[Ffmpeg-devel] GCC optimizations

Guillaume POIRIER poirierg
Sun May 28 23:06:05 CEST 2006


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).

> I get the same good results using tcc.

Ahem... tcc is just a fast compiler that holds in a very few number of
lines. It should not be considered more seriously than that...

> Could I have a bug somewhere in my
> code that would cause this or is it more likely a bug in gcc?

Did you identify the file/function in your code that isn't compiled
into fast code?

> My gcc
> version is 3.3.4.

In my own experience, gcc-3.3 isn't too good of a gcc milestone. Gcc
3.4, gcc-4.0 or 4.1 are much, much better.

"Success consists of going from failure to failure without loss of enthusiasm."
 -- Winston Churchill

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