[PATCH] D79799: [VectorCombine] add loop to enable iterative folding

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D79799#2038887 <https://reviews.llvm.org/D79799#2038887>, @nikic wrote:

> My only concern here would be whether this can degenerate quadratically. Without particular familiarity with this pass, and looking at example `@ins1_ins1_iterate`, it seems like this would work by scalarizing one operation on each iteration. Is that right? If so, that seems potentially problematic, as the pass becomes quadratic for longer instruction chains.


Yes, in the worst case it could become quadratic, and you're seeing the test correctly. I think we could undo the reverse basic-block walk in this patch to make that particular case more efficient.

If I'm seeing it correctly, other iterative passes like SimplifyCFG and CodeGenPrepare allow for quadratic-time possibility too. I'm not sure if there's a theoretical way to draw the line on that, or if we just accept that potential risk (assume that anything this pass will ever do is rare, so it can't be too expensive). As I wrote in the description, I think the alternative is to revise things to be more like InstCombine's user-based worklist. We could also put in a cl::opt flag to bail out and/or assert if we go overboard. Any other suggestions?


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D79799/new/

https://reviews.llvm.org/D79799