[FFmpeg-devel] Re: [PATCH] Miscellaneous aacdec_usac cleanups (PR #20627)

[Kieran Kunhya via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

On Mon, 29 Sept 2025, 16:04 Lynne via ffmpeg-devel, <ffmpeg-devel@ffmpeg.org>
wrote:

> PR #20627 opened by Lynne
> URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20627
> Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20627.patch
>
> Nothing major.
>
>
> >From 485939f5ba2051d3091bb130f0879d3b1ed83e6a Mon Sep 17 00:00:00 2001
> From: Lynne <dev@lynne.ee>
> Date: Tue, 30 Sep 2025 00:02:19 +0900
> Subject: [PATCH 1/2] aacdec_usac: memset uncoded coeffs instead of
> iterating
>
> Microoptimization.
> ---
>  libavcodec/aac/aacdec_usac.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/libavcodec/aac/aacdec_usac.c b/libavcodec/aac/aacdec_usac.c
> index c4b821bbba..38d5328029 100644
> --- a/libavcodec/aac/aacdec_usac.c
> +++ b/libavcodec/aac/aacdec_usac.c
> @@ -670,10 +670,8 @@ static int decode_spectrum_ac(AACDecContext *s, float
> coef[1024],
>
>      ff_aac_ac_finish(state, i, N);
>
> -    for (; i < N/2; i++) {
> -        coef[2*i + 0] = 0;
> -        coef[2*i + 1] = 0;
> -    }
> +    /* Zero out uncoded coeffs */
> +    memset(coef + 2*i, 0, N*sizeof(*coef)));
>

This looks wrong.

It is only correct if i is guaranteed to be zero before the for loop. The
number of elements to zero is not N*sizeof(...) otherwise.

Kieran

>
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