[llvm-dev] Speculation and control dependent no wrap flags
Artur Pilipenko via llvm-dev
llvm-dev at lists.llvm.org
Fri Feb 3 05:28:33 PST 2017
I'm looking at the bug (https://llvm.org/bugs/show_bug.cgi?id=31181) which was triggered by my change to make CVP mark adds as no wrap (https://reviews.llvm.org/rL278220) and I'd like to have some broader discussion of the problem. In this bug CVP correctly marks an add as nuw basing on the loop latch check, but later loop rotation pass moves the add to a point before the check. In the new context nuw is no longer valid and leads to an incorrect transformation of the loop. See https://llvm.org/bugs/show_bug.cgi?id=31181#c5 comment in the bug for more details.
Since nsw, nuw flags can be control dependent, it seems like we should be treating them as metadata, i.e. we should be stripping them when we speculate the instruction. I don’t think that we are doing this now anywhere. The problem was noticed on loop rotation, but I expect any other pass which speculates overflowing operations is suffering from the same problem.
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