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      1 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 // This file defines some bit utilities.
      6 
      7 #ifndef BASE_BITS_H_
      8 #define BASE_BITS_H_
      9 
     10 #include <stddef.h>
     11 #include <stdint.h>
     12 
     13 #include "base/compiler_specific.h"
     14 #include "base/logging.h"
     15 
     16 #if defined(COMPILER_MSVC)
     17 #include <intrin.h>
     18 #endif
     19 
     20 namespace base {
     21 namespace bits {
     22 
     23 // Returns the integer i such as 2^i <= n < 2^(i+1)
     24 inline int Log2Floor(uint32_t n) {
     25   if (n == 0)
     26     return -1;
     27   int log = 0;
     28   uint32_t value = n;
     29   for (int i = 4; i >= 0; --i) {
     30     int shift = (1 << i);
     31     uint32_t x = value >> shift;
     32     if (x != 0) {
     33       value = x;
     34       log += shift;
     35     }
     36   }
     37   DCHECK_EQ(value, 1u);
     38   return log;
     39 }
     40 
     41 // Returns the integer i such as 2^(i-1) < n <= 2^i
     42 inline int Log2Ceiling(uint32_t n) {
     43   if (n == 0) {
     44     return -1;
     45   } else {
     46     // Log2Floor returns -1 for 0, so the following works correctly for n=1.
     47     return 1 + Log2Floor(n - 1);
     48   }
     49 }
     50 
     51 // Round up |size| to a multiple of alignment, which must be a power of two.
     52 inline size_t Align(size_t size, size_t alignment) {
     53   DCHECK_EQ(alignment & (alignment - 1), 0u);
     54   return (size + alignment - 1) & ~(alignment - 1);
     55 }
     56 
     57 // These functions count the number of leading zeros in a binary value, starting
     58 // with the most significant bit. C does not have an operator to do this, but
     59 // fortunately the various compilers have built-ins that map to fast underlying
     60 // processor instructions.
     61 #if defined(COMPILER_MSVC)
     62 
     63 ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
     64   unsigned long index;
     65   return LIKELY(_BitScanReverse(&index, x)) ? (31 - index) : 32;
     66 }
     67 
     68 #if defined(ARCH_CPU_64_BITS)
     69 
     70 // MSVC only supplies _BitScanForward64 when building for a 64-bit target.
     71 ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
     72   unsigned long index;
     73   return LIKELY(_BitScanReverse64(&index, x)) ? (63 - index) : 64;
     74 }
     75 
     76 #endif
     77 
     78 #elif defined(COMPILER_GCC)
     79 
     80 // This is very annoying. __builtin_clz has undefined behaviour for an input of
     81 // 0, even though there's clearly a return value that makes sense, and even
     82 // though some processor clz instructions have defined behaviour for 0. We could
     83 // drop to raw __asm__ to do better, but we'll avoid doing that unless we see
     84 // proof that we need to.
     85 ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
     86   return LIKELY(x) ? __builtin_clz(x) : 32;
     87 }
     88 
     89 ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
     90   return LIKELY(x) ? __builtin_clzll(x) : 64;
     91 }
     92 
     93 #endif
     94 
     95 #if defined(ARCH_CPU_64_BITS)
     96 
     97 ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x) {
     98   return CountLeadingZeroBits64(x);
     99 }
    100 
    101 #else
    102 
    103 ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x) {
    104   return CountLeadingZeroBits32(x);
    105 }
    106 
    107 #endif
    108 
    109 }  // namespace bits
    110 }  // namespace base
    111 
    112 #endif  // BASE_BITS_H_
    113