X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FAnalysis%2FInstructionSimplify.cpp;h=459fc92bce1a856a2d621b6aedc24c76c18c2d1a;hb=25529b337f75a4b9b174592e2c95136e781bd824;hp=fd7a4de6903786b7ef7c44093799a1a0c57bb32c;hpb=312646b71e415bb543ea1c143cca5b9009364b0d;p=oota-llvm.git

diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index fd7a4de6903..459fc92bce1 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -676,6 +676,18 @@ static Value *SimplifySubInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
   if (Op0 == Op1)
     return Constant::getNullValue(Op0->getType());
 
+  // X - (0 - Y) -> X if the second sub is NUW.
+  // If Y != 0, 0 - Y is a poison value.
+  // If Y == 0, 0 - Y simplifies to 0.
+  if (BinaryOperator::isNeg(Op1)) {
+    if (const auto *BO = dyn_cast<BinaryOperator>(Op1)) {
+      assert(BO->getOpcode() == Instruction::Sub &&
+             "Expected a subtraction operator!");
+      if (BO->hasNoUnsignedWrap())
+        return Op0;
+    }
+  }
+
   // (X + Y) - Z -> X + (Y - Z) or Y + (X - Z) if everything simplifies.
   // For example, (X + Y) - Y -> X; (Y + X) - Y -> X
   Value *X = nullptr, *Y = nullptr, *Z = Op1;
@@ -1346,6 +1358,11 @@ static Value *SimplifyAShrInst(Value *Op0, Value *Op1, bool isExact,
       cast<OverflowingBinaryOperator>(Op0)->hasNoSignedWrap())
     return X;
 
+  // Arithmetic shifting an all-sign-bit value is a no-op.
+  unsigned NumSignBits = ComputeNumSignBits(Op0, Q.DL);
+  if (NumSignBits == Op0->getType()->getScalarSizeInBits())
+    return Op0;
+
   return nullptr;
 }