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Re: Loop peeling


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?





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