* [FFmpeg-devel] [PR] avcodec/notchlc: lz4_decompress() avoid byte loops, try to work in blocks (PR #22287)
@ 2026-02-26 1:38 michaelni via ffmpeg-devel
0 siblings, 0 replies; only message in thread
From: michaelni via ffmpeg-devel @ 2026-02-26 1:38 UTC (permalink / raw)
To: ffmpeg-devel; +Cc: michaelni
PR #22287 opened by michaelni
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/22287
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/22287.patch
47720380 decicycles -> 34335598 decicycles
>From b14a62a84da87062bf9a1c63e06773b819cbf7b4 Mon Sep 17 00:00:00 2001
From: Michael Niedermayer <michael@niedermayer.cc>
Date: Wed, 25 Feb 2026 16:57:01 +0100
Subject: [PATCH 1/2] avcodec/notchlc: Avoid clearing history in
lz4_decompress()
instead of clearing history before "byte 0", we error out
clearing 64kb takes time, which this commit avoids
Benchmark with big_buck_bunny\ NotchLC\ Optimal\ Encoding\ 1920.mov shows no meassurable difference though
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
---
libavcodec/notchlc.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
diff --git a/libavcodec/notchlc.c b/libavcodec/notchlc.c
index c28fddcea0..278499f450 100644
--- a/libavcodec/notchlc.c
+++ b/libavcodec/notchlc.c
@@ -79,8 +79,9 @@ static int lz4_decompress(AVCodecContext *avctx,
PutByteContext *pb)
{
unsigned reference_pos, delta, pos = 0;
- uint8_t history[HISTORY_SIZE] = { 0 };
+ uint8_t history[HISTORY_SIZE];
int match_length;
+ int wraped = 0;
while (bytestream2_get_bytes_left(gb) > 0) {
uint8_t token = bytestream2_get_byte(gb);
@@ -106,6 +107,7 @@ static int lz4_decompress(AVCodecContext *avctx,
if (pos == HISTORY_SIZE) {
bytestream2_put_buffer(pb, history, HISTORY_SIZE);
pos = 0;
+ wraped = 1;
}
}
}
@@ -125,7 +127,13 @@ static int lz4_decompress(AVCodecContext *avctx,
match_length += current;
} while (current == 255);
}
- reference_pos = (pos >= delta) ? (pos - delta) : (HISTORY_SIZE + pos - delta);
+ reference_pos = pos - delta;
+ if (pos < delta) {
+ if (!wraped)
+ return AVERROR_INVALIDDATA;
+ reference_pos += HISTORY_SIZE;
+ }
+
if (pos + match_length < HISTORY_SIZE && reference_pos + match_length < HISTORY_SIZE) {
if (pos >= reference_pos + match_length || reference_pos >= pos + match_length) {
memcpy(history + pos, history + reference_pos, match_length);
@@ -140,6 +148,7 @@ static int lz4_decompress(AVCodecContext *avctx,
if (pos == HISTORY_SIZE) {
bytestream2_put_buffer(pb, history, HISTORY_SIZE);
pos = 0;
+ wraped = 1;
}
reference_pos %= HISTORY_SIZE;
}
--
2.52.0
>From b889dbc8dc1451b75e4db67c3c3bd7d4eef7e067 Mon Sep 17 00:00:00 2001
From: Michael Niedermayer <michael@niedermayer.cc>
Date: Wed, 25 Feb 2026 17:47:11 +0100
Subject: [PATCH 2/2] avcodec/notchlc: lz4_decompress() avoid byte loops, try
to work in blocks
47720380 decicycles -> 34335598 decicycles
---
libavcodec/notchlc.c | 44 ++++++++++++++++++++++----------------------
1 file changed, 22 insertions(+), 22 deletions(-)
diff --git a/libavcodec/notchlc.c b/libavcodec/notchlc.c
index 278499f450..079e821d6c 100644
--- a/libavcodec/notchlc.c
+++ b/libavcodec/notchlc.c
@@ -98,18 +98,16 @@ static int lz4_decompress(AVCodecContext *avctx,
if (bytestream2_get_bytes_left(gb) < num_literals)
return AVERROR_INVALIDDATA;
- if (pos + num_literals < HISTORY_SIZE) {
- bytestream2_get_buffer(gb, history + pos, num_literals);
- pos += num_literals;
- } else {
- while (num_literals-- > 0) {
- history[pos++] = bytestream2_get_byte(gb);
- if (pos == HISTORY_SIZE) {
- bytestream2_put_buffer(pb, history, HISTORY_SIZE);
- pos = 0;
- wraped = 1;
- }
+ while (num_literals) {
+ int max_literals = FFMIN(num_literals, HISTORY_SIZE - pos);
+ bytestream2_get_buffer(gb, history + pos, max_literals);
+ pos += max_literals;
+ if (pos == HISTORY_SIZE) {
+ bytestream2_put_buffer(pb, history, HISTORY_SIZE);
+ pos = 0;
+ wraped = 1;
}
+ num_literals -= max_literals;
}
if (bytestream2_get_bytes_left(gb) <= 0)
@@ -134,24 +132,26 @@ static int lz4_decompress(AVCodecContext *avctx,
reference_pos += HISTORY_SIZE;
}
- if (pos + match_length < HISTORY_SIZE && reference_pos + match_length < HISTORY_SIZE) {
- if (pos >= reference_pos + match_length || reference_pos >= pos + match_length) {
- memcpy(history + pos, history + reference_pos, match_length);
- pos += match_length;
+ while (match_length) {
+ int max_match;
+ if (reference_pos > pos) {
+ max_match = FFMIN(match_length, HISTORY_SIZE - reference_pos);
+ memmove(history + pos, history + reference_pos, max_match);
+ pos += max_match;
+ reference_pos += max_match;
+ if (reference_pos == HISTORY_SIZE) reference_pos = 0;
} else {
- while (match_length-- > 0)
- history[pos++] = history[reference_pos++];
- }
- } else {
- while (match_length-- > 0) {
- history[pos++] = history[reference_pos++];
+ max_match = FFMIN(match_length, HISTORY_SIZE - pos);
+ av_memcpy_backptr(history + pos, pos - reference_pos, max_match);
+ pos += max_match;
+ reference_pos += max_match;
if (pos == HISTORY_SIZE) {
bytestream2_put_buffer(pb, history, HISTORY_SIZE);
pos = 0;
wraped = 1;
}
- reference_pos %= HISTORY_SIZE;
}
+ match_length -= max_match;
}
}
--
2.52.0
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2026-02-26 1:38 [FFmpeg-devel] [PR] avcodec/notchlc: lz4_decompress() avoid byte loops, try to work in blocks (PR #22287) michaelni via ffmpeg-devel
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