[PATCH] D14496: X86: More efficient codegen for 64-bit compare-and-branch

[Hans Wennborg via llvm-commits]

hans added a comment.

David, Michael: thank you very much for your input. Not only is the code you suggest better, but it should be easier to generate too since it doesn't change the cfg.

I've been trying to make this work today, but am not sure exactly where to put it. The approach of pattern-matching for "(setcc (or (xor hi1 hi2) (xor lo1 lo2)) 0 {eq,ne})" is brittle because some nodes can get "simplified" e.g .(setcc a 0 ugt) will become (setcc a 0 ne). We could of course try to unsimplify it but I wonder if there's a better way.

Michael mentioned maybe we could do a target-specific DAG-combine. But that would need to be done before legalization, so the types would be illegal. Is it OK to legalize them in a combine? I don't think the combine has access to the logic for expanding integers etc?

Can I make this a target-specific legalization somehow? Where would I do that?

Or, could we make this a generic thing? We have SUBE and SUBC that would let us do subtract with borrow, but I don't think we can pick up the flags from that. Is there some way we could represent this in the generic selection dag?

I'll keep working on this tomorrow, just wanted to put down my notes in case you have any comments. The backend is still new territory for me.


http://reviews.llvm.org/D14496