llvm dev - Oct 2014 - [LLVMdev] llvm.loop metadata placement and critical edge splitting

While reviewing a fix for maintaining loop metadata
(http://reviews.llvm.org/D5539) I noticed that we make a strict assumption about
the metadata being attached to the branch that is an immediate predecessor of
the loop header. This does not work well with LLVM's approach of lazy
critical edge splitting. I've proposed working around this with heroics
inside the critical edges splitting utility, but feel that the workaround is
really unnecessary because the design could be fixed more easily. I was not
involved in the original design discussion for llvm.loop metadata, so I want to
get feedback before proposing a direction.

My question is: Why can't we define requirements of loop metadata such that
*critical edge splitting does not invalidate loop metadata*.

I think fixing this may be an easy change to LoopInfo get/setLoopID. The rule
would be simple, if the loop back branch is unconditional, and it has a single
predecessor, the metadata is expected on the predecessaor's conditional
branch:

loop.body:
  ...
  br i1 %c, label %loop.tail, label %exit, !llvm.loop

loop.tail:
  ...
  br label %loop.body

exit:
  ret

If the loop tail does not have a single predecessor (complex control flow occurs
after the loop test), then the metadata can still be placed on the unconditional
branch. Either way, we don't need to worry about edge splitting. Only a
signficant loop restructuring will invalidate the metadata.

I don't think any change is necessary when clang emits a loop with an
unconditional backedge, but someone will want to verify with some test cases. If
a change is needed it should also be easy.

With that design change we can remove any current workarounds from
SplitCriticalEdge and simply preserve loop metadata, which remains valid by
definition.

-Andy

----- Original Message -----
> From: "Andrew Trick" <atrick at apple.com>
> To: "llvmdev at cs.uiuc.edu Dev" <llvmdev at cs.uiuc.edu>
> Cc: doerfert at cs.uni-saarland.de, "zinovy nis" <zinovy.nis
at gmail.com>, "Hal Finkel" <hfinkel at anl.gov>,
"Arnold
> Schwaighofer" <aschwaighofer at apple.com>
> Sent: Monday, October 6, 2014 12:55:11 PM
> Subject: llvm.loop metadata placement and critical edge splitting
> 
> While reviewing a fix for maintaining loop metadata
> (http://reviews.llvm.org/D5539) I noticed that we make a strict
> assumption about the metadata being attached to the branch that is
> an immediate predecessor of the loop header. This does not work well
> with LLVM's approach of lazy critical edge splitting. I've proposed
> working around this with heroics inside the critical edges splitting
> utility, but feel that the workaround is really unnecessary because
> the design could be fixed more easily. I was not involved in the
> original design discussion for llvm.loop metadata, so I want to get
> feedback before proposing a direction.
> 
> My question is: Why can't we define requirements of loop metadata
> such that *critical edge splitting does not invalidate loop
> metadata*.
> 
> I think fixing this may be an easy change to LoopInfo get/setLoopID.
> The rule would be simple, if the loop back branch is unconditional,
> and it has a single predecessor, the metadata is expected on the
> predecessaor's conditional branch:
> 
> loop.body:
>   ...
>   br i1 %c, label %loop.tail, label %exit, !llvm.loop
> 
> loop.tail:
>   ...
>   br label %loop.body
> 
> exit:
>   ret
> 
> If the loop tail does not have a single predecessor (complex control
> flow occurs after the loop test), then the metadata can still be
> placed on the unconditional branch. Either way, we don't need to
> worry about edge splitting. Only a signficant loop restructuring
> will invalidate the metadata.
> 
> I don't think any change is necessary when clang emits a loop with an
> unconditional backedge, but someone will want to verify with some
> test cases. If a change is needed it should also be easy.
> 
> With that design change we can remove any current workarounds from
> SplitCriticalEdge and simply preserve loop metadata, which remains
> valid by definition.
I think this makes sense. cc'ing the folks who did the OpenMP codegen work
in case they have an opinion. Also, maybe the polly and/or POCL folks have an
opinion (not sure exactly who to ask for those).

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

On 06/10/2014 23:56, Hal Finkel wrote:
> ----- Original Message -----
>> From: "Andrew Trick" <atrick at apple.com>
>> To: "llvmdev at cs.uiuc.edu Dev" <llvmdev at
cs.uiuc.edu>
>> Cc: doerfert at cs.uni-saarland.de, "zinovy nis"
<zinovy.nis at gmail.com>, "Hal Finkel" <hfinkel at
anl.gov>, "Arnold
>> Schwaighofer" <aschwaighofer at apple.com>
>> Sent: Monday, October 6, 2014 12:55:11 PM
>> Subject: llvm.loop metadata placement and critical edge splitting
>>
>> While reviewing a fix for maintaining loop metadata
>> (http://reviews.llvm.org/D5539) I noticed that we make a strict
>> assumption about the metadata being attached to the branch that is
>> an immediate predecessor of the loop header. This does not work well
>> with LLVM's approach of lazy critical edge splitting. I've
proposed
>> working around this with heroics inside the critical edges splitting
>> utility, but feel that the workaround is really unnecessary because
>> the design could be fixed more easily. I was not involved in the
>> original design discussion for llvm.loop metadata, so I want to get
>> feedback before proposing a direction.
>>
>> My question is: Why can't we define requirements of loop metadata
>> such that *critical edge splitting does not invalidate loop
>> metadata*.
>>
>> I think fixing this may be an easy change to LoopInfo get/setLoopID.
>> The rule would be simple, if the loop back branch is unconditional,
>> and it has a single predecessor, the metadata is expected on the
>> predecessaor's conditional branch:
>>
>> loop.body:
>>    ...
>>    br i1 %c, label %loop.tail, label %exit, !llvm.loop
>>
>> loop.tail:
>>    ...
>>    br label %loop.body
>>
>> exit:
>>    ret
>>
>> If the loop tail does not have a single predecessor (complex control
>> flow occurs after the loop test), then the metadata can still be
>> placed on the unconditional branch. Either way, we don't need to
>> worry about edge splitting. Only a signficant loop restructuring
>> will invalidate the metadata.
>>
>> I don't think any change is necessary when clang emits a loop with
an
>> unconditional backedge, but someone will want to verify with some
>> test cases. If a change is needed it should also be easy.
>>
>> With that design change we can remove any current workarounds from
>> SplitCriticalEdge and simply preserve loop metadata, which remains
>> valid by definition.
>
> I think this makes sense. cc'ing the folks who did the OpenMP codegen
work in case they have an opinion. Also, maybe the polly and/or POCL folks have
an opinion (not sure exactly who to ask for those).
 From my side, this makes a lot of sense.

@Johannes, what do you think?

Cheers,
Tobias