[PATCH] D135541: [TargetLowering][RISCV][X86] Support even divisors in expandDIVREMByConstant.

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


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Comment at: llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp:7327
   APInt Mod = APInt::getSignedMinValue(BitWidth + 1);
   APInt MulFactor = Divisor.zext(BitWidth + 1);
   MulFactor = MulFactor.multiplicativeInverse(Mod);
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craig.topper wrote:
> craig.topper wrote:
> > jrtc27 wrote:
> > > Should this not be using the original divisor (whether by shifting back by TrailingZeros or making a copy you use elsewhere instead of doing an lshrInPlace)?
> > Yes! Thank you, that's probably the bug.
> Well I think that just exposed worse problems in the algorithm. The multiplicative inverse doesn't exist if Divisor is even. It needs to be coprime with 1<<BitWidth. But they will both have 2 as a common factor.
After the subtraction, you have something like "udiv exact X, Y".  So you can use the same algorithm as BuildExactSDIV (i.e. shift right X and Y, then multiply X by the inverse of Y).


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  https://reviews.llvm.org/D135541/new/

https://reviews.llvm.org/D135541