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

[Krzysztof Parzyszek via llvm-dev]

Outlook removed llvm-dev from the recipient list, so re-replying.

-- 
Krzysztof Parzyszek  kparzysz at quicinc.com   LLVM compiler development

-----Original Message-----
From: Krzysztof Parzyszek <kparzysz at quicinc.com> 
Sent: Friday, May 31, 2019 10:41 AM
To: Krzysztof Parzyszek <kparzysz at quicinc.com>; Philip Reames <listmail at philipreames.com>; Finkel, Hal J. <hfinkel at anl.gov>; Kit Barton <kit.barton at gmail.com>
Subject: RE: [EXT] Re: [llvm-dev] Making loop guards part of canonical loop structure

Actually, having guards inserted in codegen would require loop analysis to be able to compute iteration counts of MI loops (which we don't really do generically right now).  So, this isn't really an applicable argument at the moment.

Still, on the IR level, having a consistent loop structure could simplify code that analyzes loops (as in Kit's example).

-- 
Krzysztof Parzyszek  kparzysz at quicinc.com   LLVM compiler development

-----Original Message-----
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Krzysztof Parzyszek via llvm-dev
Sent: Friday, May 31, 2019 10:32 AM
To: Philip Reames <listmail at philipreames.com>; Finkel, Hal J. <hfinkel at anl.gov>; Kit Barton <kit.barton at gmail.com>; llvm-dev at lists.llvm.org
Subject: [EXT] Re: [llvm-dev] Making loop guards part of canonical loop structure

I don't really care that much about that particular case.  We generate hardware loops somewhat late in codegen, so this is long past IR-level loop transformations.  It's just a case where having guarded loops would make things easier to analyze, regardless of how the loop code may have been reorganized in the meantime.

-- 
Krzysztof Parzyszek  kparzysz at quicinc.com   LLVM compiler development

-----Original Message-----
From: Philip Reames <listmail at philipreames.com> 
Sent: Thursday, May 30, 2019 4:57 PM
To: Krzysztof Parzyszek <kparzysz at quicinc.com>; Finkel, Hal J. <hfinkel at anl.gov>; Kit Barton <kit.barton at gmail.com>; llvm-dev at lists.llvm.org
Subject: [EXT] Re: [llvm-dev] Making loop guards part of canonical loop structure

Seems like a case which should be handled by loop rotation.  If you find a reproducer for it, that's where I'd start looking rather than anything specific to guarded loops. 

Philip

On 5/30/19 1:44 PM, Krzysztof Parzyszek wrote:
> I don't remember the details of the particular case where we encountered this, but I think the loop started with the condition check and ended with an unconditional branch back to the beginning.
>
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