[polly] r252726 - ScopDetection: Tighten the check for always executed 'error blocks'

[Johannes Doerfert via llvm-commits]

On 11/11, Tobias Grosser wrote:
> On 11/11/2015 06:31 PM, Johannes Doerfert wrote:
> >We should not go for dominance at all but for post-dominance instead and
> >use it to compute control conditions (see below). Hence, if the
> >(immediate) control condition of a error block is a conditional we should
> >consider it to be an error block, otherwise not.
> >
> >Below:
> >- copied from https://www.cs.utexas.edu/~pingali/CS380C/2013/lectures/Dominators.pdf
> >
> >Control dependence: given a CFG G, a node w is control-dependent on an edge (u -> v) if
> >   – w postdominates v
> >   – w does not strictly postdominate u
> 
> Right. As Chandler said, making use of post-dominance makes sense at a
> couple of different places. However, we currently do not yet preserve
> post-dominance in polly. We should probably add this (and also fix LLVM's
> post-dominance pass to correctly model unreachables), but this might take a
> little. To get our bots green quickly, without disabling all the error-block
> features, I decided to go for a patch with minimal impact. To my
> understanding the patch should be conservatively correct.
Should be, yes, but again something we have to keep in mind...
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