[PATCH] D75233: [LoopTerminology] LCSSA Form

[Stefanos Baziotis via Phabricator via llvm-commits]

baziotis added a comment.

In D75233#1948305 <https://reviews.llvm.org/D75233#1948305>, @Meinersbur wrote:

> In D75233#1944176 <https://reviews.llvm.org/D75233#1944176>, @baziotis wrote:
>
> > Yes. Could you explain the part about dominator nodes ? I understand how dominators help in the identification
> >  of loops and how dominance frontiers help in the placement of normal PHI nodes (i.e. when 2 or more paths converge). But I don't see how dominators
> >  help with the placement of LCSSA phi nodes.
>
>
> It is simple if the loop has just a single exit, but much more involved with multiple exits. Example CFG:
>
>   Header:
>     %val = ...
>  
>   Header->{Latch1,Latch2,Latch3}->Header
>  
>   Header->Latch1->Exit1
>   Header->Latch2->Exit2
>   Header->Latch3->Exit3
>  
>   Exit->Successor1
>  
>   Successor1->SharedSucc
>   Exit2->SharedSucc
>  
>   SharedSucc->CommonSucc
>   Exit3->CommonSucc
>  
>   CommonSucc:
>     use(%val)
>
>
> Header dominates all other BBs in this example, hence without LCSSA, this is well-formed. However, with LCSSA we have to insert phis into Exit1,Exit2,Exit3 such that the end result has to look like:
>
>   Exit1:
>     %lcssa1 = phi [%val, %Latch1]
>  
>   Exit2:
>     %lcssa2 = phi [%val, %Latch2]
>  
>   Exit3:
>     %lcssa3 = phi [%val, %Latch3]
>  
>   SharedSucc:
>     %sharedval = phi [%lcssa1 , %Exit1], [%lcssa2, %Exit2]
>  
>   CommonSucc:
>     %commonval = phi [%lcssa3 , %Exit3], [%sharedval , %SharedSucc]
>     use(%commonval)
>
>
> For computing %sharedval and %commonval, DominanceFrontier is needed again, hence LCSSA is not less complicated that SSA itself.


Ok, I get it now, thanks! So with multiple exits one has to compute dominator info again.


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