compiler.h" 42 43 #include <stdint.h> 44 45 #if COMPILER(MSVC) 46 #include <intrin.h> 47 #endif 48 49 namespace WTF { 50 51 #if COMPILER(MSVC) 52 53 ALWAYS_INLINE uint32_t countLeadingZeros32(uint32_t x) 54 { 55 DWORD index; 56 return LIKELY(_BitScanReverse(&index, x)) ? (31 - index) : 32; 57 } 58 59 #if CPU(64BIT) 60 61 // MSVC only supplies _BitScanForward64 when building for a 64-bit target. 62 ALWAYS_INLINE uint64_t countLeadingZeros64(uint64_t x) 63 { 64 DWORD index; 65 return LIKELY(_BitScanReverse64(&index, x)) ? (63 - index) : 64; 66 } 67 68 #endif 69 70 #elif COMPILER(GCC) 71 72 // This is very annoying. __builtin_clz has undefined behaviour for an input of 73 // 0, even though these's clearly a return value that makes sense, and even 74 // though nascent processor clz instructions have defined behaviour for 0. 75 // We could drop to raw __asm__ to do better, but we'll avoid doing that unless 76 // we see proof that we need to. 77 ALWAYS_INLINE uint32_t countLeadingZeros32(uint32_t x) 78 { 79 return LIKELY(x) ? __builtin_clz(x) : 32; 80 } 81 82 ALWAYS_INLINE uint64_t countLeadingZeros64(uint64_t x) 83 { 84 return LIKELY(x) ? __builtin_clzll(x) : 64; 85 } 86 87 #endif 88 89 #if CPU(64BIT) 90 91 ALWAYS_INLINE size_t countLeadingZerosSizet(size_t x) { return countLeadingZeros64(x); } 92 93 #else 94 95 ALWAYS_INLINE size_t countLeadingZerosSizet(size_t x) { return countLeadingZeros32(x); } 96 97 #endif 98 99 } // namespace WTF 100 101 #endif // WTF_BitwiseOperations_h 102
