[llvm] r303779 - [InstCombine] Use less bitwise operations to handle Instruction::SExt in SimplifyDemandedUseBits. Other improvements.

[Craig Topper via llvm-commits]

Author: ctopper
Date: Wed May 24 12:33:30 2017
New Revision: 303779

URL: http://llvm.org/viewvc/llvm-project?rev=303779&view=rev
Log:
[InstCombine] Use less bitwise operations to handle Instruction::SExt in SimplifyDemandedUseBits. Other improvements.

The current code created a NewBits mask and used it as a mask several times. One of them just before a call to trunc making it unnecessary. A call to getActiveBits can get us the same information for the case. We also ORed with this mask later when we should have just sign extended the known bits.

We also called trunc on the guaranteed to be zero KnownZeros/Ones masks entering this code. Creating appropriately sized temporary APInts is probably better.

Differential Revision: https://reviews.llvm.org/D32098

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp?rev=303779&r1=303778&r2=303779&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp Wed May 24 12:33:30 2017
@@ -374,37 +374,32 @@ Value *InstCombiner::SimplifyDemandedUse
   }
   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)
+    if (DemandedMask.getActiveBits() > SrcBitWidth)
       InputDemandedBits.setBit(SrcBitWidth-1);
 
-    InputDemandedBits = InputDemandedBits.trunc(SrcBitWidth);
-    Known = Known.trunc(SrcBitWidth);
-    if (SimplifyDemandedBits(I, 0, InputDemandedBits, Known, Depth + 1))
+    KnownBits InputKnown(SrcBitWidth);
+    if (SimplifyDemandedBits(I, 0, InputDemandedBits, InputKnown, Depth + 1))
       return I;
-    InputDemandedBits = InputDemandedBits.zext(BitWidth);
-    Known = Known.zext(BitWidth);
-    assert(!Known.hasConflict() && "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 (Known.Zero[SrcBitWidth-1] || (NewBits & ~DemandedMask) == NewBits) {
-      // Convert to ZExt cast
+    if (InputKnown.isNonNegative() ||
+        DemandedMask.getActiveBits() <= SrcBitWidth) {
+      // Convert to ZExt cast.
       CastInst *NewCast = new ZExtInst(I->getOperand(0), VTy, I->getName());
       return InsertNewInstWith(NewCast, *I);
-    } else if (Known.One[SrcBitWidth-1]) {    // Input sign bit known set
-      Known.One |= NewBits;
-    }
+     }
+
+    // If the sign bit of the input is known set or clear, then we know the
+    // top bits of the result.
+    Known = InputKnown.sext(BitWidth);
+    assert(!Known.hasConflict() && "Bits known to be one AND zero?");
     break;
   }
   case Instruction::Add: