[LLVMdev] Enabling the vectorizer for -Os -- ping

[Chandler Carruth]

All I have to say is *wow*. The vectorizer performs *remarkably* better now
than it did the last time I benchmarked it. I'm stunned.

I measured -O2 and -Os, as well as -march=x86-64 and -march=corei7-avx. My
hope with the latter two was to cover both worst-case and best-case in
terms of the quality of the vector ISA available.

First, binary size growth. This is measured on average across a reasonably
wide selection of binaries including large servers, video codecs, image
processing, etc.

O2, x86-64: 1% larger w/ vectorizer
O2, corei7-avx: 1.2% larger
Os, x86-64: 0.1% larger
Os, corei7-avx: < 0.1% larger

This is incredibly impressive IMO. =]

The performance numbers are also pretty good. There are a couple of minor
regressions, only one significant one. That one happens to be open source:
https://code.google.com/p/snappy/source/browse/trunk/snappy.cc this slows
down -- the vectorizer vectorizes a cold loop, which then gets inlined and
blocks subsequent inlining. (Many thanks to Ben Kramer for pointing out the
cause so quickly for me.) But there are a lot of potential solutions to
this problem:

1) vectorize after inlining -- this has some problems (code growth mostly)
but we might be able to solve them.
2) mark the cold path as cold so the optimizer is aware of it (tested this,
it seems to work, but i'm still experimenting)
3) rewrite this part of snappy to be fundamentally better (the code as it
is doesn't make a lot of sense to me, but i'm not an expert on it and will
need time to figure out the best way to solve the issue)

I'm actually happy with any of the 3, although #2 isn't terribly
satisfying. But even if that's the result, I can live with it.

So essentially, I think you should turn the vectorizer on completely.
What's left seem very much like small isolated issues.

Thanks for driving this whole thing and giving me time to do some
evaluation. I'm really thrilled by the result.
-Chandler


On Fri, Jun 14, 2013 at 11:53 AM, Chandler Carruth <chandlerc at google.com>wrote:

> Sorry for the delays here. I am running our benchmark suite and will have
> data in a day or so.
> On Jun 13, 2013 9:40 PM, "Nadav Rotem" <nrotem at apple.com> wrote:
>
>> Hi,
>>
>> Last week I wrote llvm-dev and presented data that shows how enabling the
>> vectorizer on -Os can improve the performance of many workloads and that it
>> has negligible effects on code size.  I also added a command line switch to
>> make it easier for people to benchmark the vectorizer using -Os directly
>> from clang without changing LLVM.  Has anyone done any benchmarks on -Os +
>> vectorization ?
>>
>> Thanks,
>> Nadav
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>
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