diff options
Diffstat (limited to 'chromium/v8/src/base/bits.h')
| -rw-r--r-- | chromium/v8/src/base/bits.h | 29 |
1 files changed, 16 insertions, 13 deletions
diff --git a/chromium/v8/src/base/bits.h b/chromium/v8/src/base/bits.h index b74b98e1069..cf4b77fa186 100644 --- a/chromium/v8/src/base/bits.h +++ b/chromium/v8/src/base/bits.h @@ -32,22 +32,27 @@ constexpr inline return sizeof(T) == 8 ? __builtin_popcountll(static_cast<uint64_t>(value)) : __builtin_popcount(static_cast<uint32_t>(value)); #else + // Fall back to divide-and-conquer popcount (see "Hacker's Delight" by Henry + // S. Warren, Jr.), chapter 5-1. constexpr uint64_t mask[] = {0x5555555555555555, 0x3333333333333333, 0x0f0f0f0f0f0f0f0f}; - // Start with 1 bit wide buckets of [0,1]. + // Start with 64 buckets of 1 bits, holding values from [0,1]. value = ((value >> 1) & mask[0]) + (value & mask[0]); - // Having 2 bit wide buckets of [0,2] now. + // Having 32 buckets of 2 bits, holding values from [0,2] now. value = ((value >> 2) & mask[1]) + (value & mask[1]); - // Having 4 bit wide buckets of [0,4] now. - value = (value >> 4) + value; - // Having 4 bit wide buckets of [0,8] now. - if (sizeof(T) > 1) - value = ((value >> (sizeof(T) > 1 ? 8 : 0)) & mask[2]) + (value & mask[2]); - // Having 8 bit wide buckets of [0,16] now. + // Having 16 buckets of 4 bits, holding values from [0,4] now. + value = ((value >> 4) & mask[2]) + (value & mask[2]); + // Having 8 buckets of 8 bits, holding values from [0,8] now. + // From this point on, the buckets are bigger than the number of bits + // required to hold the values, and the buckets are bigger the maximum + // result, so there's no need to mask value anymore, since there's no + // more risk of overflow between buckets. + if (sizeof(T) > 1) value = (value >> (sizeof(T) > 1 ? 8 : 0)) + value; + // Having 4 buckets of 16 bits, holding values from [0,16] now. if (sizeof(T) > 2) value = (value >> (sizeof(T) > 2 ? 16 : 0)) + value; - // Having 8 bit wide buckets of [0,32] now. + // Having 2 buckets of 32 bits, holding values from [0,32] now. if (sizeof(T) > 4) value = (value >> (sizeof(T) > 4 ? 32 : 0)) + value; - // Having 8 bit wide buckets of [0,64] now. + // Having 1 buckets of 64 bits, holding values from [0,64] now. return static_cast<unsigned>(value & 0xff); #endif } @@ -140,9 +145,7 @@ constexpr inline bool IsPowerOfTwo(T value) { template <typename T, typename = typename std::enable_if<std::is_integral<T>::value>::type> inline constexpr int WhichPowerOfTwo(T value) { -#if V8_HAS_CXX14_CONSTEXPR - DCHECK(IsPowerOfTwo(value)); -#endif + CONSTEXPR_DCHECK(IsPowerOfTwo(value)); #if V8_HAS_BUILTIN_CTZ STATIC_ASSERT(sizeof(T) <= 8); return sizeof(T) == 8 ? __builtin_ctzll(static_cast<uint64_t>(value)) |