Re: strange behaviour of -O2 optimization
On Tue, Jan 06, 2004 at 12:02:29AM +0100, Nicolas François wrote:
> Hello,
>
> I had a look at the debian bug #225313. This bug only raise raise when
> optimization is used. I've submitted a patch, but would like to
> understand if gcc behaviour was normal.
>
>
> I've selected a sample of code that should raise the same issue:
>
> =======================================================================
> typedef unsigned long word;
> typedef unsigned long long dword;
>
> #define LOW_WORD(x) (word)(x)
> #define HIGH_WORD(x) (*(((word *)&(x))+1))
You can't do this. I recommend you google for "strict aliasing" for a
number of good discussions of the problem.
> gcc warn about:
> test.c: In function `test':
> test.c:14: warning: dereferencing type-punned pointer will break
> strict-aliasing rules
> Is it related?
Yes.
--
Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer
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