PR #21207 opened by Rémi Denis-Courmont (Courmisch) URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21207 Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21207.patch
From 87750fc41d88c6fd80f7cfe7485e9cc640e76367 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 1/2] 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 such as x86 and Arm, but notably not Arm64 and RVA22. Besides, using C code allows the compiler to schedule instruction properly. Even on architectures with neither conditional moves nor min/max, this leads to a visible performance improvement for C code, as seen here for RVA20 code running on SiFive-U74: Before: sub_median_pred_c: 1657.5 ( 1.00x) sub_median_pred_rvb_b: 875.9 ( 1.89x) After: sub_median_pred_c: 1331.9 ( 1.00x) sub_median_pred_rvb_b: 881.8 ( 1.51x) --- 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 From 2cf5e492568bd06ac829f0f743477f42c665fcc4 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?R=C3=A9mi=20Denis-Courmont?= <[email protected]> Date: Sun, 14 Dec 2025 14:45:16 +0200 Subject: [PATCH 2/2] lavc/mathops: R-V B optimisation for mid_pred If Zbb is enabled at compilation (e.g. Ubuntu), the compiler should compile the new C mid_pred() function correctly. But if Zbb is *not* enabled (e.g. Debian), then we can at least fallback at run-time. On SiFive-U74, before: sub_median_pred_c: 1331.9 ( 1.00x) sub_median_pred_rvb_b: 881.8 ( 1.51x) After: sub_median_pred_c: 1133.1 ( 1.00x) sub_median_pred_rvb_b: 875.7 ( 1.29x) --- libavcodec/mathops.h | 2 ++ libavcodec/riscv/mathops.h | 54 ++++++++++++++++++++++++++++++++++++++ 2 files changed, 56 insertions(+) create mode 100644 libavcodec/riscv/mathops.h diff --git a/libavcodec/mathops.h b/libavcodec/mathops.h index 4411d138b4..64431b8a15 100644 --- a/libavcodec/mathops.h +++ b/libavcodec/mathops.h @@ -44,6 +44,8 @@ extern const uint8_t ff_zigzag_scan[16+1]; # include "mips/mathops.h" #elif ARCH_PPC # include "ppc/mathops.h" +#elif ARCH_RISCV +# include "riscv/mathops.h" #elif ARCH_X86 # include "x86/mathops.h" #endif diff --git a/libavcodec/riscv/mathops.h b/libavcodec/riscv/mathops.h new file mode 100644 index 0000000000..c2258f49d7 --- /dev/null +++ b/libavcodec/riscv/mathops.h @@ -0,0 +1,54 @@ +/* + * Copyright © 2025 Rémi Denis-Courmont. + * + * This file is part of FFmpeg. + * + * FFmpeg is free software; you can redistribute it and/or + * modify it under the terms of the GNU Lesser General Public + * License as published by the Free Software Foundation; either + * version 2.1 of the License, or (at your option) any later version. + * + * FFmpeg is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + * Lesser General Public License for more details. + * + * You should have received a copy of the GNU Lesser General Public + * License along with FFmpeg; if not, write to the Free Software + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA + */ + +#ifndef AVCODEC_RISCV_MATHOPS_H +#define AVCODEC_RISCV_MATHOPS_H + +#include "config.h" +#include <stdbool.h> +#include "libavutil/attributes_internal.h" +#include "libavutil/riscv/cpu.h" + +#if HAVE_RV && !defined(__riscv_zbb) +static inline int median3_c(int a, int b, int c); + +static inline av_const int median3_rv(int a, int b, int c) +{ + if (__builtin_expect(ff_rv_zbb_support(), true)) { + int min2, max2; + + __asm__ ( + ".option push\n" + ".option arch, +zbb\n" + "max %1, %2, %3\n" + "min %0, %2, %3\n" + "min %1, %4, %1\n" + "max %0, %0, %1\n" + ".option pop\n" + : "=&r" (min2), "=&r" (max2) : "r" (a), "r" (b), "r" (c)); + + return min2; + } + return median3_c(a, b, c); +} +#define mid_pred median3_rv +#endif + +#endif /* HAVE_RVV */ -- 2.49.1 _______________________________________________ ffmpeg-devel mailing list -- [email protected] To unsubscribe send an email to [email protected]
