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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 "third_party/base/compiler_specific.h"
     14 #include "third_party/base/logging.h"
     15 
     16 #if defined(COMPILER_MSVC)
     17 #include <intrin.h>
     18 #endif
     19 
     20 namespace pdfium {
     21 namespace base {
     22 namespace bits {
     23 
     24 // Returns the integer i such as 2^i <= n < 2^(i+1)
     25 inline int Log2Floor(uint32_t n) {
     26   if (n == 0)
     27     return -1;
     28   int log = 0;
     29   uint32_t value = n;
     30   for (int i = 4; i >= 0; --i) {
     31     int shift = (1 << i);
     32     uint32_t x = value >> shift;
     33     if (x != 0) {
     34       value = x;
     35       log += shift;
     36     }
     37   }
     38   DCHECK_EQ(value, 1u);
     39   return log;
     40 }
     41 
     42 // Returns the integer i such as 2^(i-1) < n <= 2^i
     43 inline int Log2Ceiling(uint32_t n) {
     44   if (n == 0) {
     45     return -1;
     46   } else {
     47     // Log2Floor returns -1 for 0, so the following works correctly for n=1.
     48     return 1 + Log2Floor(n - 1);
     49   }
     50 }
     51 
     52 // Round up |size| to a multiple of alignment, which must be a power of two.
     53 inline size_t Align(size_t size, size_t alignment) {
     54   DCHECK_EQ(alignment & (alignment - 1), 0u);
     55   return (size + alignment - 1) & ~(alignment - 1);
     56 }
     57 
     58 // These functions count the number of leading zeros in a binary value, starting
     59 // with the most significant bit. C does not have an operator to do this, but
     60 // fortunately the various compilers have built-ins that map to fast underlying
     61 // processor instructions.
     62 #if defined(COMPILER_MSVC)
     63 
     64 ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
     65   unsigned long index;
     66   return LIKELY(_BitScanReverse(&index, x)) ? (31 - index) : 32;
     67 }
     68 
     69 #if defined(ARCH_CPU_64_BITS)
     70 
     71 // MSVC only supplies _BitScanForward64 when building for a 64-bit target.
     72 ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
     73   unsigned long index;
     74   return LIKELY(_BitScanReverse64(&index, x)) ? (63 - index) : 64;
     75 }
     76 
     77 #endif
     78 
     79 #elif defined(COMPILER_GCC)
     80 
     81 // This is very annoying. __builtin_clz has undefined behaviour for an input of
     82 // 0, even though there's clearly a return value that makes sense, and even
     83 // though some processor clz instructions have defined behaviour for 0. We could
     84 // drop to raw __asm__ to do better, but we'll avoid doing that unless we see
     85 // proof that we need to.
     86 ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
     87   return LIKELY(x) ? __builtin_clz(x) : 32;
     88 }
     89 
     90 ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
     91   return LIKELY(x) ? __builtin_clzll(x) : 64;
     92 }
     93 
     94 #endif
     95 
     96 #if defined(ARCH_CPU_64_BITS)
     97 
     98 ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x) {
     99   return CountLeadingZeroBits64(x);
    100 }
    101 
    102 #else
    103 
    104 ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x) {
    105   return CountLeadingZeroBits32(x);
    106 }
    107 
    108 #endif
    109 
    110 }  // namespace bits
    111 }  // namespace base
    112 }  // namespace pdfium
    113 
    114 #endif  // BASE_BITS_H_
    115