[llvm-dev] questionabout loop rotation

[Aditya K via llvm-dev]

Hi Stefanos,

Thanks for your comments. I added both as reviewer.

> One question though. Are you sure that this:
> This helps with LICM when instructions inside a conditional is loop invariant 
> is not achieved with the current LoopRotate pass? Because AFAIK, it does. Basically it inserts
> a guard (that branches to the preheader) and then passes like LICM hoist invariant instructions to the preheader.
> Also, it does itself host some invariant instructions but only on the header and very limited ones.

Currently LICM does some of it but not all. If there are more than 2 conditionals,
LICM would fail to move loop invariant from the second conditional because loop rotation didn't peel all the conditionals.
My observation is from a while ago so happy to be corrected.

-Aditya









From: Stefanos Baziotis <stefanos.baziotis at gmail.com>

Sent: Saturday, March 21, 2020 1:42 PM

To: Aditya K <hiraditya at msn.com>; 林政宗 <jackie_linzz at 126.com>; Johannes Doerfert <johannesdoerfert at gmail.com>

Cc: llvm-dev <llvm-dev at lists.llvm.org>

Subject: Re: [llvm-dev] questionabout loop rotation

 


Hi Aditya,



Glad to hear that. I want to read the patch but it's kind of big and unfortunately right now I so don't have time.

I'll try to take a look when there is some time. In the meantime, you may want to add Kit Barton and Michael Kruse

as reviewers.



One question though. Are you sure that this:

> This helps with LICM when instructions inside a conditional is loop invariant

is not achieved with the current LoopRotate pass? Because AFAIK, it does. Basically it inserts

a guard (that branches to the preheader) and then passes like LICM hoist invariant instructions to the preheader.

Also, it does itself host some invariant instructions but only on the header and very limited ones.



Kind regards,

Stefanos Baziotis



P.S. I re-CC'd the other participants.








Στις Σάβ, 21 Μαρ 2020 στις 8:41 μ.μ., ο/η Aditya K via llvm-dev <llvm-dev at lists.llvm.org> έγραψε:



hi,

I had an implementation of loop-rotation that peels the loop such that each conditional is executed at least once.

https://reviews.llvm.org/D22630



This helps with LICM when instructions inside a conditional is loop invariant. The patch has decent number of test cases.

All the requested comments were addressed at that time. I'm happy to rebase and put the patch again.



-Aditya

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