[FFmpeg-devel] [PATCH] configure: Fix ARM architecture feature detection for ARMv6 (PR #21256)

[Jun Zhao via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

PR #21256 opened by Jun Zhao (mypopydev)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21256
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21256.patch

When configuring for ARMv6 targets (e.g., Raspberry Pi 1 with
ARM1176JZF-S), the configure script incorrectly enabled ARMv6T2
and NEON features that are not supported by plain ARMv6 cores.

All ARM architecture extensions are enabled by default, and the
subarch-based case statement only enabled optimization flags
without explicitly disabling incompatible features.

This fix adds explicit feature disabling based on the detected ARM
subarchitecture:
- ARMv5TE: Disable all ARMv6+ features
- ARMv6 variants: Disable ARMv6T2, NEON, and VFPv3
- ARMv6T2: Disable NEON and VFPv3
- ARMv7/8: Let compiler tests determine feature support

Without this fix, compiled binaries would contain unsupported
instructions and crash with SIGILL on ARMv6 devices.

Fixes trac ticket #11695

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>


>From 212753d3dc2c31456019345a1718e273f6677dd5 Mon Sep 17 00:00:00 2001
From: Jun Zhao <barryjzhao@tencent.com>
Date: Sun, 21 Dec 2025 20:28:26 +0800
Subject: [PATCH] configure: Fix ARM architecture feature detection for ARMv6

When configuring for ARMv6 targets (e.g., Raspberry Pi 1 with
ARM1176JZF-S), the configure script incorrectly enabled ARMv6T2
and NEON features that are not supported by plain ARMv6 cores.

All ARM architecture extensions are enabled by default, and the
subarch-based case statement only enabled optimization flags
without explicitly disabling incompatible features.

This fix adds explicit feature disabling based on the detected ARM
subarchitecture:
- ARMv5TE: Disable all ARMv6+ features
- ARMv6 variants: Disable ARMv6T2, NEON, and VFPv3
- ARMv6T2: Disable NEON and VFPv3
- ARMv7/8: Let compiler tests determine feature support

Without this fix, compiled binaries would contain unsupported
instructions and crash with SIGILL on ARMv6 devices.

Fixes trac ticket #11695

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>
---
 configure | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/configure b/configure
index 4791a67562..0f07757f83 100755
--- a/configure
+++ b/configure
@@ -5570,10 +5570,33 @@ elif enabled arm; then
     esac
 
     case "$subarch" in
-        armv5t*)    enable fast_clz                ;;
-        armv[6-8]*)
+        armv5t*)
+            enable fast_clz
+            # ARMv5TE doesn't support ARMv6+ features
+            disable armv6 armv6t2 neon vfp vfpv3
+            ;;
+        armv6)
             enable fast_clz
             disabled fast_unaligned || enable fast_unaligned
+            # Plain ARMv6 doesn't support ARMv6T2 or NEON
+            disable armv6t2 neon vfpv3
+            ;;
+        armv6j|armv6k|armv6z|armv6zk)
+            enable fast_clz
+            disabled fast_unaligned || enable fast_unaligned
+            # These ARMv6 variants don't support ARMv6T2 or NEON
+            disable armv6t2 neon vfpv3
+            ;;
+        armv6t2*)
+            enable fast_clz
+            disabled fast_unaligned || enable fast_unaligned
+            # ARMv6T2 doesn't support NEON
+            disable neon vfpv3
+            ;;
+        armv[7-8]*)
+            enable fast_clz
+            disabled fast_unaligned || enable fast_unaligned
+            # ARMv7/ARMv8 features are checked later via compiler tests
             ;;
     esac
 
-- 
2.49.1

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