[polly] r244606 - Revise the simplification of regions

[Johannes Doerfert via llvm-commits]

On 08/12, Michael Kruse via llvm-commits wrote:
> Two corrections:
> 
> First, such PHI nodes can only happen in the entry node, since
> buildPHIAccesses checks the origin of the incoming edges (not of the
> operands which I assumed).
The operands are not the relevant part of the PHI semantics but the
control flow is. There are examples for this in the PHI codegen
discussion(s) we had on the list.

> Sich PHI nodes can only appear in the entry
> node. So I think solutions 1 and 3 do work.
What is wrong with adding a pre SCoP block for all entry block PHI
nodes? That solved the problem before and I cannot see the negative side
effects and it needs only a minimal amount of code.

> This all doesn't look like designed deliberately. And it is what I
> meant with make simplifyRegion "stable" in D11867.
In the original PHI codegen patch there was a detection of PHI nodes
that were at some point part of the region (in the entry block) and
split out of it by one of the region simplification methods just before
the codegen part. Once detected these PHI's were deliberately ignored in
the block codegen as they were no longer part of the region (SCoP).

I cannot argue for the simplification and splitting stuff but the PHI
codegen was aware of such PHI nodes and was handling them at some point.


Cheers,
  Johannes
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