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

[Kit Barton via llvm-dev]

Hi Hal,

You're right, for perfect loop nests I think simply knowing that the
inner loop has a guard will make things easier. This ties in with
Philip's earlier reply, that being able to reliably identify guarded
loops is a good first step.

Your comments about continually modifying the IR and thus invalidating
analysis makes a lot of sense. I just replied to the list with a summary
of a discussion that Philip and I had off the list (because I forgot to
use reply-all) and we're thinking to start by focusing on accurately
identifying existing guards before trying to add new (unnecessary)
guards. Please let me know if you're OK with this direction to start.

Kit


"Finkel, Hal J." <hfinkel at anl.gov> writes:

> On 5/28/19 8:46 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.
>
>
> Hi, Kit,
>
> Maybe I'm missing something obvious, but how does this help? I 
> understand how guards can make checking for perfect nesting, etc. 
> harder, but does always having guards make this easier?
>
> Also, we already have a situation with LCSSA form where trivial 
> constructs are introduced, and then eliminated by local simplifications, 
> sometimes multiple times as the pipeline progresses. Having more of them 
> seems undesirable - it leads to more IR changes, which causes more 
> analysis invalidation, and so on.
>
> Thanks again,
>
> Hal
>
>
>>
>>
>>
>> Any comments/feedback are greatly appreciated.
>>
>> Thanks,
>>
>> Kit
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