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

[Hal Finkel via llvm-commits]

----- Original Message -----

> From: "Xinliang David Li" <davidxl at google.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Dehao Chen" <danielcdh at gmail.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: Wednesday, May 11, 2016 12:58:24 PM
> Subject: Re: [PATCH] D19950: Use frequency info to guide Loop
> Invariant Code Motion.

> On Tue, May 10, 2016 at 3:14 PM, Hal Finkel < hfinkel at anl.gov >
> wrote:

> > ----- 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.
> 
> I think this case can be/should be handled by more general profile
> driver speculative PRE. The above case may not be profitable even
> after GVN CSEed two expressions.
Why not? It is performance neutral and a code-size improvement. 

> On the other hand,

> ... = a * b;

> for (...) {
> if (cold) {
> .... = a * b;
> }
> }

> It will be good to hoist and CSE. Though in this case, we do not need
> LICM to enable this CSE. Another case:

> if (....) {
> ... = a*b;
> }

> for (....) {
> if (cold) {
> ... = a * b;
> }
> }

> Depending on the profile, it might be profitable to do:

> t = a * b;
> if (...) {
> .. = t;
> }
> for (...) {
> if (cold) {
> .. = t ;
> }
> }

> Again, LICM won't be necessary to enable this.

> > One might also imagine cases where the two hoistable sections are
> > SLP
> > vectorized.
> 

> Will that make it harder to undo the damage later ?
This depends on the speedup gained by vectorizing, and is a code-size improvement. 

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

> There are always existing cleanups that can only happen after
> loop-unswitching happens. IMO, loop unswiitching, like inliner
> should also look at the code state if the transformation happens.
Loop unswitching could certainly be smarter about how it estimates the cost of the loop body, but if there are nested conditions, I don't think that always helps. For example, if we have this: 

for (...) { 
if (c1) { 
... 
if (c2) { 
hoistable 
cold_stuff 
} 
... 
} 

... 
} 

then the code-size cost of unswitching on condition c1 is rightly impacted by whether the hoistable code is still in the loop body. 

> > 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 ;)
> 

> SLP vectorize? Any example like this? Can vectorizor be enhanced so
> that it can be done in absence of the hoisting?
I'm referring to the loop vectorizer. I'm not sure it is an issue of potential enhancement, the issue seems fundamental. The "cost" of if-conversion is proportional to the increased execution cost, and that's proportional to the number of instructions in the conditionally-executed block. Even more so if the hoistable operations would need to be scalarized under the targeted ISA. The more instructions need to be if-converted, and the colder the block, the lower the profitability of vectorizing (because, once vectorized, the instructions now execute on every iteration). 

In any case, for the same reason we don't want to throttle LICM based on register pressure, I don't think we want to do it here either. There is value is having a canonical form where things execute as soon as possible, because it means that we can use dominance to determine the frontier of legal placement. Otherwise we need to teach a myriad of other passes how to hoist in combination with their core functionality. Maybe we'll come up with some fundamental reason why sinking late is insufficient, but I don't think I've seen one yet. 

Thanks again, 
Hal 

> thanks,

> David

> > Thanks again,
> 
> > Hal
> 

> > >
> 
> > > Thanks,
> 
> > > Dehao
> 
> > >
> 
> > >
> 
> > > On Tue, May 10, 2016 at 1:58 PM, Hal Finkel < hfinkel at anl.gov >
> 
> > > wrote:
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > > 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.
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > > On Tue, May 10, 2016 at 1:03 PM, Dehao Chen < danielcdh at gmail.com
> > > >
> 
> > > wrote:
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > > On Tue, May 10, 2016 at 11:48 AM, Xinliang David Li <
> 
> > > davidxl at google.com > wrote:
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > > 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.
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > > Perhaps you can do profile driven sinking CGP separately to
> > > handle
> 
> > > manually hoisted code situation mentioned by Hal.
> 
> > >
> 
> > >
> 
> > > 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?
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > > 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
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > > David
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > > Dehao
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > > David
> 
> > >
> 
> > >
> 
> > >
> 
> > > 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
> 
> > >
> 
> > >
> 
> > > http://reviews.llvm.org/D19950
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > > --
> 
> > >
> 
> > > Hal Finkel
> 
> > > Assistant Computational Scientist
> 
> > > Leadership Computing Facility
> 
> > > Argonne National Laboratory
> 
> > >
> 
> > >
> 

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

-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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