[llvm-dev] LSR/SCEV problem/question

[Michael Zolotukhin via llvm-dev]

Hi Geoff,

How was it handled before your change? Were we just lucky that insert points always matched?

Would it be possible to move all the logic for finding a proper insertion into SCEVExpander? I think that is ultimately the best solution for this, but I have to admit I looked into this code a while ago.

Michael

> On Mar 23, 2016, at 9:22 AM, Geoff Berry via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi All,
>  
> I’ve run into what appears to be a bug in ScalarEvolution, but I’m not sure if it is instead caused by me missing an implicit assumption between LSR and SCEV.
> This issue is caused by the change D18001 <http://reviews.llvm.org/D18001>, which is an attempt to increase SCEV-inserted instruction re-use by picking a more canonical insertion position in the case where a new insert position block is not found.
> The problem I have run into with this change is that it causes an insert position instruction to be chosen which is later hoisted into a different block by  SCEVExpander::hoistIVInc().  This in turn makes the SCEVExpander Builder InsertPt inconsistent (the instruction and block end up not matching), which leads to an invalid instruction being inserted (with its parent != its containing block).
>  
> Does anyone more familiar with the design of the LSR/SCEV interaction have an opinion on what the right approach to fixing this is?  The two approaches I’ve considered are: 1) avoid choosing an insert point that SCEV will move or 2) make SCEVExpander handle the case where it moves an instruction that is an insert point.  I’ve attempted (2), but it gets ugly pretty fast since there are quite a few different insert points and builders saved/restored.  (1) also seems tricky, since it essentially means LSR will have to predict what SCEVExpander is going to do and work around this case.
>  
> Any thoughts would be most appreciated,
>  
> --
> Geoff Berry
> Employee of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>  
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