fa2b6ea092
ef9a4e05ba(libjpeg-turbo 1.4.x), which was based on https://bug815473.bmoattachments.org/attachment.cgi?id=692126 (https://bugzilla.mozilla.org/show_bug.cgi?id=815473), modified the C baseline Huffman encoder so that it precomputes jpeg_nbits_table, in order to facilitate sharing the table among multiple processes. However, libjpeg-turbo never shared the table, and because the table was implemented as a static array,f3a8684cd1(libjpeg-turbo 1.5.x) and37bae1a0e9(libjpeg-turbo 2.0.x) each introduced a duplicate copy of the table for (respectively) the SSE2 baseline Huffman encoder and the C progressive Huffman encoder. This commit does the following: - Move the duplicated code in jchuff.c and jcphuff.c, originally introduced in0cfc4c17b7and37bae1a0e9, into a header (jpeg_nbits.h). - Credit the co-author of0cfc4c17b7. (Refer to https://sourceforge.net/p/libjpeg-turbo/patches/57). - Modify the SSE2 baseline Huffman encoder so that the C Huffman encoders can share its definition of jpeg_nbits_table. - Move the definition of jpeg_nbits_table into a C source file (jpeg_nbits.c) rather than a header, and define the table only if USE_CLZ_INTRINSIC is undefined and the SSE2 baseline Huffman encoder will not be built. - Apply hidden symbol visibility to the shared definition of jpeg_nbits_table, if the compiler supports the necessary attribute. (In practice, only Visual C++ doesn't.) Closes #114 See also: https://bugzilla.mozilla.org/show_bug.cgi?id=1501523
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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