Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."

[Daniel Berlin]

On 12/29/06, Daniel Berlin <address@hidden> wrote:
> On 12/29/06, Richard Kenner <address@hidden> wrote:
> > > I'm not sure what data you're asking for.
> >
> > Here's the data *I'd* like to see:
> >
> > (1) What is the maximum performance loss that can be shown using a real
> > program (e.g,. one in SPEC) and some compiler (not necessarily GCC) when
> > one assumes wrapping semantics?
>
> The XLC numbers i was given about a year ago (i assume it was version 8)
>
> SpecINT with undefined signed overflow at -O5 on a P5 2100mhz running
> linux: 1634
> SpecFP with undefined signed overflow at -O5 on a P5 2100mhz running linux: 3010
>
> SpecINT with wrapping signed overflow at -O5 on a P5 2100mhz running
> linux: 1319
> SpecFP with wrapping signed overflow at -O5 on a P5 2100mhz running linux: 1624
>
> >
> > (2) In the current SPEC, how many programs benefit from undefined overflow
> > semantics and how much does each benefit?
>
> All of the fortran programs (IE SpecFP) benefit from undefined
> *unsigned* overflow semantics due to 32 bit iv vs 64 bit array index
> issues.
> The same is true of the SpecFP C programs.

^^^^
Just to be clear, the above behavior is not standards conformant, and
they do give a warning that they are doing it.
It is however, the default at -O3 for XLC, and AFAIK, at all opt levels for icc.