[FFmpeg-devel] FATE & Regressions (and PPC is broken)

[Måns Rullgård]

Mike Melanson <mike at multimedia.cx> writes:

> Hi,
>
> The following regression test is broken on PowerPC (gcc 4.1.2):
>
> diff -u -w
> "/home/melanson/ffmpeg/ffmpeg-main"/tests/ffmpeg.regression.ref
> tests/data/vsynth.regression
> --- /home/melanson/ffmpeg/ffmpeg-main/tests/ffmpeg.regression.ref
> 2008-03-08 19:10:07.000000000 -0800
> +++ tests/data/vsynth.regression        2008-03-12 14:27:53.000000000 -0700
> @@ -197,7 +197,7 @@
>  353368 ./tests/data/a-flac.flac
>  c4228df189aad9567a037727d0e763e4 *./tests/data/flac.vsynth.out.wav
>  stddev: 33.31 PSNR:65.87 bytes:1040384
> -a1e56bb034c2773438ba1c830c4cea07 *./tests/data/a-wmav1.asf
> +59575b4756d674cd8c8d53d86c08e8cb *./tests/data/a-wmav1.asf
>  106004 ./tests/data/a-wmav1.asf
>  stddev:12251.50 PSNR:14.56 bytes:1056768
>  stddev:2106.00 PSNR:29.85 bytes:1048576
> make: *** [codectest] Error 1

OK, I tracked this one down.  It turns out lrint() rounds slightly
differently for whatever reason (bug or otherwise).  Attached patch
changes it to lround() (which has well-defined rounding behaviour).
With this change, the results are the same on x86_64 and ppc64.

This raises the broader question of lrint() usage in FFmpeg.  The
documentation for the lrint() family says:

  These functions shall round their argument to the nearest integer
  value, rounding according to the current rounding direction.

Clearly, in a library we do not know the current rounding direction,
and it would be rude of us to change it.  The lround() family, on the
other hand, always rounds the same way:

  These functions shall round their argument to the nearest integer
  value, rounding halfway cases away from zero, regardless of the
  current rounding direction.

-- 
M?ns Rullg?rd
mans at mansr.com
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