[PATCH] D36395: [InstCombine] narrow rotate left/right patterns to eliminate zext/trunc (PR34046)

Craig Topper via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Mon Aug 7 12:41:43 PDT 2017

craig.topper added a comment.

I think we miss the and x, 15 because we use the same pattern we do for shifts where we look for 5-bit masking even in 16-bit mode because hardware doesn't ignore bit 4. For shift the implication of that is obvious, if bit 4 is set then the shift will fill with 0s on 16-bit shift. For rotate, I think it just means we'd rotate around a second time and still get the correct result, but I'm not sure.


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