[PATCH] D32098: [InstCombine] Use less bitwise operations to handle Instruction::SExt in SimplifyDemandedUseBits. Other improvements.

[Craig Topper via Phabricator via llvm-commits]

craig.topper updated this revision to Diff 96635.
craig.topper added a comment.

Remove the early conversion to ZExtInst.


https://reviews.llvm.org/D32098

Files:
  lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp


Index: lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -391,40 +391,34 @@
   }
   case Instruction::SExt: {
     // Compute the bits in the result that are not present in the input.
-    unsigned SrcBitWidth =I->getOperand(0)->getType()->getScalarSizeInBits();
+    unsigned SrcBitWidth = I->getOperand(0)->getType()->getScalarSizeInBits();
 
-    APInt InputDemandedBits = DemandedMask &
-                              APInt::getLowBitsSet(BitWidth, SrcBitWidth);
+    APInt InputDemandedBits = DemandedMask.trunc(SrcBitWidth);
 
-    APInt NewBits(APInt::getBitsSetFrom(BitWidth, SrcBitWidth));
     // If any of the sign extended bits are demanded, we know that the sign
     // bit is demanded.
-    if ((NewBits & DemandedMask) != 0)
-      InputDemandedBits.setBit(SrcBitWidth-1);
+    if (DemandedMask.getActiveBits() > SrcBitWidth)
+      InputDemandedBits.setSignBit();
 
-    InputDemandedBits = InputDemandedBits.trunc(SrcBitWidth);
-    KnownZero = KnownZero.trunc(SrcBitWidth);
-    KnownOne = KnownOne.trunc(SrcBitWidth);
-    if (SimplifyDemandedBits(I, 0, InputDemandedBits, KnownZero, KnownOne,
-                             Depth + 1))
+    APInt InputKnownZero(SrcBitWidth, 0), InputKnownOne(SrcBitWidth, 0);
+    if (SimplifyDemandedBits(I, 0, InputDemandedBits, InputKnownZero,
+                             InputKnownOne, Depth + 1))
       return I;
-    InputDemandedBits = InputDemandedBits.zext(BitWidth);
-    KnownZero = KnownZero.zext(BitWidth);
-    KnownOne = KnownOne.zext(BitWidth);
-    assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
-
-    // If the sign bit of the input is known set or clear, then we know the
-    // top bits of the result.
 
     // If the input sign bit is known zero, or if the NewBits are not demanded
     // convert this into a zero extension.
-    if (KnownZero[SrcBitWidth-1] || (NewBits & ~DemandedMask) == NewBits) {
-      // Convert to ZExt cast
+    if (InputKnownZero.isSignBitSet() ||
+        DemandedMask.getActiveBits() <= SrcBitWidth) {
+      // Convert to ZExt cast.
       CastInst *NewCast = new ZExtInst(I->getOperand(0), VTy, I->getName());
       return InsertNewInstWith(NewCast, *I);
-    } else if (KnownOne[SrcBitWidth-1]) {    // Input sign bit known set
-      KnownOne |= NewBits;
     }
+
+    // If the sign bit of the input is known set or clear, then we know the
+    // top bits of the result.
+    KnownZero = InputKnownZero.sext(BitWidth);
+    KnownOne = InputKnownOne.sext(BitWidth);
+    assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
     break;
   }
   case Instruction::Add:


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