On Wednesday, 12 de January de 2011 16:01:31 Carsten Munk wrote: > 2011/1/12 Arjan van de Ven <ar...@linux.intel.com>: > > On 1/12/2011 1:06 AM, Carsten Munk wrote: > >> Hi (ARM toolchain group mostly) > >> > >> Do we have a patch for glibc-2.11-12-g24c0bf7 and/or glibc-2.12.1 that > >> enables ARM RunFast[1] mode by default anywhere? Would be good to push > >> it along with hardfp while we're at it and getting things tested > >> through. > > > > can this be turned into something that's passed in via CFLAGS ? > > that way apps will not be surprised, and there is an easy way for us to > > toggle
Right now, it's a context configuration, so there's nothing that will really
work from CFLAGS. Without changing gcc, the only thing we could do is supply
different crt1.o, one that puts the FPU in RunFast, the other doesn't.
But this will, like I said, apply to all code within a process, so it doesn't
help the library case. Libraries will need to cope with running in both modes.
> > of course we can have a default in our OBS that you pick, but it becomes
> > an easy-to-manage (from a distro perspective)
> > property
>
> That would be a better way than patching glibc, I would believe?
Not necessarily. To do the right thing, the compiler would need to emit the
code that changes FPSCR before any FP operation, so this means an increase in
code size. I can also bet that there's a pipeline delay in modifying this
register.
And there's no such GCC patch.
> Wouldn't -ffast-math correspond to this on x86 side at least?
>
> Leonid, does this correspond to an auto-setup of RunFast on ARM, when
> used there?
No, it's different.
By the way, I should point out that on Cortex-A8, RunFast only has a
perceptible improvement for float. If you use double, you still have
performance issues.
On Cortex-A9, both are fast.
--
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
Senior Product Manager - Nokia, Qt Development Frameworks
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