[PATCH][AArch64] Enable -frename-registers at -O2 and higher

[Evandro Menezes]

> On Fri, May 27, 2016 at 02:50:15PM +0100, Kyrill Tkachov wrote:
> >
> > As mentioned in
> > https://gcc.gnu.org/ml/gcc-patches/2016-05/msg00297.html,
> > frename-registers registers can be beneficial for aarch64 and the
> > patch at https://gcc.gnu.org/ml/gcc-patches/2016-05/msg01618.html
> > resolves the AESE/AESMC fusion issue that it exposed in the aarch64
> > backend. So this patch enables the pass for aarch64 at -O2 and above.
> >
> > Ok for trunk?
> 
> As you're proposing to have this on by default, I'd like to give a chance
to
> hear whether there is consensus as to this being the right choice for the
> thunderx, xgene1, exynos-m1 and qdf24xx subtargets.

Though there's a slight (<1%) overall improvement on Exynos M1, there just
were too many significant (<-3%) regressions for a few significant
improvements for me to be comfortable with -frename-registers being a
generic default for AArch64.

I'll run some larger benchmarks tonight, but I'm leaning towards having it
as a target specific extra tuning option.

Thank you,

-- 
Evandro Menezes                              Austin, TX

PS: I'm fine with refactoring aarch_option_optimization_table to
aarch64_option_optimization_table.