Re: Loop peeling

[Tejas Belagod <tejas dot belagod at arm dot com>]

On 29/10/14 09:32, Richard Biener wrote:
> On Tue, Oct 28, 2014 at 4:55 PM, Evandro Menezes <e.menezes@samsung.com> wrote:
> > While doing some benchmark flag mining on AArch64, I noticed that
> > -fpeel-loops was a mined option often.  As a matter of fact, when using it
> > always, even without FDO, it seemed to raise most benchmarks and to leave
> > almost all of the rest flat, with a barely noticeable cost in code-size.  It
> > seems to me that it might be safe enough to be implied perhaps at -O3.  Is
> > there any reason why this never came into being?

Loop peeling is done by default on AArch64 unless, IIRC, 
-fvect-cost-model=cheap is specified which switches it off. There was a 
general thread on loop peeling around the same time last year 
(https://gcc.gnu.org/ml/gcc/2013-11/msg00307.html) where Richard 
suggested that peeling vs. non-peeling should be factored into the 
vector cost model and is a more generic improvement.

Thanks,
Tejas.

> Not sure, but peeling is/was very stupid (peeling 8 times unconditionally
> or not at all).  At least without FDO (and with -fprofile-use it is enabled).
> Similar case for -funroll-loops.
>
> For GCC 5 peeling now moved to GIMPLE, so maybe things changed
> for that (but I'd doubt that).  Honza?