[InstCombine] Use isSignBitCheck to simplify an if statement. Directly create new sign bit compares instead of manipulating the constant. NFCI

Since we no longer had the direct constant compares, manipulating the constant seemeded less clear.

llvm-svn: 314830

1 files changed, 8 insertions, 12 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 5155fa4b0fd..00feb01de40 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1457,8 +1457,8 @@ Instruction *InstCombiner::foldICmpXorConstant(ICmpInst &Cmp,
   // If this is a comparison that tests the signbit (X < 0) or (x > -1),
   // fold the xor.
   ICmpInst::Predicate Pred = Cmp.getPredicate();
-  if ((Pred == ICmpInst::ICMP_SLT && C.isNullValue()) ||
-      (Pred == ICmpInst::ICMP_SGT && C.isAllOnesValue())) {
+  bool TrueIfSigned = false;
+  if (isSignBitCheck(Cmp.getPredicate(), C, TrueIfSigned)) {
 
     // If the sign bit of the XorCst is not set, there is no change to
     // the operation, just stop using the Xor.
@@ -1468,17 +1468,13 @@ Instruction *InstCombiner::foldICmpXorConstant(ICmpInst &Cmp,
       return &Cmp;
     }
 
-    // Was the old condition true if the operand is positive?
-    bool isTrueIfPositive = Pred == ICmpInst::ICMP_SGT;
-
-    // If so, the new one isn't.
-    isTrueIfPositive ^= true;
-
-    Constant *CmpConstant = cast<Constant>(Cmp.getOperand(1));
-    if (isTrueIfPositive)
-      return new ICmpInst(ICmpInst::ICMP_SGT, X, SubOne(CmpConstant));
+    // Emit the opposite comparison.
+    if (TrueIfSigned)
+      return new ICmpInst(ICmpInst::ICMP_SGT, X,
+                          ConstantInt::getAllOnesValue(X->getType()));
     else
-      return new ICmpInst(ICmpInst::ICMP_SLT, X, AddOne(CmpConstant));
+      return new ICmpInst(ICmpInst::ICMP_SLT, X,
+                          ConstantInt::getNullValue(X->getType()));
   }
 
   if (Xor->hasOneUse()) {