GCC bug

Leo Cacciari leo.cacciari at gmail.com
Wed Nov 21 15:53:14 UTC 2007


Il giorno mar, 20/11/2007 alle 12.31 +0000, Steve Flynn ha scritto:
> On Nov 19, 2007 6:06 PM, Unai Uribarri <unaiur at gmail.com> wrote:
> > I've filled a bug report in the lauchpad six weeks ago,
> > https://bugs.launchpad.net/ubuntu/+source/gcc-4.1/+bug/158799.
> >
> > GCC is generating bad code when compiled with -O2. I believe that it's an
> > important issue, but so far the bug report has attracted no attention.
> 
> Actually, upon lookling at your code on launchpad, I suspect you are
> falling victim to improper aliasing. You should refer to the ISO
> standard for more information on the rules for aliasing and compiler
> optimization.
> 
> If the code works as expected without Optimzation, I'd suspect your
> code before I'd suspect GCC...
> 
Infact, if you run the code in the bug report with -O2 _and_ -Wall
--pedantic, you'll see a warning that

dereferencing type-punned pointer will break strict-aliasing

This is why, with --no-strict-aliasing you get "right" executable. As
Steve said, suspect your code before suspecting gcc ;)

-- 
Leo "TheHobbit" Cacciari

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