On Wed, 21 May 2025, Andreas Rheinhardt wrote:
Jiawei:
This patch modifies the FFmpeg build system to remove the explicit disabling
of GCC's auto-vectorization feature.
Modern GCC versions (>= 10.0) have demonstrated stable auto-vectorization
capabilities through extensive optimizations in loop analysis and SIMD
code generation. The explicit -fno-tree-vectorize flag originally added
in commit 973859f (2009) to workaround early GCC vectorization instability
is no longer necessary.
Key improvements justifying this change:
1. Enhanced heuristics for loop vectorization cost models
2. Mature handling of alignment and memory access patterns
3. Robust fallback mechanisms for unsupported architectures
This change allows FFmpeg to benefit from automated SIMD optimizations
when built with -O3 optimization level, particularly improving
performance on x86_64 (AVX), ARM64 (SVE) and RISC-V(RVV) architectures.
[1]
https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/973859f5230e77beea7bb59dc081870689d6d191
---
configure | 1 -
1 file changed, 1 deletion(-)
diff --git a/configure b/configure
index 3730b0524c..b9e95ce4ec 100755
--- a/configure
+++ b/configure
@@ -7656,7 +7656,6 @@ if enabled icc; then
disable aligned_stack
fi
elif enabled gcc; then
- check_optflags -fno-tree-vectorize
check_cflags -Werror=format-security
check_cflags -Werror=implicit-function-declaration
check_cflags -Werror=missing-prototypes
FYI: The last discussion about auto-vectorization is here:
https://ffmpeg.org/pipermail/ffmpeg-devel/2022-July/299405.html
It contains a report about a failing build with vectorization enabled:
https://ffmpeg.org/pipermail/ffmpeg-devel/2022-July/299421.html
I don't know whether this is still reproducible with the latest GCC.
The issue which was reported last time, when compiling for i686 mingw32
with --cpu=haswell, seems to have gone away in
182663a58a7a099e02e76da3b0f96d63e5c26a6d, where we made the whole
problematic x86 inline cabac assembly noinline on i386. (That whole inline
assembly block has been problematic in a large number of cases anyway.)
// Martin
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