зеркало из https://github.com/mozilla/gecko-dev.git
79 строки
1.9 KiB
C
79 строки
1.9 KiB
C
/*
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* Copyright (c) 2016, Alliance for Open Media. All rights reserved
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*
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* This source code is subject to the terms of the BSD 2 Clause License and
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* the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
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* was not distributed with this source code in the LICENSE file, you can
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* obtain it at www.aomedia.org/license/software. If the Alliance for Open
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* Media Patent License 1.0 was not distributed with this source code in the
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* PATENTS file, you can obtain it at www.aomedia.org/license/patent.
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*/
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#ifndef AOM_AOM_PORTS_BITOPS_H_
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#define AOM_AOM_PORTS_BITOPS_H_
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#include <assert.h>
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#include "aom_ports/msvc.h"
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#include "config/aom_config.h"
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#ifdef _MSC_VER
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#if defined(_M_X64) || defined(_M_IX86)
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#include <intrin.h>
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#define USE_MSC_INTRINSICS
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#endif
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#endif
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#ifdef __cplusplus
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extern "C" {
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#endif
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// get_msb:
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// Returns (int)floor(log2(n)). n must be > 0.
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// These versions of get_msb() are only valid when n != 0 because all
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// of the optimized versions are undefined when n == 0:
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// https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
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// use GNU builtins where available.
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#if defined(__GNUC__) && \
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((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || __GNUC__ >= 4)
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static INLINE int get_msb(unsigned int n) {
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assert(n != 0);
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return 31 ^ __builtin_clz(n);
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}
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#elif defined(USE_MSC_INTRINSICS)
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#pragma intrinsic(_BitScanReverse)
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static INLINE int get_msb(unsigned int n) {
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unsigned long first_set_bit;
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assert(n != 0);
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_BitScanReverse(&first_set_bit, n);
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return first_set_bit;
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}
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#undef USE_MSC_INTRINSICS
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#else
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static INLINE int get_msb(unsigned int n) {
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int log = 0;
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unsigned int value = n;
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int i;
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assert(n != 0);
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for (i = 4; i >= 0; --i) {
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const int shift = (1 << i);
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const unsigned int x = value >> shift;
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if (x != 0) {
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value = x;
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log += shift;
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}
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}
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return log;
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}
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#endif
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#ifdef __cplusplus
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} // extern "C"
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#endif
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#endif // AOM_AOM_PORTS_BITOPS_H_
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