Re: [FFmpeg-devel] [PATCH] enable auto vectorization for gcc 7 and higher

[Soft Works <softworkz@hotmail.com>]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
> Swinney, Jonathan
> Sent: Wednesday, July 27, 2022 7:35 PM
> To: ffmpeg-devel@ffmpeg.org
> Subject: [FFmpeg-devel] [PATCH] enable auto vectorization for gcc 7
> and higher
> 
> I recognize that this patch is going to be somewhat controversial.
> I'm submitting it mostly to see what the opinions are and evaluate
> options. I am working on improving performance for aarch64. On that
> architecture, there are fewer hand written assembly implementations
> of hot functions than there are for x86_64 and allowing gcc to auto-
> vectorize yields noticeable improvements.
> 
> Gcc vectorization has improved recently and it hasn't been evaluated
> on the mailing list for a few years. This is the latest discussion I
> found in my searches: http://ffmpeg.org/pipermail/ffmpeg-devel/2016-
> May/193977.html
> 
> If the community is not comfortable accepting a patch like this
> outright, would you be willing to accept a new option to the
> configure script, something like --enable-auto-vectorization?
> 
> Thanks!
> 
> Signed-off-by: Jonathan Swinney <jswinney@amazon.com>
> ---
>  configure | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/configure b/configure
> index 6629d14099..c63c9348ad 100755
> --- a/configure
> +++ b/configure
> @@ -7173,7 +7173,9 @@ if enabled icc; then
>              disable aligned_stack
>      fi
>  elif enabled gcc; then
> -    check_optflags -fno-tree-vectorize
> +    case $gcc_basever in
> +        2|2.*|3.*|4.*|5.*|6.*) check_optflags -fno-tree-vectorize ;;
> +    esac
>      check_cflags -Werror=format-security
>      check_cflags -Werror=implicit-function-declaration
>      check_cflags -Werror=missing-prototypes
> --

LGTM - basically. I had removed that flag about two years ago and never
seen an issue (Win,Linux,BSD x x86_64,armv7,aarch64). 
But it has always been with quite recent versions of gcc, so I can't say 
whether it's safe already with 7.x

One exception I've seen was with an Android NDK build in gcc compatibility
mode, where I got a clang compilation error. But that's nothing of concern
I think.

sw




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