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liblzma: Choose the range decoder variants using a bitmask macro.

This commit is contained in:
Lasse Collin 2024-02-13 14:05:13 +02:00
parent 0709c2b2d7
commit de55485cb2

View file

@ -16,6 +16,35 @@
#include "range_common.h"
// Choose the range decoder variants to use using a bitmask.
// If no bits are set, only the basic version is used.
// If more than one version is selected for the same feature,
// the last one on the list below is used.
//
// Bitwise-or of the following enable branchless C versions:
// 0x01 normal bittrees
// 0x02 fixed-sized reverse bittrees
// 0x04 variable-sized reverse bittrees (disabled by default, not faster?)
// 0x08 matched literal (disabled by default, not faster?)
//
// GCC & Clang compatible x86-64 inline assembly:
// 0x010 normal bittrees
// 0x020 fixed-sized reverse bittrees
// 0x040 variable-sized reverse bittrees
// 0x080 matched literal
// 0x100 direct bits
//
// The default can be overriden at build time by defining
// LZMA_RANGE_DECODER_CONFIG to the desired mask.
#ifndef LZMA_RANGE_DECODER_CONFIG
# if defined(__x86_64__) && (defined(__GNUC__) || defined(__clang__))
# define LZMA_RANGE_DECODER_CONFIG 0x1F0
# else
# define LZMA_RANGE_DECODER_CONFIG 0x03
# endif
#endif
// Negative RC_BIT_MODEL_TOTAL but the lowest RC_MOVE_BITS are flipped.
// This is useful for updating probability variables in branchless decoding:
//
@ -369,20 +398,24 @@ do { \
} while (0)
// TODO: Testing on x86-64 give an impression that only the main bittrees are
// worth the branchless C code. It should be tested on other archs for which
// there isn't assembly code in this file.
// Testing on x86-64 give an impression that only the normal bittrees and
// the fixed-sized reverse bittrees are worth the branchless C code.
// It should be tested on other archs for which there isn't assembly code
// in this file.
// Using addition in "(symbol << 1) + rc_mask" allows use of x86 LEA
// or RISC-V SH1ADD instructions. Compilers might infer it from
// "(symbol << 1) | rc_mask" too if they see that mask is 0 or 1 but
// the use of addition doesn't require such analysis from compilers.
#if LZMA_RANGE_DECODER_CONFIG & 0x01
#undef rc_bittree_bit
#define rc_bittree_bit(prob) \
rc_c_bit(prob, \
symbol = (symbol << 1) + rc_mask, \
)
#endif // LZMA_RANGE_DECODER_CONFIG & 0x01
#if LZMA_RANGE_DECODER_CONFIG & 0x02
#undef rc_bittree_rev4
#define rc_bittree_rev4(probs) \
do { \
@ -392,20 +425,18 @@ do { \
rc_c_bit(probs[symbol + 4], symbol += rc_mask << 2, ); \
rc_c_bit(probs[symbol + 8], symbol += rc_mask << 3, ); \
} while (0)
#endif // LZMA_RANGE_DECODER_CONFIG & 0x02
// TODO: Test performance on platforms for which there is no assembly code.
/*
#if LZMA_RANGE_DECODER_CONFIG & 0x04
#undef rc_bit_add_if_1
#define rc_bit_add_if_1(probs, dest, value_to_add_if_1) \
rc_c_bit(probs[symbol], \
symbol = (symbol << 1) + rc_mask, \
dest += (value_to_add_if_1) & rc_mask)
*/
#endif // LZMA_RANGE_DECODER_CONFIG & 0x04
// TODO: Test on platforms for which there is no assembly code.
/*
#if LZMA_RANGE_DECODER_CONFIG & 0x08
#undef decode_with_match_bit
#define decode_with_match_bit \
t_match_byte <<= 1; \
@ -414,14 +445,14 @@ do { \
rc_c_bit(probs[t_subcoder_index], \
symbol = (symbol << 1) + rc_mask, \
t_offset &= ~t_match_bit ^ rc_mask)
*/
#endif // LZMA_RANGE_DECODER_CONFIG & 0x08
////////////
// x86-64 //
////////////
#if defined(__x86_64__) && (defined(__GNUC__) || defined(__clang__))
#if LZMA_RANGE_DECODER_CONFIG & 0x1F0
// rc_asm_y and rc_asm_n are used as arguments to macros to control which
// strings to include or omit.
@ -625,6 +656,8 @@ do { \
"cc", "memory"); \
} while (0)
#if LZMA_RANGE_DECODER_CONFIG & 0x010
#undef rc_bittree3
#define rc_bittree3(probs_base_var, final_add) \
rc_asm_bittree_n(probs_base_var, final_add, \
@ -656,6 +689,7 @@ do { \
rc_asm_bittree(0, 1, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree(1, 0, rc_asm_n, rc_asm_n, rc_asm_y) \
)
#endif // LZMA_RANGE_DECODER_CONFIG & 0x010
// Fixed-sized reverse bittree
@ -717,6 +751,7 @@ do { \
#dcur "(%[probs_base], %q[t1], 2)\n\t" \
)
#if LZMA_RANGE_DECODER_CONFIG & 0x020
#undef rc_bittree_rev4
#define rc_bittree_rev4(probs_base_var) \
rc_asm_bittree_n(probs_base_var, 4, \
@ -725,8 +760,10 @@ rc_asm_bittree_n(probs_base_var, 4, \
rc_asm_bittree_rev(0, 1, 4, 8, 16, 24, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree_rev(1, 0, 8, 16, -, -, rc_asm_n, rc_asm_n, rc_asm_y) \
)
#endif // LZMA_RANGE_DECODER_CONFIG & 0x020
#if LZMA_RANGE_DECODER_CONFIG & 0x040
#undef rc_bit_add_if_1
#define rc_bit_add_if_1(probs_base_var, dest_var, value_to_add_if_1) \
do { \
@ -778,6 +815,7 @@ do { \
: \
"cc", "memory"); \
} while (0)
#endif // LZMA_RANGE_DECODER_CONFIG & 0x040
// Literal decoding uses a normal 8-bit bittree but literal with match byte
@ -826,6 +864,7 @@ do { \
"mov %w[prob], (%[probs_base], %q[t1], 1)\n\t"
#if LZMA_RANGE_DECODER_CONFIG & 0x080
#undef rc_matched_literal
#define rc_matched_literal(probs_base_var, match_byte_value) \
do { \
@ -867,9 +906,11 @@ do { \
: \
"cc", "memory"); \
} while (0)
#endif // LZMA_RANGE_DECODER_CONFIG & 0x080
// Doing the loop in asm instead of C seems to help a little.
#if LZMA_RANGE_DECODER_CONFIG & 0x100
#undef rc_direct
#define rc_direct(dest_var, count_var) \
do { \
@ -904,6 +945,7 @@ do { \
: \
"cc", "memory"); \
} while (0)
#endif // LZMA_RANGE_DECODER_CONFIG & 0x100
#endif // x86_64