[Ffmpeg-devel] GCC optimizations

Justin Ruggles jruggle
Sun May 28 23:54:28 CEST 2006

Guillaume POIRIER wrote:
> Hi,
> On 5/28/06, Justin Ruggles <jruggle at earthlink.net> wrote:
>>Guillaume POIRIER wrote:
>>>On 5/28/06, Justin Ruggles <jruggle at earthlink.net> wrote:
>>>>While working on the FLAC encoder, I have come across a problem I've
>>>>never encountered.  I am getting completely different results using gcc
>>>>with optimization vs. without optimization.  Using -O0 gives me good
>>>>results, but using -O1 to -O3 gives bad (but still valid) results.
>>>You mean that the performance of you code decrease as you increase the
>>>optimization level? This seems odd. -O1, -O2, -Os should always be
>>>better than -O0 (which is pretty much like macro-expand).
>>Actually, it's not the performance that's the issue.  I get different
>>output from the encoder.  File sizes and contents are different.
> I imagine that it's quite possible. The C norm is very complex,
> therefore when you get into some corner cases, you can have the
> compiler do what you meant with -O0 and do what you wrote with
> -O1/2/3. So I'd say the bug _may_ be in your code.
> The best way to find out is obviously to test with other versions of
> gcc, and/or compile each file with/without optimizations and test...
> Guillaume

I did an upgrade to gcc-3.4 and the problem persisted.  Then I did some
more bug hunting and I think I found the bug...in my code.  I don't know
how it ended up working w/o optimization though.  It seems to have been
a data type overflow error.

Thanks for the help.


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