[FFmpeg-devel] [PATCH] riscv/cpu: add V subset feature detection (PR #20873)

["Rémi Denis-Courmont via ffmpeg-devel" <ffmpeg-devel@ffmpeg.org>]

PR #20873 opened by Rémi Denis-Courmont (Courmisch)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20873
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20873.patch

This adds support for detecting Zve32x, Zve32f, Zve64x and Zve64d on
a system that does not support (or declare) the full V feature set.

Because the code assumes that vector detected at runtime are at least
128-bit long, we have to manually check that too (in theory, vectors
can be 32-bit or 64-bit, though that makes little sense in 2025).


From 3ea7c5449d2e35c01debdac8fb981675eb861644 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?R=C3=A9mi=20Denis-Courmont?= <remi@remlab.net>
Date: Sun, 9 Nov 2025 15:01:03 +0200
Subject: [PATCH] riscv/cpu: add V subset feature detection

This adds support for detecting Zve32x, Zve32f, Zve64x and Zve64d on
a system that does not support (or declare) the full V feature set.

Because the code assumes that vector detected at runtime are at least
128-bit long, we have to manually check that too (in theory, vectors
can be 32-bit or 64-bit, though that makes little sense in 2025).
---
 libavutil/riscv/cpu.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/libavutil/riscv/cpu.c b/libavutil/riscv/cpu.c
index 379703651a..8cc91625c7 100644
--- a/libavutil/riscv/cpu.c
+++ b/libavutil/riscv/cpu.c
@@ -22,6 +22,7 @@
 # define _GNU_SOURCE
 #endif
 #include "libavutil/cpu.h"
+#include "libavutil/riscv/cpu.h"
 #include "libavutil/cpu_internal.h"
 #include "libavutil/macros.h"
 #include "libavutil/log.h"
@@ -64,6 +65,21 @@ int ff_get_cpu_flags_riscv(void)
         if (pairs[1].value & RISCV_HWPROBE_IMA_V)
             ret |= AV_CPU_FLAG_RVV_I32 | AV_CPU_FLAG_RVV_I64
                  | AV_CPU_FLAG_RVV_F32 | AV_CPU_FLAG_RVV_F64;
+#ifdef RISCV_HWPROBE_EXT_ZVE32X
+        else if ((pairs[1].value & RISCV_HWPROBE_EXT_ZVE32X) &&
+                 ff_get_rv_vlenb() >= 16) { // runtime detect assumes 128+ bits
+            ret |= AV_CPU_FLAG_RVV_I32;
+
+            if (pairs[1].value & RISCV_HWPROBE_EXT_ZVE32F)
+                ret |= AV_CPU_FLAG_RVV_F32;
+            if (pairs[1].value & RISCV_HWPROBE_EXT_ZVE64X) {
+                ret |= AV_CPU_FLAG_RVV_I64;
+
+                if (pairs[1].value & RISCV_HWPROBE_EXT_ZVE64D)
+                    ret |= AV_CPU_FLAG_RVV_F64;
+            }
+        }
+#endif
 #endif
 #ifdef RISCV_HWPROBE_EXT_ZBB
         if (pairs[1].value & RISCV_HWPROBE_EXT_ZBB)
-- 
2.49.1

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