[PATCH] D43349: [InstCombine] Make SimplifyDemandedUseBits handle PhiNode

[Craig Topper via Phabricator via llvm-commits]

craig.topper added inline comments.


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Comment at: lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp:479
       if (cast<LShrOperator>(I)->isExact())
         DemandedMaskIn.setLowBits(ShiftAmt);
 
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xur wrote:
> craig.topper wrote:
> > DemandedMaskIn isn't used after this.
> I actually did not understand why DemandedMaskIn is reset here. So I moved it down (after processed the operand 0).
> 
> To me, since we  are doing the do the shift right, ShiftAmt of the lowbits is not needed (whatever value it might be).
> The original code seems to say these lowbits are needed. 
> 
> If the lowbits in DemandMaskin are indeed needed, I would have to set the Known.zero and check Known.zero in AND instruction.
> 
If we dont' preserve the bits in the input to the shift, then the exact flag could become incorrect. I'm not sure why we would prefer to preserve the bits rather than clear the exact flag.


https://reviews.llvm.org/D43349