[LLVMdev] RFC: Should we have (something like) -extra-vectorizer-passes in -O2?

[Andrew Trick]

> On Oct 14, 2014, at 8:53 AM, Arnold Schwaighofer <aschwaighofer at apple.com> wrote:
> 
> 
>> On Oct 13, 2014, at 5:56 PM, Chandler Carruth <chandlerc at gmail.com> wrote:
>> 
>> I've added a straw-man of some extra optimization passes that help specific benchmarks here or there by either preparing code better on the way into the vectorizer or cleaning up afterward. These are off by default until there is some consensus on the right path forward, but this way we can all test out the same set of flags, and collaborate on any tweaks to them.
>> 
>> The primary principle here is that the vectorizer expects the IR input to be in a certain canonical form, and produces IR output that may not yet be in that form. The primary alternative to this is to make the vectorizers both extra powerful (able to recognize many variations on things like loop structure) and extra cautious about their emitted code (so that it is always already optimized). I much prefer the solution of using passes rather than this unless compile time is hurt too drastically. It makes it much easier to test, validate, and compose all of the various components of the core optimizer.
>> 
>> Here is the structural diff:
>> 
>> + loop-rotate
>>  loop-vectorize
>> + early-cse
>> + correlated-propagation
>> + instcombine
>> + licm
>> + loop-unswitch
>> + simplifycfg
>> + instcombine
>>  slp-vectorize
>> + early-cse
>> 
> 
> I think a late loop optimization (vectorization) pipeline makes sense. I think we just have to carefully evaluate benefit over compile time.
> 
> Runing loop rotation makes sense. Critical edge splitting can transform loops into a form that prevents loop vectorization.
> 
> Both the loop vectorizer and the SLPVectorizer perform limited (restricted in region) forms of CSE to cleanup. EarlyCSE runs across the whole function and so might catch more opportunities.
> 
> The downside of always running passes is that we pay the cost irrespective of benefit. There might not be much to cleanup if we don’t vectorize a loop but we still have to pay for running the cleanup passes. This has been the motivator to have “pass local” CSE but this also stems from a time where we ran within the inlining pass manager which meant running over and over again.
> 
> I think we will just have to look at compile time and decide what makes sense.

It’s great that we’re running the vectorizers late, outside CGSCC. Regarding the set of passes that we rerun, I completely agree with Arnold. Naturally, iterating over the pass pipeline produces speedups, and I understand the engineering advantage. But rerunning several expensive function passes on the slim chance that a loop was transformed is an awful design for compile time.

>> + loop-rotate

I have no concern about loop-rotate. It should be very fast.

>>  loop-vectorize
>> + early-cse

Passes like loop-vectorize should be able to do their own CSE without much engineering effort.

>> + correlated-propagation

A little worried about this.

>> + instcombine

I'm *very* concerned about rerunning instcombine, but understand it may help cleanup the vectorized preheader.

>> + licm
>> + loop-unswitch

These should limited to the relevant loop nest. 

>> + simplifycfg

OK if the CFG actually changed.

>> + instcombine

instcombine again! This can’t be good.

>>  slp-vectorize
>> + early-cse

SLP should do its own CSE.

—

I think it’s generally useful to have an “extreme” level of optimization without much regard for compile time, and in that scenario this pipeline makes sense. But this is hardly something that should happen at -O2/-Os, unless real data shows otherwise.

If the pass manager were designed to conditionally invoke late passes triggered by certain transformation passes, that would solve my immediate concern.

Long term, I think a much better design is for function transformations to be conditionally rerun within a scope/region. For example, loop-vectorize should be able to trigger instcombine on the loop preheader, which I think is the real problem here.

-Andy

>> The rationale I have for this:
>> 
>> 1) Zinovy pointed out that the loop vectorizer really needs the input loops to still be rotated. One counter point is that perhaps we should prevent any pass from un-rotating loops?
>> 
>> 2) I cherrypicked the core of the scalar optimization pipeline that seems like it would be relevant to code which looks like runtime checks. Things like correlated values for overlap predicates, loop invariant code, or predicates that can be unswitched out of loops. Then I added the canonicalizing passes that might be relevant given those passes.
>> 
>> 3) I pulled the EarlyCSE from the BB vectorize stuff. Maybe it isn't relevant for SLP vectorize, no idea. I did say this was a straw man. =D
>> 
>> 
>> My benchmarking has shown some modest improvements to benchmarks, but nothing huge. However, it shows only a 2% slowdown for building the 'opt' binary, which I'm actually happy with so that we can work to improve the loop vectorizer's overhead *knowing* that these passes will clean up stuff. Thoughts? I'm currently OK with this, but it's pretty borderline so I just wanted to start the discussion and see what other folks observe in their benchmarking.
>> 
>> -Chandler
>