Abseil Common Libraries (C++) (grcp 依赖)
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53 lines
1.6 KiB
53 lines
1.6 KiB
// Copyright 2018 The Abseil Authors. |
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// |
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// Licensed under the Apache License, Version 2.0 (the "License"); |
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// you may not use this file except in compliance with the License. |
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// You may obtain a copy of the License at |
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// |
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// http://www.apache.org/licenses/LICENSE-2.0 |
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// |
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// Unless required by applicable law or agreed to in writing, software |
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// distributed under the License is distributed on an "AS IS" BASIS, |
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
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// See the License for the specific language governing permissions and |
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// limitations under the License. |
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#ifndef ABSL_STRINGS_INTERNAL_BITS_H_ |
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#define ABSL_STRINGS_INTERNAL_BITS_H_ |
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#include <cstdint> |
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#if defined(_MSC_VER) && defined(_M_X64) |
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#include <intrin.h> |
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#pragma intrinsic(_BitScanReverse64) |
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#endif |
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namespace absl { |
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namespace strings_internal { |
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// Returns the number of leading 0 bits in a 64-bit value. |
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inline int CountLeadingZeros64(uint64_t n) { |
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#if defined(__GNUC__) |
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static_assert(sizeof(unsigned long long) == sizeof(n), // NOLINT(runtime/int) |
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"__builtin_clzll does not take 64bit arg"); |
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return n == 0 ? 64 : __builtin_clzll(n); |
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#elif defined(_MSC_VER) && defined(_M_X64) |
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unsigned long result; // NOLINT(runtime/int) |
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if (_BitScanReverse64(&result, n)) { |
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return 63 - result; |
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} |
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return 64; |
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#else |
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int zeroes = 60; |
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if (n >> 32) zeroes -= 32, n >>= 32; |
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if (n >> 16) zeroes -= 16, n >>= 16; |
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if (n >> 8) zeroes -= 8, n >>= 8; |
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if (n >> 4) zeroes -= 4, n >>= 4; |
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return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0\0"[n] + zeroes; |
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#endif |
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} |
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} // namespace strings_internal |
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} // namespace absl |
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#endif // ABSL_STRINGS_INTERNAL_BITS_H_
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