[PATCH] D19950: Use frequency info to guide Loop Invariant Code Motion.

Tue May 10 15:14:12 PDT 2016

----- Original Message -----
> From: "Dehao Chen" <danielcdh at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Xinliang David Li" <davidxl at google.com>, reviews+D19950+public+38ba22078c2035b8 at reviews.llvm.org, "David
> Majnemer" <david.majnemer at gmail.com>, "Junbum Lim" <junbuml at codeaurora.org>, mcrosier at codeaurora.org, "llvm-commits"
> <llvm-commits at lists.llvm.org>, "amara emerson" <amara.emerson at arm.com>
> Sent: Tuesday, May 10, 2016 4:10:49 PM
> Subject: Re: [PATCH] D19950: Use frequency info to guide Loop Invariant Code Motion.
> 
> 
> Thanks for the comment. I spent quite a while to think, but still
> cannot think of an optimization that could be unblocked by
> speculatively hoisting an loop invariant from an unlikely executed
> path. Can you give some hint (or an example) on what type of
> optimization can benefit from this case?

I'm specifically thinking about this case (although I suspect there are others):

 for (...) {
   if (...) {
     hoistable
     cold_stuff
   }
 }

 for (...) {
   if (...) {
     hoistable
     hot_stuff
   }
 }

I expect that 'hoistable' will be hoisted by LICM out of both loops, and then CSE'd by GVN. One might also imagine cases where the two hoistable sections are SLP vectorized.

Failing to host the code might also prevent loop unswitching (by failing to reduce the size of the loop body below the threshold size).

Another potential issue is that the hoistable code might be cold, and relatively cheap to hoist, but expensive to vectorize. As a result, failing to hoist the code might block otherwise-profitable vectorization. Which reminds me, we need to fix the vectorizer's if-conversion heuristic to use profiling information too ;)

Thanks again,
Hal

> 
> Thanks,
> Dehao
> 
> 
> On Tue, May 10, 2016 at 1:58 PM, Hal Finkel < hfinkel at anl.gov >
> wrote:
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> From: "Xinliang David Li" < davidxl at google.com >
> To: "Dehao Chen" < danielcdh at gmail.com >
> Cc: reviews+D19950+public+38ba22078c2035b8 at reviews.llvm.org , "David
> Majnemer" < david.majnemer at gmail.com >, "Hal Finkel" <
> hfinkel at anl.gov >, "Junbum Lim" < junbuml at codeaurora.org >,
> mcrosier at codeaurora.org , "llvm-commits" <
> llvm-commits at lists.llvm.org >, "amara emerson" <
> amara.emerson at arm.com >
> Sent: Tuesday, May 10, 2016 3:15:24 PM
> Subject: Re: [PATCH] D19950: Use frequency info to guide Loop
> Invariant Code Motion.
> 
> 
> 
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> On Tue, May 10, 2016 at 1:03 PM, Dehao Chen < danielcdh at gmail.com >
> wrote:
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> On Tue, May 10, 2016 at 11:48 AM, Xinliang David Li <
> davidxl at google.com > wrote:
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> On Tue, May 10, 2016 at 11:01 AM, Dehao Chen < danielcdh at gmail.com >
> wrote:
> 
> 
> danielcdh added a comment.
> 
> In http://reviews.llvm.org/D19950#425287 , @hfinkel wrote:
> 
> > In http://reviews.llvm.org/D19950#425286 , @hfinkel wrote:
> > 
> > > In http://reviews.llvm.org/D19950#425285 , @davidxl wrote:
> > > 
> > > > Static prediction has been conservative in estimating loop trip
> > > > count -- it produces something like 30ish iterations. If the a
> > > > very hot loop has a big if-then-else (or switch), it is very
> > > > likely to mark many bbs' to be colder than the loop header.
> > > > Turning on this for static prediction really depends on the
> > > > false rate. It seems to be this can get wrong pretty easily
> > > > for very hot loops (which is also the most important thing to
> > > > optimize for).
> > > 
> > > 
> > > This is a good point. There's no universal conservative choice
> > > (assuming a small trip count is conservative in some cases, and
> > > assuming a large trip count is conservative in other cases).
> > 
> > 
> > Would it be better (and practical) if there were some way for the
> > BFI client to specify which kind of 'conservative' is desired?
> > 
> > Also, why are we doing this instead of sinking later (in CGP or
> > similar)? LICM can expose optimization opportunities, plus
> > represents a code pattern the user might input manually. Sinking
> > later seems more robust.
> 
> 
> I looked at CGP pass, looks like it's handling the sinking
> case-by-case (e.g. there is separate routine to handle sinking of
> load, gep, etc. I'm afraid this would miss opportunities.
> Additionally, the file-level comment of CGP pass says "This works
> around limitations in it's basic-block-at-a-time approach. It should
> eventually be removed."
> Yes, but it will be "removed" when the entire subsystem is replaced
> by GlobalISel, and we'll certainly need to make GlobalISel
> profiling-data aware, so I expect this is the right path forward
> regardless. I agree, however, that we want a general sinking here
> based on profiling data, not just the specific existing heuristics
> for loads, GEPs, etc.
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> Perhaps you can do profile driven sinking CGP separately to handle
> manually hoisted code situation mentioned by Hal.
> 
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> Do you mean we still use frequency to decide whether to hoist code in
> LICM, additionally use frequency info to check if we want to sink
> instructions in CGP?
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> yes -- that is the suggestion. I'd prefer that we try to sink late
> first, and only if there are use cases that we can't handle this
> way, we consider throttling hoisting early. If we come across such
> use cases, I'd like to understand them better. Hoisting can expose
> other optimization opportunities, and you lose those opportunities
> if you don't hoist in the first place.
> 
> -Hal
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> David
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> Dehao
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> David
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> I'm not quite clear why it helps to move code out of loop early and
> later sink it inside. Could you give an example or some more
> context?
> 
> Thanks,
> Dehao
> 
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> http://reviews.llvm.org/D19950
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> --
> 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory