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Hash : fa2b6ea0
Author :
Date : 2024-01-12T18:21:41

Eliminate duplicate copies of jpeg_nbits_table

ef9a4e05ba919494cbebe50e15f332de5ab97e82 (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, f3a8684cd1c28e557d394470962a7a224c76ddbc
(libjpeg-turbo 1.5.x) and 37bae1a0e977ee1ba769e6f0aa27e519ab6e58c6
(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 in 0cfc4c17b740cb2cbb11f9d85c8ab3745d5b913a and
  37bae1a0e977ee1ba769e6f0aa27e519ab6e58c6, into a header
  (jpeg_nbits.h).
- Credit the co-author of 0cfc4c17b740cb2cbb11f9d85c8ab3745d5b913a.
  (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

jpeg_nbits.h

/*
 * Copyright (C) 2014, 2021, 2024, D. R. Commander.
 * Copyright (C) 2014, Olle Liljenzin.
 * Copyright (C) 2020, Arm Limited.
 *
 * For conditions of distribution and use, see the accompanying README.ijg
 * file.
 */

/*
 * NOTE: If USE_CLZ_INTRINSIC is defined, then clz/bsr instructions will be
 * used for bit counting rather than the lookup table.  This will reduce the
 * memory footprint by 64k, which is important for some mobile applications
 * that create many isolated instances of libjpeg-turbo (web browsers, for
 * instance.)  This may improve performance on some mobile platforms as well.
 * This feature is enabled by default only on Arm processors, because some x86
 * chips have a slow implementation of bsr, and the use of clz/bsr cannot be
 * shown to have a significant performance impact even on the x86 chips that
 * have a fast implementation of it.  When building for Armv6, you can
 * explicitly disable the use of clz/bsr by adding -mthumb to the compiler
 * flags (this defines __thumb__).
 */

/* NOTE: Both GCC and Clang define __GNUC__ */
#if (defined(__GNUC__) && (defined(__arm__) || defined(__aarch64__))) || \
    defined(_M_ARM) || defined(_M_ARM64)
#if !defined(__thumb__) || defined(__thumb2__)
#define USE_CLZ_INTRINSIC
#endif
#endif

#ifdef USE_CLZ_INTRINSIC
#if defined(_MSC_VER) && !defined(__clang__)
#define JPEG_NBITS_NONZERO(x)  (32 - _CountLeadingZeros(x))
#else
#define JPEG_NBITS_NONZERO(x)  (32 - __builtin_clz(x))
#endif
#define JPEG_NBITS(x)          (x ? JPEG_NBITS_NONZERO(x) : 0)
#else
extern const unsigned char jpeg_nbits_table[65536];
#define JPEG_NBITS(x)          (jpeg_nbits_table[x])
#define JPEG_NBITS_NONZERO(x)  JPEG_NBITS(x)
#endif

kc3-lang/libjpeg-turbo/jpeg_nbits.h

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jpeg_nbits.h