[llvm-dev] InstCombiner bug

[Rob Springer via llvm-dev]

I believe there's a bug
in InstCombiner::reassociateShiftAmtsOfTwoSameDirectionShifts(),
illustrated by https://rise4fun.com/Alive/ne8y. I'll post a bug on the
tracker as soon as I can get an account set up.

Short form: when identifying the two shifts to combine, it does so while
ignoring any zero-extensions of those values. In this case, the values are
both i1 zero-extended to i3s, so when taking their sum, SimplifyAddInst
emits an xor, causing the shift to basically disappear.

I could write more, but the Alive link is probably more concise.

Let me know what you think / if there's some followup needed, etc.
Thanks!
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