Teach instcombine to look for booleans in wider integers when it encounters a

zext(icmp). It may be able to optimize that away. This fixes one of the cases
in PR5438.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@88830 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index efda6608e44d1a67d44ead393a6f611c730ab34e..14a482dd6e8ad499fde282f79151ea1445740250 100644 (file)
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -8539,6 +8539,34 @@ Instruction *InstCombiner::transformZExtICmp(ICmpInst *ICI, Instruction &CI,
     }
   }
 
+  // icmp ne A, B is equal to xor A, B when A and B only really have one bit.
+  // It is also profitable to transform icmp eq into not(xor(A, B)) because that
+  // may lead to additional simplifications.
+  if (CI.getType() == ICI->getOperand(0)->getType()) {
+    if (const IntegerType *ITy = dyn_cast<IntegerType>(CI.getType())) {
+      Value *LHS = ICI->getOperand(0);
+      Value *RHS = ICI->getOperand(1);
+
+      uint32_t BitWidth = ITy->getBitWidth();
+      APInt KnownZeroLHS(BitWidth, 0), KnownOneLHS(BitWidth, 0);
+      APInt KnownZeroRHS(BitWidth, 0), KnownOneRHS(BitWidth, 0);
+      APInt TypeMask(APInt::getAllOnesValue(BitWidth));
+      ComputeMaskedBits(LHS, TypeMask, KnownZeroLHS, KnownOneLHS);
+      ComputeMaskedBits(RHS, TypeMask, KnownZeroRHS, KnownOneRHS);
+
+      if (KnownZeroLHS.countLeadingOnes() == BitWidth-1 &&
+          KnownZeroRHS.countLeadingOnes() == BitWidth-1) {
+        if (!DoXform) return ICI;
+
+        Value *Xor = Builder->CreateXor(LHS, RHS);
+        if (ICI->isTrueWhenEqual())
+          Xor = Builder->CreateXor(Xor, ConstantInt::get(ITy, 1));
+        Xor->takeName(ICI);
+        return ReplaceInstUsesWith(CI, Xor);
+      }
+    }
+  }
+
   return 0;
 }
 

diff --git a/test/Transforms/InstCombine/compare-signs.ll b/test/Transforms/InstCombine/compare-signs.ll
new file mode 100644 (file)
index 0000000..991020a
--- /dev/null
+++ b/test/Transforms/InstCombine/compare-signs.ll
@@ -0,0 +1,28 @@
+; RUN: opt %S -instcombine -S | FileCheck %S
+; PR5438
+
+; TODO: This should also optimize down.
+;define i32 @bar(i32 %a, i32 %b) nounwind readnone {
+;entry:
+;        %0 = icmp sgt i32 %a, -1        ; <i1> [#uses=1]
+;        %1 = icmp slt i32 %b, 0         ; <i1> [#uses=1]
+;        %2 = xor i1 %1, %0              ; <i1> [#uses=1]
+;        %3 = zext i1 %2 to i32          ; <i32> [#uses=1]
+;        ret i32 %3
+;}
+
+define i32 @qaz(i32 %a, i32 %b) nounwind readnone {
+; CHECK: @qaz
+entry:
+; CHECK: xor i32 %a, %b
+; CHECK; lshr i32 %0, 31
+; CHECK: xor i32 %1, 1
+        %0 = lshr i32 %a, 31            ; <i32> [#uses=1]
+        %1 = lshr i32 %b, 31            ; <i32> [#uses=1]
+        %2 = icmp eq i32 %0, %1         ; <i1> [#uses=1]
+        %3 = zext i1 %2 to i32          ; <i32> [#uses=1]
+        ret i32 %3
+; CHECK-NOT: icmp
+; CHECK-NOT: zext
+; CHECK: ret i32 %2
+}