Re: [FFmpeg-devel] [PATCH] tx_float_neon: Do not access outside stack. - Rémi Denis-Courmont

From: "Rémi Denis-Courmont" <remi@remlab.net>
To: ffmpeg-devel@ffmpeg.org
Subject: Re: [FFmpeg-devel] [PATCH] tx_float_neon: Do not access outside stack.
Date: Sun, 09 Oct 2022 17:11:04 +0300
Message-ID: <2648779.mvXUDI8C0e@basile.remlab.net> (raw)
In-Reply-To: <20221009131447.5001-1-Reimar.Doeffinger@gmx.de>

Le sunnuntaina 9. lokakuuta 2022, 16.14.47 EEST Reimar Döffinger a écrit :
> Use load/store instructions that modify sp to save
> registers to stack, like it is done for all other
> functions.
> At least valgrind complains about the current code.
> ---
>  libavutil/aarch64/tx_float_neon.S | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/libavutil/aarch64/tx_float_neon.S
> b/libavutil/aarch64/tx_float_neon.S index 4126c3b812..4be93cc963 100644
> --- a/libavutil/aarch64/tx_float_neon.S
> +++ b/libavutil/aarch64/tx_float_neon.S
> @@ -866,10 +866,10 @@ FFT16_FN ns_float, 1
> 
>  .macro FFT32_FN name, no_perm
>  function ff_tx_fft32_\name\()_neon, export=1
> -        stp             d8,  d9,  [sp, #-16]
> -        stp             d10, d11, [sp, #-32]
> -        stp             d12, d13, [sp, #-48]
> -        stp             d14, d15, [sp, #-64]
> +        stp             d8,  d9,  [sp, #-16]!
> +        stp             d10, d11, [sp, #-16]!
> +        stp             d12, d13, [sp, #-16]!
> +        stp             d14, d15, [sp, #-16]!

While this fixes the ABI violation, it introduces multiple data dependencies on 
stack pointer due to write-back.

The idiomatic way to do this is to allocate the entire needed stack space in 
the first store / last load, and use positive offsets elsewhence.

> 
>          LOAD_SUBADD
>          SETUP_SR_RECOMB 32, x7, x8, x9
> @@ -911,10 +911,10 @@ function ff_tx_fft32_\name\()_neon, export=1
>          zip2            v31.2d, v11.2d, v15.2d
>          st1             { v28.4s, v29.4s, v30.4s, v31.4s }, [x1]
> 
> -        ldp             d14, d15, [sp, #-64]
> -        ldp             d12, d13, [sp, #-48]
> -        ldp             d10, d11, [sp, #-32]
> -        ldp             d8,  d9,  [sp, #-16]
> +        ldp             d14, d15, [sp], #16
> +        ldp             d12, d13, [sp], #16
> +        ldp             d10, d11, [sp], #16
> +        ldp             d8,  d9,  [sp], #16
> 
>          ret
>  endfunc

-- 
Rémi Denis-Courmont
http://www.remlab.net/

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