[PATCH] D35267: Pass Divergence Analysis data to selection DAG to drive divergence dependent instruction selection

[Alexander via Phabricator via llvm-commits]

alex-t added a comment.

In https://reviews.llvm.org/D35267#998083, @efriedma wrote:

> I'd like to see a verifier somewhere that the divergence bit is still correct after DAGCombine (it could be different from what SelectionDAG::createOperands would compute given how ReplaceAllUsesWith works).


Could you please clarify the goal of the verification? Let's say we managed to transform the DAG in such a way that uniform pattern has been changed to divergent one.
Then the approach depends on our attitude to the transformation.
If we agree that transformation that change the pattern divergence is illegal? like it is in case we change uniform to divergent, we should assert and bail out.
If we assume that transformation is legal? like in your example of folding divergent variable to zero constant (x*0 => 0), we should recompute the divergence bits instead.
To handle both cases we need one more re-computation over all DAG nodes like it was done in my previous implementation but with error message if the uniform node becomes divergent.

I would like to just re-compute the bits just before selection and leave the legality of the DAG transformation issues to that transformations authors. In other words we compute what we have. 
If someone transform the DAG incorrectly it is his own problem.


https://reviews.llvm.org/D35267