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 49namespace WTF { 50 51#if COMPILER(MSVC) 52 53ALWAYS_INLINE uint32_t countLeadingZeros32(uint32_t x) 54{ 55 unsigned long 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. 62ALWAYS_INLINE uint64_t countLeadingZeros64(uint64_t x) 63{ 64 unsigned long 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. 77ALWAYS_INLINE uint32_t countLeadingZeros32(uint32_t x) 78{ 79 return LIKELY(x) ? __builtin_clz(x) : 32; 80} 81 82ALWAYS_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 91ALWAYS_INLINE size_t countLeadingZerosSizet(size_t x) { return countLeadingZeros64(x); } 92 93#else 94 95ALWAYS_INLINE size_t countLeadingZerosSizet(size_t x) { return countLeadingZeros32(x); } 96 97#endif 98 99} // namespace WTF 100 101#endif // WTF_BitwiseOperations_h 102