RFC: [ARM] Disable peeling

[Christophe Lyon <christophe dot lyon at linaro dot org>]

Hi,

As ARM supports unaligned vector accesses for almost no penalty, I'd
like to disable loop peeling on ARM targets.

I have ran benchmarks on cortex-A9 (hard-float) and noticed these
significant improvements:
* 1.5% improvement on a popular embedded benchmark (with peaks at +20% and +29%)
* 2.1% on spec2k mesa
* 9.2% on spec2k eon
* up to 3.4% on some part of another embedded benchmark

The largest regression I noticed is 1%.

I have attached a preliminary patch to discuss how acceptable it would
be, and to discuss the needed changes in the testsuite. Indeed; quite
a few tests now fail because they count the number of "vectorizing an
unaligned access" and "alignment of access forced using peeling"
occurrences in the vectorizer traces.

I could add a property to target-supports.exp, which would currently
be only true on ARM to select whether to rely on peeling or not, and
updated all the affected tests accordingly.

As there are quite a few tests to update, I'd like opinions first.

Thanks,

Christophe.

Attachment: disable-peeling.changelog.txt
Description: Text document

Attachment: disable-peeling.patch.txt
Description: Text document