InstSimplify: Optimize away useless unsigned comparisons

Code like X < Y && Y == 0 should always be folded away to false.

llvm-svn: 223583

1 files changed, 49 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 45290fdcc00..e58998897c2 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -1443,12 +1443,57 @@ Value *llvm::SimplifyAShrInst(Value *Op0, Value *Op1, bool isExact,
                             RecursionLimit);
 }
 
+static Value *simplifyUnsignedRangeCheck(ICmpInst *ZeroICmp,
+                                         ICmpInst *UnsignedICmp, bool IsAnd) {
+  Value *X, *Y;
+
+  ICmpInst::Predicate EqPred;
+  if (!match(ZeroICmp, m_ICmp(EqPred, m_Value(Y), m_Zero())) &&
+      ICmpInst::isEquality(EqPred))
+    return nullptr;
+
+  ICmpInst::Predicate UnsignedPred;
+  if (match(UnsignedICmp, m_ICmp(UnsignedPred, m_Value(X), m_Specific(Y))) &&
+      ICmpInst::isUnsigned(UnsignedPred))
+    ;
+  else if (match(UnsignedICmp,
+                 m_ICmp(UnsignedPred, m_Value(Y), m_Specific(X))) &&
+           ICmpInst::isUnsigned(UnsignedPred))
+    UnsignedPred = ICmpInst::getSwappedPredicate(UnsignedPred);
+  else
+    return nullptr;
+
+  // X < Y && Y != 0  -->  X < Y
+  // X < Y || Y != 0  -->  Y != 0
+  if (UnsignedPred == ICmpInst::ICMP_ULT && EqPred == ICmpInst::ICMP_NE)
+    return IsAnd ? UnsignedICmp : ZeroICmp;
+
+  // X >= Y || Y != 0  -->  true
+  // X >= Y || Y == 0  -->  X >= Y
+  if (UnsignedPred == ICmpInst::ICMP_UGE && !IsAnd) {
+    if (EqPred == ICmpInst::ICMP_NE)
+      return getTrue(UnsignedICmp->getType());
+    return UnsignedICmp;
+  }
+
+  // X < Y && Y == 0  -->  false
+  if (UnsignedPred == ICmpInst::ICMP_ULT && EqPred == ICmpInst::ICMP_EQ &&
+      IsAnd)
+    return getFalse(UnsignedICmp->getType());
+
+  return nullptr;
+}
+
 // Simplify (and (icmp ...) (icmp ...)) to true when we can tell that the range
 // of possible values cannot be satisfied.
 static Value *SimplifyAndOfICmps(ICmpInst *Op0, ICmpInst *Op1) {
   ICmpInst::Predicate Pred0, Pred1;
   ConstantInt *CI1, *CI2;
   Value *V;
+
+  if (Value *X = simplifyUnsignedRangeCheck(Op0, Op1, /*IsAnd=*/true))
+    return X;
+
   if (!match(Op0, m_ICmp(Pred0, m_Add(m_Value(V), m_ConstantInt(CI1)),
                          m_ConstantInt(CI2))))
    return nullptr;
@@ -1602,6 +1647,10 @@ static Value *SimplifyOrOfICmps(ICmpInst *Op0, ICmpInst *Op1) {
   ICmpInst::Predicate Pred0, Pred1;
   ConstantInt *CI1, *CI2;
   Value *V;
+
+  if (Value *X = simplifyUnsignedRangeCheck(Op0, Op1, /*IsAnd=*/false))
+    return X;
+
   if (!match(Op0, m_ICmp(Pred0, m_Add(m_Value(V), m_ConstantInt(CI1)),
                          m_ConstantInt(CI2))))
    return nullptr;