[FFmpeg-devel] [PATCH] avutil/timestamp: avoid using INFINITY for log10 result in av_ts_make_time_string2

[Rémi Denis-Courmont]

Le 27 août 2024 03:03:37 GMT+03:00, Marton Balint <cus at passwd.hu> a écrit :
>Using INFINITY can cause issues with -ffast-math, and since we only use this
>value to decide the formatting, we can just as easily use 0 for log10 of zero.

FFmpeg does not enable fast-math and last I tried it won't work at all anyway: FATE commits seppuku by OoM.

The statement above is completely false if applied to the whole code base rather than just this function. Notably FFMAX and FFMIN behave differently under -fno-infinite-math (which is implied by -ffast-math).

I suspect that *this* code is just the tip of the iceberg. It's not hard to imagine that ignoring infinities will break stuff, for one: reading raw binary content as float can lead to infinities and other NaNs.

So IMO this patch is wrong. Either we've audited the whole code base, and then we should enable -ffast-math, or at least -fno-infinite-math by default, or we haven't and then we should not be papering over the issue like this.

>
>Signed-off-by: Marton Balint <cus at passwd.hu>
>---
> libavutil/timestamp.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/libavutil/timestamp.c b/libavutil/timestamp.c
>index 6c231a517d..be4540d4c8 100644
>--- a/libavutil/timestamp.c
>+++ b/libavutil/timestamp.c
>@@ -24,7 +24,7 @@ char *av_ts_make_time_string2(char *buf, int64_t ts, AVRational tb)
>         snprintf(buf, AV_TS_MAX_STRING_SIZE, "NOPTS");
>     } else {
>         double val = av_q2d(tb) * ts;
>-        double log = (fpclassify(val) == FP_ZERO ? -INFINITY : floor(log10(fabs(val))));
>+        double log = (fpclassify(val) == FP_ZERO ? 0 : floor(log10(fabs(val))));
>         int precision = (isfinite(log) && log < 0) ? -log + 5 : 6;
>         int last = snprintf(buf, AV_TS_MAX_STRING_SIZE, "%.*f", precision, val);
>         last = FFMIN(last, AV_TS_MAX_STRING_SIZE - 1) - 1;