[PATCH] D31120: [InstCombine] Teach SimplifyDemandedUseBits that adding or subtractings 0s from every bit below the highest demanded bit can be simplified

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.

If we are adding/subtractings 0s below the highest demanded bit we can just use the other operand and remove the operation.

My primary motivation is observing that we can call ShrinkDemandedConstant for the add/sub and create a 0 constant, rather than removing the add completely. In the case I saw, we modified the constant on an add instruction to a 0, but the add is not put into the worklist. So we didn't revisit it until the next InstCombine iteration. This caused an IR modification to remove add and a subsequent iteration to be ran.

With this change we get bypass the add in the first iteration and prevent the second iteration from changing anything.

Not sure how to test this because we do eventually optimize it. Is there any way to bound instcombine iterations from the command line?


https://reviews.llvm.org/D31120

Files:
  lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp


Index: lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -537,15 +537,24 @@
                                LHSKnownZero, LHSKnownOne, Depth + 1) ||
           ShrinkDemandedConstant(I, 1, DemandedFromOps) ||
           SimplifyDemandedBits(I->getOperandUse(1), DemandedFromOps,
-                               LHSKnownZero, LHSKnownOne, Depth + 1)) {
+                               RHSKnownZero, RHSKnownOne, Depth + 1)) {
         // Disable the nsw and nuw flags here: We can no longer guarantee that
         // we won't wrap after simplification. Removing the nsw/nuw flags is
         // legal here because the top bit is not demanded.
         BinaryOperator &BinOP = *cast<BinaryOperator>(I);
         BinOP.setHasNoSignedWrap(false);
         BinOP.setHasNoUnsignedWrap(false);
         return I;
       }
+
+      // If we are known to be adding/subtracting zeros to every bit below
+      // the highest demanded bit, we just return the other side.
+      if ((DemandedFromOps & RHSKnownZero) == DemandedFromOps)
+        return I->getOperand(0);
+      // We can't do this with the LHS for subtraction.
+      if (I->getOpcode() == Instruction::Add &&
+          (DemandedFromOps & LHSKnownZero) == DemandedFromOps)
+        return I->getOperand(1);
     }
 
     // Otherwise just hand the add/sub off to computeKnownBits to fill in


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D31120.92264.patch
Type: text/x-patch
Size: 1553 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20170319/fe3e9e31/attachment.bin>