Kit Barton via llvm-dev
2019-May-28 13:46 UTC
[llvm-dev] Making loop guards part of canonical loop structure
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
Philip Reames via llvm-dev
2019-May-28 17:49 UTC
[llvm-dev] Making loop guards part of canonical loop structure
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 > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
Finkel, Hal J. via llvm-dev
2019-May-28 21:02 UTC
[llvm-dev] Making loop guards part of canonical loop structure
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 > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev-- Hal Finkel Lead, Compiler Technology and Programming Languages Leadership Computing Facility Argonne National Laboratory
Kit Barton via llvm-dev
2019-May-28 23:37 UTC
[llvm-dev] Making loop guards part of canonical loop structure
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 >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
Kit Barton via llvm-dev
2019-May-28 23:43 UTC
[llvm-dev] Making loop guards part of canonical loop structure
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 >> _______________________________________________ >> LLVM Developers mailing list >> llvm-dev at lists.llvm.org >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
David Greene via llvm-dev
2019-May-29 14:04 UTC
[llvm-dev] Making loop guards part of canonical loop structure
This seems like potentially a good thing. There have been cases where
I've wanted to know the guard associated with the loop.
Loop versioning can create many guards, including guards nested within
other guards. I don't know how/if we want to capture that level of
complexity, but it was the first question that popped into my mind.
-David
Kit Barton via llvm-dev <llvm-dev at lists.llvm.org> writes:
> 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
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
Krzysztof Parzyszek via llvm-dev
2019-May-30 18:28 UTC
[llvm-dev] Making loop guards part of canonical loop structure
On Hexagon, unguarded loops cannot be converted to hardware loops. If the loop's latch branch alone handles the iteration count, including the possibility of 0, then the loop cannot be converted to a hardware loop, because hardware loops must iterate at least once. If the entire loop is guarded against zero iteration count, we can put the loop setup in the preheader, since at that point the loop is guaranteed to execute at least once. I am strongly in favor of having some way to create loop guards, even if they are trivial. -- Krzysztof Parzyszek kparzysz at quicinc.com LLVM compiler development -----Original Message----- From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Finkel, Hal J. via llvm-dev Sent: Tuesday, May 28, 2019 4:02 PM To: Kit Barton <kit.barton at gmail.com>; LLVM Dev Mailing List <llvm-dev at lists.llvm.org> Subject: [EXT] Re: [llvm-dev] Making loop guards part of canonical loop structure 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 > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev-- Hal Finkel Lead, Compiler Technology and Programming Languages Leadership Computing Facility Argonne National Laboratory _______________________________________________ LLVM Developers mailing list llvm-dev at lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev