[PATCH] D42424: [InstCombine] Allow common type conversions to i8/i16

[Dave Green via Phabricator via llvm-commits]

dmgreen added a comment.

Yes, making them legal types in the datalayout was one of the things I tried. It made things (a little) worse in the quick tests I ran, but didn't seem to fail on anything. It was only a quick set of tests, and the changes vs this were only quite small. It obviously has larger reaching implications that this just in instcombine.

The purist in me is looking for something a little better, but I think what we have here is a sensible approach to take. We can always change it if not.

Krzysztof, thanks for the info. It sounds like having a canonical representation we all agree on would be a hard problem to solve :) I thinks that is how this all began, wanting a canonicalisation of selects vs cfg that played better with GVN/similar passes.

It appears that the loop idiom recogniser is not picking this up because it does not know how to handle the trunc's that are now part of the CFG. There seem to be a number of changes in the IR going into the idiom recogniser, like doing icmp eq 1 vs icmp eq 0, doing and(xor()) vs xor(and()), etc. They are all being handled fine, except for the new trunk nodes it just doesn't know anything about.


https://reviews.llvm.org/D42424