I'm submitting a new revision since i don't understand how to reclaim/reopen/take over the existing one, D50222.
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This implements an optimization described in Hacker's Delight 10-17: when C is constant,
the result of X % C == 0 can be computed more cheaply without actually calculating the remainder.
The motivation is discussed here: https://bugs.llvm.org/show_bug.cgi?id=35479.
- In principle, it's possible to also handle the X % C1 == C2 case, as discussed on bugzilla. This seems to require an extra branch on overflow, so I refrained from implementing this for now.
- An explicit check for when the REM can be reduced to just its LHS is included: the X % C == 0 optimization breaks test1 in test/CodeGen/X86/jump_sign.ll otherwise. I hadn't managed to find a better way to not generate worse output in this case.
- The test/CodeGen/X86/jump_sign.ll regresses, and is being fixed by a followup patch D63390.