[PATCH] D154953: [InstSimplify] Remove the remainder loop if we know the mask is always true

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


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Comment at: llvm/lib/Analysis/InstructionSimplify.cpp:1285
+    KnownBits Known = computeKnownBits(Op1, Q.DL, 0, Q.AC, Q.CxtI, Q.DT);
+    if (RemC->getActiveBits() >= Known.getMaxValue().getActiveBits())
+      return ConstantInt::getNullValue(Op0->getType());
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This should be just `RemC->uge(Known.getMaxValue())`. Same below.


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Comment at: llvm/lib/Analysis/InstructionSimplify.cpp:1288
+        return ConstantInt::getNullValue(Op0->getType());
+    }
+  }
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goldstein.w.n wrote:
> Since we are already here, might as well also add an `else if(Known.getMinValue().ugt(*RemC)) { return Op0; }`
> 
> Proofs: https://alive2.llvm.org/ce/z/FkTMoy
It looks like these proofs also work with urem replaced by srem: https://alive2.llvm.org/ce/z/-8RHjq So we should move this into simplifyRem and support both.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D154953/new/

https://reviews.llvm.org/D154953