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44 lines
1.7 KiB
C
44 lines
1.7 KiB
C
/*
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* Copyright (C) 2014, 2021, 2024, D. R. Commander.
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* Copyright (C) 2014, Olle Liljenzin.
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* Copyright (C) 2020, Arm Limited.
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*
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* For conditions of distribution and use, see the accompanying README.ijg
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* file.
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*/
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/*
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* NOTE: If USE_CLZ_INTRINSIC is defined, then clz/bsr instructions will be
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* used for bit counting rather than the lookup table. This will reduce the
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* memory footprint by 64k, which is important for some mobile applications
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* that create many isolated instances of libjpeg-turbo (web browsers, for
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* instance.) This may improve performance on some mobile platforms as well.
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* This feature is enabled by default only on Arm processors, because some x86
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* chips have a slow implementation of bsr, and the use of clz/bsr cannot be
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* shown to have a significant performance impact even on the x86 chips that
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* have a fast implementation of it. When building for Armv6, you can
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* explicitly disable the use of clz/bsr by adding -mthumb to the compiler
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* flags (this defines __thumb__).
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*/
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/* NOTE: Both GCC and Clang define __GNUC__ */
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#if (defined(__GNUC__) && (defined(__arm__) || defined(__aarch64__))) || \
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defined(_M_ARM) || defined(_M_ARM64)
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#if !defined(__thumb__) || defined(__thumb2__)
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#define USE_CLZ_INTRINSIC
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#endif
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#endif
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#ifdef USE_CLZ_INTRINSIC
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#if defined(_MSC_VER) && !defined(__clang__)
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#define JPEG_NBITS_NONZERO(x) (32 - _CountLeadingZeros(x))
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#else
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#define JPEG_NBITS_NONZERO(x) (32 - __builtin_clz(x))
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#endif
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#define JPEG_NBITS(x) (x ? JPEG_NBITS_NONZERO(x) : 0)
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#else
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extern const unsigned char jpeg_nbits_table[65536];
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#define JPEG_NBITS(x) (jpeg_nbits_table[x])
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#define JPEG_NBITS_NONZERO(x) JPEG_NBITS(x)
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#endif
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