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Diffstat (limited to 'absl/base/internal/bits.h')
| -rw-r--r-- | absl/base/internal/bits.h | 195 |
1 files changed, 195 insertions, 0 deletions
diff --git a/absl/base/internal/bits.h b/absl/base/internal/bits.h new file mode 100644 index 00000000..29657426 --- /dev/null +++ b/absl/base/internal/bits.h @@ -0,0 +1,195 @@ +// Copyright 2018 The Abseil Authors. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +#ifndef ABSL_BASE_INTERNAL_BITS_H_ +#define ABSL_BASE_INTERNAL_BITS_H_ + +// This file contains bitwise ops which are implementation details of various +// absl libraries. + +#include <cstdint> + +// Clang on Windows has __builtin_clzll; otherwise we need to use the +// windows intrinsic functions. +#if defined(_MSC_VER) +#include <intrin.h> +#if defined(_M_X64) +#pragma intrinsic(_BitScanReverse64) +#pragma intrinsic(_BitScanForward64) +#endif +#pragma intrinsic(_BitScanReverse) +#pragma intrinsic(_BitScanForward) +#endif + +#include "absl/base/attributes.h" + +#if defined(_MSC_VER) +// We can achieve something similar to attribute((always_inline)) with MSVC by +// using the __forceinline keyword, however this is not perfect. MSVC is +// much less aggressive about inlining, and even with the __forceinline keyword. +#define ABSL_BASE_INTERNAL_FORCEINLINE __forceinline +#else +// Use default attribute inline. +#define ABSL_BASE_INTERNAL_FORCEINLINE inline ABSL_ATTRIBUTE_ALWAYS_INLINE +#endif + + +namespace absl { +inline namespace lts_2018_12_18 { +namespace base_internal { + +ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros64Slow(uint64_t n) { + int zeroes = 60; + if (n >> 32) zeroes -= 32, n >>= 32; + if (n >> 16) zeroes -= 16, n >>= 16; + if (n >> 8) zeroes -= 8, n >>= 8; + if (n >> 4) zeroes -= 4, n >>= 4; + return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[n] + zeroes; +} + +ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros64(uint64_t n) { +#if defined(_MSC_VER) && defined(_M_X64) + // MSVC does not have __buitin_clzll. Use _BitScanReverse64. + unsigned long result = 0; // NOLINT(runtime/int) + if (_BitScanReverse64(&result, n)) { + return 63 - result; + } + return 64; +#elif defined(_MSC_VER) + // MSVC does not have __buitin_clzll. Compose two calls to _BitScanReverse + unsigned long result = 0; // NOLINT(runtime/int) + if ((n >> 32) && _BitScanReverse(&result, n >> 32)) { + return 31 - result; + } + if (_BitScanReverse(&result, n)) { + return 63 - result; + } + return 64; +#elif defined(__GNUC__) + // Use __builtin_clzll, which uses the following instructions: + // x86: bsr + // ARM64: clz + // PPC: cntlzd + static_assert(sizeof(unsigned long long) == sizeof(n), // NOLINT(runtime/int) + "__builtin_clzll does not take 64-bit arg"); + + // Handle 0 as a special case because __builtin_clzll(0) is undefined. + if (n == 0) { + return 64; + } + return __builtin_clzll(n); +#else + return CountLeadingZeros64Slow(n); +#endif +} + +ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros32Slow(uint64_t n) { + int zeroes = 28; + if (n >> 16) zeroes -= 16, n >>= 16; + if (n >> 8) zeroes -= 8, n >>= 8; + if (n >> 4) zeroes -= 4, n >>= 4; + return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[n] + zeroes; +} + +ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros32(uint32_t n) { +#if defined(_MSC_VER) + unsigned long result = 0; // NOLINT(runtime/int) + if (_BitScanReverse(&result, n)) { + return 31 - result; + } + return 32; +#elif defined(__GNUC__) + // Use __builtin_clz, which uses the following instructions: + // x86: bsr + // ARM64: clz + // PPC: cntlzd + static_assert(sizeof(int) == sizeof(n), + "__builtin_clz does not take 32-bit arg"); + + // Handle 0 as a special case because __builtin_clz(0) is undefined. + if (n == 0) { + return 32; + } + return __builtin_clz(n); +#else + return CountLeadingZeros32Slow(n); +#endif +} + +ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero64Slow(uint64_t n) { + int c = 63; + n &= ~n + 1; + if (n & 0x00000000FFFFFFFF) c -= 32; + if (n & 0x0000FFFF0000FFFF) c -= 16; + if (n & 0x00FF00FF00FF00FF) c -= 8; + if (n & 0x0F0F0F0F0F0F0F0F) c -= 4; + if (n & 0x3333333333333333) c -= 2; + if (n & 0x5555555555555555) c -= 1; + return c; +} + +ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero64(uint64_t n) { +#if defined(_MSC_VER) && defined(_M_X64) + unsigned long result = 0; // NOLINT(runtime/int) + _BitScanForward64(&result, n); + return result; +#elif defined(_MSC_VER) + unsigned long result = 0; // NOLINT(runtime/int) + if (static_cast<uint32_t>(n) == 0) { + _BitScanForward(&result, n >> 32); + return result + 32; + } + _BitScanForward(&result, n); + return result; +#elif defined(__GNUC__) + static_assert(sizeof(unsigned long long) == sizeof(n), // NOLINT(runtime/int) + "__builtin_ctzll does not take 64-bit arg"); + return __builtin_ctzll(n); +#else + return CountTrailingZerosNonZero64Slow(n); +#endif +} + +ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero32Slow(uint32_t n) { + int c = 31; + n &= ~n + 1; + if (n & 0x0000FFFF) c -= 16; + if (n & 0x00FF00FF) c -= 8; + if (n & 0x0F0F0F0F) c -= 4; + if (n & 0x33333333) c -= 2; + if (n & 0x55555555) c -= 1; + return c; +} + +ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero32(uint32_t n) { +#if defined(_MSC_VER) + unsigned long result = 0; // NOLINT(runtime/int) + _BitScanForward(&result, n); + return result; +#elif defined(__GNUC__) + static_assert(sizeof(int) == sizeof(n), + "__builtin_ctz does not take 32-bit arg"); + return __builtin_ctz(n); +#else + return CountTrailingZerosNonZero32Slow(n); +#endif +} + +#undef ABSL_BASE_INTERNAL_FORCEINLINE + +} // namespace base_internal +} // inline namespace lts_2018_12_18 +} // namespace absl + +#endif // ABSL_BASE_INTERNAL_BITS_H_ |