PR #21188 opened by Rémi Denis-Courmont (Courmisch) URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21188 Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21188.patch
This reduces mid_pred() (i.e. median of 3) down to the minimum: - 3 comparisons and 4 conditional moves, or - 4 min/max, whilst eliminating all branches. The same algorithm is already implemented via inline assembler for some architectures, but notably not Arm64 and RVA22. Besides, using C code allows the compiler to schedule instruction properly. See also https://godbolt.org/z/YKbabMG74 for comparisons. From aa6c8d167f43c17fd7a324125f7bf668aec6d08a Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?R=C3=A9mi=20Denis-Courmont?= <[email protected]> Date: Sun, 14 Dec 2025 11:06:13 +0200 Subject: [PATCH] lavc/mathops: simplify mid_pred() This reduces mid_pred() (i.e. median of 3) down to the minimum: - 3 comparisons and 4 conditional moves, or - 4 min/max, whilst eliminating all branches. The same algorithm is already implemented via inline assembler for some architectures, but notably not Arm64 and RVA22. Besides, using C code allows the compiler to schedule instruction properly. --- libavcodec/mathops.h | 29 +++++++++++++++-------------- 1 file changed, 15 insertions(+), 14 deletions(-) diff --git a/libavcodec/mathops.h b/libavcodec/mathops.h index aa0bdfe956..4411d138b4 100644 --- a/libavcodec/mathops.h +++ b/libavcodec/mathops.h @@ -93,23 +93,24 @@ static av_always_inline unsigned UMULH(unsigned a, unsigned b){ #endif /* median of 3 */ -#ifndef mid_pred -#define mid_pred mid_pred -static inline av_const int mid_pred(int a, int b, int c) +static inline av_const int median3_c(int a, int b, int c) { - if(a>b){ - if(c>b){ - if(c>a) b=a; - else b=c; - } - }else{ - if(b>c){ - if(c>a) b=c; - else b=a; - } + int max2, min2, m; + + if (a >= b) { + max2 = a; + min2 = b; + } else { + max2 = b; + min2 = a; } - return b; + m = (c >= max2) ? max2 : c; + + return (m >= min2) ? m : min2; } + +#ifndef mid_pred +#define mid_pred median3_c #endif #ifndef median4 -- 2.49.1 _______________________________________________ ffmpeg-devel mailing list -- [email protected] To unsubscribe send an email to [email protected]
