Faster LZMA decoder for x86 CPUs (patch for XZ Utils).

faster_lzma_decoder_x86.patch

From 387fd25f57f41009fc317f7922e957de9f370ff2 Mon Sep 17 00:00:00 2001
From: Ilya Kurdyukov <[email protected]>
Date: Tue, 14 Dec 2021 21:54:32 +0700
Subject: [PATCH] faster lzma_decoder for x86

Notice: Uses inline assembly with CMOV instruction.

Another change that removes the comparison with in_size can give a few
percent speedup for architectures with a small number of registers.
---
 src/liblzma/lzma/lzma_decoder.c        | 78 +++++++++++++-------------
 src/liblzma/rangecoder/range_decoder.h | 78 ++++++++++++++++++++++++--
 2 files changed, 113 insertions(+), 43 deletions(-)

diff --git a/src/liblzma/lzma/lzma_decoder.c b/src/liblzma/lzma/lzma_decoder.c
index e605a0a..ecb804b 100644
--- a/src/liblzma/lzma/lzma_decoder.c
+++ b/src/liblzma/lzma/lzma_decoder.c
@@ -415,9 +415,7 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
 							= offset + match_bit
 							+ symbol;
 
-					rc_bit(probs[subcoder_index],
-							offset &= ~match_bit,
-							offset &= match_bit,
+					rc_bit_matched(probs[subcoder_index],
 							SEQ_LITERAL_MATCHED);
 
 					// It seems to be faster to do this
@@ -437,10 +435,7 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
 		case seq: \
 			match_bit = len & offset; \
 			subcoder_index = offset + match_bit + symbol; \
-			rc_bit(probs[subcoder_index], \
-					offset &= ~match_bit, \
-					offset &= match_bit, \
-					seq)
+			rc_bit_matched(probs[subcoder_index], seq)
 
 				d(SEQ_LITERAL_MATCHED0);
 				len <<= 1;
@@ -564,40 +559,43 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
 					probs = coder->pos_special + rep0
 							- symbol - 1;
 					symbol = 1;
-					offset = 0;
-	case SEQ_DIST_MODEL:
+					offset = 1;
 #ifdef HAVE_SMALL
+					limit = 1 << limit;
+	case SEQ_DIST_MODEL:
 					do {
-						rc_bit(probs[symbol], ,
-							rep0 += 1U << offset,
+						rc_bit_dist(probs[symbol],
+							offset,
 							SEQ_DIST_MODEL);
-					} while (++offset < limit);
+						offset <<= 1;
+					} while (offset < limit);
 #else
+	case SEQ_DIST_MODEL:
 					switch (limit) {
 					case 5:
 						assert(offset == 0);
-						rc_bit(probs[symbol], ,
-							rep0 += 1U,
+						rc_bit_dist(probs[symbol],
+							offset,
 							SEQ_DIST_MODEL);
-						++offset;
+						offset <<= 1;
 						--limit;
 					case 4:
-						rc_bit(probs[symbol], ,
-							rep0 += 1U << offset,
+						rc_bit_dist(probs[symbol],
+							offset,
 							SEQ_DIST_MODEL);
-						++offset;
+						offset <<= 1;
 						--limit;
 					case 3:
-						rc_bit(probs[symbol], ,
-							rep0 += 1U << offset,
+						rc_bit_dist(probs[symbol],
+							offset,
 							SEQ_DIST_MODEL);
-						++offset;
+						offset <<= 1;
 						--limit;
 					case 2:
-						rc_bit(probs[symbol], ,
-							rep0 += 1U << offset,
+						rc_bit_dist(probs[symbol],
+							offset,
 							SEQ_DIST_MODEL);
-						++offset;
+						offset <<= 1;
 						--limit;
 					case 1:
 						// We need "symbol" only for
@@ -606,8 +604,8 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
 						// rc_bit_last() here to omit
 						// the unneeded updating of
 						// "symbol".
-						rc_bit_last(probs[symbol], ,
-							rep0 += 1U << offset,
+						rc_bit_dist_last(probs[symbol],
+							offset,
 							SEQ_DIST_MODEL);
 					}
 #endif
@@ -630,30 +628,32 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
 					rep0 <<= ALIGN_BITS;
 					symbol = 1;
 #ifdef HAVE_SMALL
-					offset = 0;
+					offset = 1;
 	case SEQ_ALIGN:
 					do {
-						rc_bit(coder->pos_align[
-								symbol], ,
-							rep0 += 1U << offset,
+						rc_bit_dist(coder->pos_align[
+								symbol],
+							offset,
 							SEQ_ALIGN);
-					} while (++offset < ALIGN_BITS);
+						offset <<= 1;
+					} while (offset < (1 << ALIGN_BITS));
 #else
 	case SEQ_ALIGN0:
-					rc_bit(coder->pos_align[symbol], ,
-							rep0 += 1, SEQ_ALIGN0);
+					rc_bit_dist(coder->pos_align[symbol],
+							1, SEQ_ALIGN0);
 	case SEQ_ALIGN1:
-					rc_bit(coder->pos_align[symbol], ,
-							rep0 += 2, SEQ_ALIGN1);
+					rc_bit_dist(coder->pos_align[symbol],
+							2, SEQ_ALIGN1);
 	case SEQ_ALIGN2:
-					rc_bit(coder->pos_align[symbol], ,
-							rep0 += 4, SEQ_ALIGN2);
+					rc_bit_dist(coder->pos_align[symbol],
+							4, SEQ_ALIGN2);
 	case SEQ_ALIGN3:
 					// Like in SEQ_DIST_MODEL, we don't
 					// need "symbol" for anything else
 					// than indexing the probability array.
-					rc_bit_last(coder->pos_align[symbol], ,
-							rep0 += 8, SEQ_ALIGN3);
+					rc_bit_dist_last(coder->pos_align[
+								symbol],
+							8, SEQ_ALIGN3);
 #endif
 
 					if (rep0 == UINT32_MAX) {
diff --git a/src/liblzma/rangecoder/range_decoder.h b/src/liblzma/rangecoder/range_decoder.h
index e0b051f..cc9ac35 100644
--- a/src/liblzma/rangecoder/range_decoder.h
+++ b/src/liblzma/rangecoder/range_decoder.h
@@ -53,7 +53,8 @@ rc_read_init(lzma_range_decoder *rc, const uint8_t *restrict in,
 /// variables `in' and `in_size' to be defined.
 #define rc_to_local(range_decoder, in_pos) \
 	lzma_range_decoder rc = range_decoder; \
-	size_t rc_in_pos = (in_pos); \
+	size_t rc_in_pos = in_pos - in_size; \
+	const uint8_t *restrict rc_end = in + in_size; \
 	uint32_t rc_bound
 
 
@@ -61,7 +62,7 @@ rc_read_init(lzma_range_decoder *rc, const uint8_t *restrict in,
 #define rc_from_local(range_decoder, in_pos) \
 do { \
 	range_decoder = rc; \
-	in_pos = rc_in_pos; \
+	in_pos = rc_in_pos + in_size; \
 } while (0)
 
 
@@ -87,12 +88,13 @@ do { \
 #define rc_normalize(seq) \
 do { \
 	if (rc.range < RC_TOP_VALUE) { \
-		if (unlikely(rc_in_pos == in_size)) { \
+		if (unlikely(!rc_in_pos)) { \
 			coder->sequence = seq; \
 			goto out; \
 		} \
+		rc.code <<= RC_SHIFT_BITS; \
 		rc.range <<= RC_SHIFT_BITS; \
-		rc.code = (rc.code << RC_SHIFT_BITS) | in[rc_in_pos++]; \
+		rc.code |= rc_end[rc_in_pos++]; \
 	} \
 } while (0)
 
@@ -151,11 +153,79 @@ do { \
 
 /// Decodes one bit, updates "symbol", and runs action0 or action1 depending
 /// on the decoded bit.
+#ifndef NO_BRANCH_OPT
+#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+#define rc_bit_nobranch(prob, pre0, pre1, action0, action1, seq) \
+do { \
+	rc_normalize(seq); \
+	uint32_t cache = (prob), tmp; \
+	rc_bound = (rc.range >> RC_BIT_MODEL_TOTAL_BITS) * cache; \
+	rc.range -= rc_bound; \
+	/* tmp = rc.code < rc_bound ? rc.range = rc_bound, ~0 : 0; */ \
+	__asm__ ( \
+		"cmpl %3, %2\n\t" \
+		"cmovbl %3, %1\n\t" \
+		"sbbl %0, %0" \
+		: "=&r"(tmp), "+&r"(rc.range) \
+		: "r"(rc.code), "r"(rc_bound) \
+		: "flags" \
+	); \
+	cache += tmp & ((1 << RC_MOVE_BITS) - 1 - RC_BIT_MODEL_TOTAL); \
+	prob -= cache >> RC_MOVE_BITS; \
+	pre0; tmp = ~tmp; pre1; \
+	rc.code -= tmp & rc_bound; \
+	if (!tmp) { action0; } else { action1; }; \
+} while (0)
+#elif defined(__GNUC__) && defined(__aarch64__)
+#define rc_bit_nobranch(prob, pre0, pre1, action0, action1, seq) \
+do { \
+	rc_normalize(seq); \
+	uint32_t cache = (prob), tmp; \
+	rc_bound = (rc.range >> RC_BIT_MODEL_TOTAL_BITS) * cache; \
+	rc.range -= rc_bound; \
+	/* tmp = rc.code < rc_bound ? rc.range = rc_bound, ~0 : 0; */ \
+	__asm__ ( \
+		"cmp %w2, %w3\n\t" \
+		"csel %w1, %w3, %w1, lo\n\t" \
+		"csetm %w0, lo" \
+		: "=&r"(tmp), "+&r"(rc.range) \
+		: "r"(rc.code), "r"(rc_bound) \
+		: "cc" \
+	); \
+	cache += tmp & ((1 << RC_MOVE_BITS) - 1 - RC_BIT_MODEL_TOTAL); \
+	prob -= cache >> RC_MOVE_BITS; \
+	pre0; tmp = ~tmp; pre1; \
+	rc.code -= tmp & rc_bound; \
+	if (!tmp) { action0; } else { action1; }; \
+} while (0)
+#endif
+#endif
+#ifdef rc_bit_nobranch
+#define rc_bit(prob, action0, action1, seq) \
+	rc_bit_nobranch(prob, , symbol = (symbol << 1) - tmp, \
+		action0, action1, seq)
+#define rc_bit_matched(prob, seq) \
+	rc_bit_nobranch(prob, offset &= match_bit ^ tmp, \
+		symbol = (symbol << 1) - tmp, , , seq)
+#define rc_bit_dist(prob, offset, seq) \
+	rc_bit_nobranch(prob, , \
+		symbol = (symbol << 1) - tmp; rep0 += offset & tmp, \
+		, , seq);
+#define rc_bit_dist_last(prob, offset, seq) \
+	rc_bit_nobranch(prob, , rep0 += offset & tmp, , , seq);
+#else
 #define rc_bit(prob, action0, action1, seq) \
 	rc_bit_last(prob, \
 		symbol <<= 1; action0, \
 		symbol = (symbol << 1) + 1; action1, \
 		seq);
+#define rc_bit_matched(prob, seq) \
+	rc_bit(prob, offset &= ~match_bit, offset &= match_bit, seq)
+#define rc_bit_dist(prob, offset, seq) \
+	rc_bit(prob, , rep0 += offset, seq);
+#define rc_bit_dist_last(prob, offset, seq) \
+	rc_bit_last(prob, , rep0 += offset, seq)
+#endif
 
 
 /// Like rc_bit() but add "case seq:" as a prefix. This makes the unrolled
-- 
2.17.1

I wrote scripts to make testing easier:

# downloads test files and builds XZ
./build.sh
# tests decompression speed
./bench.sh ref
./bench.sh new

build.sh

#!/bin/bash
set -e

# patch
[ -f faster_lzma_decoder_x86.patch ] || wget https://gist.githubusercontent.com/ilyakurdyukov/f514418f3affd677e1ac408ec0c09188/raw/faster_lzma_decoder_x86.patch
# test data
[ -f linux-5.15.7.tar.xz ] || wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.15.7.tar.xz
[ -f linux-firmware-20211027.tar.xz ] || wget https://mirrors.edge.kernel.org/pub/linux/kernel/firmware/linux-firmware-20211027.tar.xz

[ -d xz ] || git clone http://git.tukaani.org/xz.git
mkdir -p ref
mkdir -p new

# build ref

cp -r xz build
cd build
# or cmake as an option
./autogen.sh --no-po4a
./configure
make
cp -L src/liblzma/.libs/liblzma.so.5 src/xz/.libs/xz ../ref/

# build new

patch -p1 < ../faster_lzma_decoder_x86.patch
make
cp -L src/liblzma/.libs/liblzma.so.5 src/xz/.libs/xz ../new/
cd ..

bench.sh

#!/bin/bash

dir=${1:-"ref"}
export LD_LIBRARY_PATH=$(pwd)/$dir
# to make sure the correct liblzma is loaded 
ldd $dir/xz | grep liblzma

# or "perf stat -r 5" instead of "for" and "time -p"

for i in 1 2 3; do
	echo "# ($dir:$i) linux-5.15.7"
	time -p $dir/xz -c -d linux-5.15.7.tar.xz > /dev/null
	echo "# ($dir:$i) linux-firmware-20211027"
	time -p $dir/xz -c -d linux-firmware-20211027.tar.xz > /dev/null
done

Kunpeng-920 (AArch64)

linux-5.15.7.tar.xz : 8.82 --> 8.78 (+0.4%)
linux-firmware-20211027.tar.xz : 8.97 --> 8.44 (+6.2%)

I got a report that this patch is slowing decoding on the Apple M1, but I am unable to investigate this on my own.

Well, at least it will be easy to exclude it for Apple M1 by adding some #ifndef.