Re: [FFmpeg-devel] [PATCH] lavc: avoid rounding errors in float constants

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

remi@remlab.net:
> From: Rémi Denis-Courmont <remi@remlab.net>
> 
> INT_MAX is (typically) a value with 31 significant bits but float can
> only represent 23 significant bits, leading to a rounding error.
> 
> This substitutes the actual rounded value to avoid a clang warning:
> 
>  warning: implicit conversion from 'int' to 'float' changes value from
>   2147483647 to 2147483648 [-Wimplicit-const-int-float-conversion]
> ---
>  libavcodec/aaccoder.c | 2 +-
>  libavcodec/imc.c      | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/aaccoder.c b/libavcodec/aaccoder.c
> index e3b6b2f02c..877558c91c 100644
> --- a/libavcodec/aaccoder.c
> +++ b/libavcodec/aaccoder.c
> @@ -531,7 +531,7 @@ static void search_for_quantizers_anmr(AVCodecContext *avctx, AACEncContext *s,
>              int nz = 0;
>  
>              bandaddr[idx] = w * 16 + g;
> -            qmin = INT_MAX;
> +            qmin = -INT_MIN;
>              qmax = 0.0f;
>              for (w2 = 0; w2 < sce->ics.group_len[w]; w2++) {
>                  FFPsyBand *band = &s->psy.ch[s->cur_channel].psy_bands[(w+w2)*16+g];
> diff --git a/libavcodec/imc.c b/libavcodec/imc.c
> index 92f9980ded..d4dfe3222c 100644
> --- a/libavcodec/imc.c
> +++ b/libavcodec/imc.c
> @@ -917,7 +917,7 @@ static int imc_decode_block(AVCodecContext *avctx, IMCContext *q, int ch)
>                                         chctx->flcoeffs1, chctx->flcoeffs2);
>  
>      for(i=0; i<BANDS; i++) {
> -        if(chctx->flcoeffs1[i] > INT_MAX) {
> +        if(chctx->flcoeffs1[i] > -INT_MIN) {
>              av_log(avctx, AV_LOG_ERROR, "scalefactor out of range\n");
>              return AVERROR_INVALIDDATA;
>          }

-INT_MIN can't be represented in an int and therefore -INT_MIN on the
right is UB by C11 6.5 (5).

- Andreas
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