[llvm-dev] Making loop guards part of canonical loop structure

[David Greene via llvm-dev]

This sounds great.  Ignore the message I just sent.  :)

                     -David

Kit Barton via llvm-dev <llvm-dev at lists.llvm.org> writes:

> I just realized that inadvertently hit reply, instead of reply-all, when
> replying to Philip. I'm going to summarize the conversation we have had
> since, so everyone can see the direction we are thinking to go.
>
> We'll start by adding a guard detection API as a loop utility that can
> identify existing guards. The initial patch will be as simple as
> possible, to establish the API. From there we can extend the detection
> to get more difficult cases, and potentially modify some parts of loop
> simplification (or other optimizations) to make the detection easier to
> do, it it makes sense. At that point we will have experience and can
> revisit this discussion if it still makes sense to do so.
>
> Kit
>
>
> Philip Reames <listmail at philipreames.com> writes:
>
>> Kit,
>>
>> I'm a bit unsure about having the guarded loop be canonical form.  In
>> particular, I'm hesitant about introducing a conditional branch on
>> constant for loops which aren't found to have an obvious guard block. 
>> (Note the distinction: no obvious guard does not imply no guard, just
>> one we didn't find.)
>>
>> I'd suggest that we maybe start with a small step in this direction,
>> rather than everything at once?
>>
>> Introducing utilities on Loop to return the loop guard block if one is
>> available seems like a reasonable and necessary first step.  Once that's
>> done, we could consider extending LoopSimplify to make guarded loops
>> "more obvious" by moving code out of guarded blocks and the like.  Note
>> that my "more obvious" here is about exposing existing guarded loops,
>> and not introducing new guards. 
>>
>> I think we need both of these pieces if we were to go all the way
>> towards your proposal right?  If so, let's start incremental and see if
>> we have enough of a problem left to justify the canonicalization. 
>>
>> Philip
>>
>> On 5/28/19 6:46 AM, Kit Barton via llvm-dev wrote:
>>> Hi all,
>>>
>>> TL;DR: Should the loop guard branch be part of the canonical loop structure?
>>>
>>> Background:
>>> -----------
>>> While working on the next set of improvements to loop fusion, we discovered that
>>> loop rotation will put a guard around a loop if it cannot prove the loop will
>>> execute at least once, as seen in the following (simplified) example:
>>>
>>> entry:
>>>   br i1 %cmp1, label %for.body.lr.ph, label %for.cond.cleanup
>>>
>>> for.body.lr.ph:                                   ; preds = %entry
>>>   br label %for.body
>>>
>>> for.body:                                         ; preds = %for.body.lr.ph, %for.inc
>>>   br label %for.inc
>>>
>>> for.inc:                                          ; preds = %for.body
>>>   br i1 %cmp, label %for.body, label %for.cond.for.cond.cleanup_crit_edge
>>>
>>> for.cond.for.cond.cleanup_crit_edge:              ; preds = %for.inc
>>>   br label %for.cond.cleanup
>>>
>>> for.cond.cleanup:                                 ; preds = %for.cond.for.cond.cleanup_crit_edge, %entry
>>>   br label %for.end
>>>
>>> for.end:                                          ; preds = %for.cond.cleanup
>>>   ret void
>>>
>>> However, for loops that can be proven to execute at least once, there is no
>>> guard.
>>>
>>> This creates complications for several loop optimizations because they need to
>>> handle both guarded loops and non-guarded loops. For example, the current loop
>>> fusion pass needs to check whether two loops are control flow equivalent before
>>> fusing them (i.e., if the first loop executes, the second loop is guaranteed to
>>> execute). This is currently done by checking that the preheader of the first
>>> loop dominates the preheader of the second loop, and the preheader of the second
>>> loop post-dominates the preheader of the first loop. When one (or both) of the
>>> loops have a guard, then this check no longer works. If the loop guard was part
>>> of the canonical form, then this check could be done using the loop guard,
>>> instead of the preheader.
>>>
>>> Similarly, when checking for perfect loop nests, a loop guard around the inner
>>> loop will make the nest appear imperfect. However, if the guard is considered to
>>> be part of the inner loop, it is easier to prove a perfect nest.
>>>
>>> Both of these examples have alternative ways to deal with the loop guards, if
>>> they are present. However, I would like to have the discussion regarding making
>>> guards part of the canonical loop structure. If people are supportive of this,
>>> we can start doing the work to implement the guards and start modify loop
>>> optimizations to deal with them appropriately. However, if this is not the
>>> direction we want to go, we can deal with the guards in the specific
>>> optimizations where they are relevant.
>>>
>>> Proposed Solution:
>>> ------------------
>>> Ensure that all loops in canonical form have a guard branch that dominates the
>>> preheader. If such a guard branch does not exist, a trivial guard will be
>>> created.
>>>
>>> I have not looked at the implementation of this
>>> extensively yet because I wanted feedback on this direction first. However, my
>>> initial thought is to modify LoopSimplify to add the concept of a loop guard,
>>> and provide similar guarantees that it currently provides for preheaders and exit
>>> blocks. Specifically, if a guard block is not found, one is created and inserted
>>> immediately before the loop preheader to guarantee a structure similar to the
>>> one in the example above. Note that as with the preheader and exit blocks, it is
>>> possible that subsequent passes (e.g., SimplifyCFG) may remove these blocks thus
>>> taking it out of canonical form. However, this is an existing situation that
>>> loop optimizations already deal with.
>>>
>>>
>>>
>>> Any comments/feedback are greatly appreciated.
>>>
>>> Thanks,
>>>
>>> Kit
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
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