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

[Daniel Berlin]

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.

All of the fortran and C programs benefit from undefined *signed*
overflow semantics because it makes dependency and loop counting
analysis possible.

Nobody has analyzed it further than that, afaik, mainly because they
don't have discussions about whether it makes sense to lose 50% of
their FP performance to do something none of *their* users ask them
for (note our users may, of course, be different).  So they generally
won't waste their cycles trying to figure out why something they
aren't going to do would hurt them.