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liblzma: Disable branchless C version in range decoder.
Thanks to Sebastian Andrzej Siewior and Sam James for benchmarking on various systems.
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1 changed files with 10 additions and 3 deletions
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@ -24,8 +24,8 @@
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// Bitwise-or of the following enable branchless C versions:
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// 0x01 normal bittrees
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// 0x02 fixed-sized reverse bittrees
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// 0x04 variable-sized reverse bittrees (disabled by default, not faster?)
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// 0x08 matched literal (disabled by default, not faster?)
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// 0x04 variable-sized reverse bittrees (not faster)
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// 0x08 matched literal (not faster)
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//
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// GCC & Clang compatible x86-64 inline assembly:
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// 0x010 normal bittrees
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@ -36,12 +36,19 @@
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//
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// The default can be overridden at build time by defining
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// LZMA_RANGE_DECODER_CONFIG to the desired mask.
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//
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// 2024-02-22: Feedback from benchmarks:
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// - Brancless C (0x003) can be better than basic on x86-64 but often it's
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// slightly worse on other archs. Since asm is much better on x86-64,
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// branchless C is not used at all.
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// - With x86-64 asm, there are slight differences between GCC and Clang
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// and different processors. Overall 0x1F0 seems to be the best choice.
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#ifndef LZMA_RANGE_DECODER_CONFIG
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# if defined(__x86_64__) && !defined(__ILP32__) \
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&& (defined(__GNUC__) || defined(__clang__))
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# define LZMA_RANGE_DECODER_CONFIG 0x1F0
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# else
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# define LZMA_RANGE_DECODER_CONFIG 0x03
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# define LZMA_RANGE_DECODER_CONFIG 0
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# endif
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#endif
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