InstSimplify: Optimize ICmpInst xform that uses computeKnownBits

A few things:
- computeKnownBits is relatively expensive, let's delay its use as long
  as we can.
- Don't create two APInt values just to run computeKnownBits on a
  ConstantInt, we already know the exact value!
- Avoid creating a temporary APInt value in order to calculate unary
  negation.

llvm-svn: 222092

1 files changed, 17 insertions, 21 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index a1b5b462b77..656768f040f 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -2422,27 +2422,6 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
     }
   }
 
-  // If a bit is known to be zero for A and known to be one for B,
-  // then A and B cannot be equal.
-  if (ICmpInst::isEquality(Pred)) {
-    if (ConstantInt *CI = dyn_cast<ConstantInt>(RHS)) {
-      uint32_t BitWidth = CI->getBitWidth();
-      APInt LHSKnownZero(BitWidth, 0);
-      APInt LHSKnownOne(BitWidth, 0);
-      computeKnownBits(LHS, LHSKnownZero, LHSKnownOne, Q.DL,
-                       0, Q.AT, Q.CxtI, Q.DT);
-      APInt RHSKnownZero(BitWidth, 0);
-      APInt RHSKnownOne(BitWidth, 0);
-      computeKnownBits(RHS, RHSKnownZero, RHSKnownOne, Q.DL,
-                       0, Q.AT, Q.CxtI, Q.DT);
-      if (((LHSKnownOne & RHSKnownZero) != 0) ||
-          ((LHSKnownZero & RHSKnownOne) != 0))
-        return (Pred == ICmpInst::ICMP_EQ)
-                   ? ConstantInt::getFalse(CI->getContext())
-                   : ConstantInt::getTrue(CI->getContext());
-    }
-  }
-
   // Special logic for binary operators.
   BinaryOperator *LBO = dyn_cast<BinaryOperator>(LHS);
   BinaryOperator *RBO = dyn_cast<BinaryOperator>(RHS);
@@ -2881,6 +2860,23 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
     }
   }
 
+  // If a bit is known to be zero for A and known to be one for B,
+  // then A and B cannot be equal.
+  if (ICmpInst::isEquality(Pred)) {
+    if (ConstantInt *CI = dyn_cast<ConstantInt>(RHS)) {
+      uint32_t BitWidth = CI->getBitWidth();
+      APInt LHSKnownZero(BitWidth, 0);
+      APInt LHSKnownOne(BitWidth, 0);
+      computeKnownBits(LHS, LHSKnownZero, LHSKnownOne, Q.DL, /*Depth=*/0, Q.AT,
+                       Q.CxtI, Q.DT);
+      const APInt &RHSVal = CI->getValue();
+      if (((LHSKnownZero & RHSVal) != 0) || ((LHSKnownOne & ~RHSVal) != 0))
+        return Pred == ICmpInst::ICMP_EQ
+                   ? ConstantInt::getFalse(CI->getContext())
+                   : ConstantInt::getTrue(CI->getContext());
+    }
+  }
+
   // If the comparison is with the result of a select instruction, check whether
   // comparing with either branch of the select always yields the same value.
   if (isa<SelectInst>(LHS) || isa<SelectInst>(RHS))