third_party/WebKit/Source/wtf/BitwiseOperations.h

   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
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   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
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  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     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.
  62 ALWAYS_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.
  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