76 lines
1.8 KiB
C++
76 lines
1.8 KiB
C++
/*
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* Copyright (c) 2010 The WebM project authors. All Rights Reserved.
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*
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* Use of this source code is governed by a BSD-style license
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* that can be found in the LICENSE file in the root of the source
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* tree. An additional intellectual property rights grant can be found
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* in the file PATENTS. All contributing project authors may
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* be found in the AUTHORS file in the root of the source tree.
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*/
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#ifndef VPX_PORTS_BITOPS_H_
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#define VPX_PORTS_BITOPS_H_
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#include <assert.h>
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#include "vpx_ports/msvc.h"
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#ifdef _MSC_VER
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# include <math.h> // the ceil() definition must precede intrin.h
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# if _MSC_VER > 1310 && (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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// 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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// Returns (int)floor(log2(n)). n must be > 0.
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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 // VPX_PORTS_BITOPS_H_
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