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

[Marton Balint]

On Tue, 27 Aug 2024, Rémi Denis-Courmont wrote:

>
>
> 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 meant the statement to the code in question, not generically.

>
> 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.

Agreed.

>
> 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.
>

IMHO the patch itself is harmless (simple, easy to follow), and if it 
helps somebody (no matter how unupported its use-case is), then why the 
hell not. But I can understand your point of view, that we should not 
bother with it, if it is unsupported in the first place, so I will just 
drop this, did not feel strongly about it anyway.

Regards,
Marton


>>
>> 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;
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