Re: [FFmpeg-devel] gcc: Remove auto-vectorization limitation.

["Rémi Denis-Courmont" <remi@remlab.net>]

Hi,

Le 21 mai 2025 09:17:50 GMT+03:00, Jiawei <jiawei@iscas.ac.cn> a écrit :
>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.

I don't mind the patch but this description is very misleading. Realistically, this will only enable SSE2 or so on x86-64 and nothing on RISC-V, because we can't simply assume that AVX and RVV are supported.

Call me when Debian, Ubuntu and Fedora require RVA23 for their official RISC-V ports... (I won't be holding my breath.)

Bluntly, I am concerned that this gives the wrong impression that AVX and RVV optimisations are no longer necessary, cut funding off. Yet GCC (and LLVM) remain incapable of selecting optimised loops depending on runtime CPU capabilities.
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