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Re: help interpreting gcc 4.1.1 optimisation bug


On Mon, Jun 12, 2006 at 12:30:50PM +0000, andrew@walrond.org wrote:
> On Mon, Jun 12, 2006 at 02:21:21PM +0200, Richard Guenther wrote:
> > On 6/12/06, andrew@walrond.org <andrew@walrond.org> wrote:
> > >On Mon, Jun 12, 2006 at 01:01:47PM +0100, Andrew Haley wrote:
> > >>
> > >> I'm starting to be a little suspicious about host2little().  I wonder
> > >> if that's well-defined.
> > >>
> > >> I'm just guessing here becasue I can't see the code.  Is it possible
> > >
> > >    template<typename T> T swap_endian(T x)
> > >    {
> > >        char* a = reinterpret_cast<char*>(&x);
> > >        char* b = a + sizeof(T) - 1;
> > >        while (a<b) {
> > >            char tmp = *a;
> > >            *a++ = *b;
> > >            *b-- = tmp;
> > >        }
> > >        return x;
> > >    }
> > >
> > >    template<typename T> T host2little(T x)
> > >    {
> > >        unsigned u = 1; return (*reinterpret_cast<char*>(&u) ? x : 
> > >        swap_endian(x));
> > >    }
> > 
> > You are violating aliasing rules.  Try -fno-strict-aliasing, which
> > should make the code
> > work, or fix it.
> 
> Hmmm - don't grok; I'll do some googling. Compiling now with
> -fno-strict-aliasing, but the manual says -O2 includes
> -strict-aliasing, so what gives?

Indeed, -fno-strict-aliasing makes no difference; its still broken.

> 
> Andrew


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