Analysis: Reformulate WillNotOverflowUnsignedMul for reusability

WillNotOverflowUnsignedMul's smarts will live in ValueTracking as
computeOverflowForUnsignedMul.  It now returns a tri-state result:
never overflows, always overflows and sometimes overflows.

llvm-svn: 225076

3 files changed, 9 insertions, 53 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombine.h b/llvm/lib/Transforms/InstCombine/InstCombine.h
index b4d1efc1a92..96edc793175 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombine.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombine.h
@@ -286,7 +286,6 @@ private:
   bool WillNotOverflowSignedSub(Value *LHS, Value *RHS, Instruction *CxtI);
   bool WillNotOverflowUnsignedSub(Value *LHS, Value *RHS, Instruction *CxtI);
   bool WillNotOverflowSignedMul(Value *LHS, Value *RHS, Instruction *CxtI);
-  bool WillNotOverflowUnsignedMul(Value *LHS, Value *RHS, Instruction *CxtI);
   Value *EmitGEPOffset(User *GEP);
   Instruction *scalarizePHI(ExtractElementInst &EI, PHINode *PN);
   Value *EvaluateInDifferentElementOrder(Value *V, ArrayRef<int> Mask);
@@ -388,6 +387,10 @@ public:
     return llvm::ComputeSignBit(V, KnownZero, KnownOne, DL, Depth, AT, CxtI,
                                 DT);
   }
+  OverflowResult computeOverflowForUnsignedMul(Value *LHS, Value *RHS,
+                                               const Instruction *CxtI) {
+    return llvm::computeOverflowForUnsignedMul(LHS, RHS, DL, AT, CxtI, DT);
+  }
 
 private:
   /// SimplifyAssociativeOrCommutative - This performs a few simplifications for
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index ec6a61307e1..34caf1a5ab9 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -440,24 +440,8 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
   }
   case Intrinsic::umul_with_overflow: {
     Value *LHS = II->getArgOperand(0), *RHS = II->getArgOperand(1);
-    unsigned BitWidth = cast<IntegerType>(LHS->getType())->getBitWidth();
-
-    APInt LHSKnownZero(BitWidth, 0);
-    APInt LHSKnownOne(BitWidth, 0);
-    computeKnownBits(LHS, LHSKnownZero, LHSKnownOne, 0, II);
-    APInt RHSKnownZero(BitWidth, 0);
-    APInt RHSKnownOne(BitWidth, 0);
-    computeKnownBits(RHS, RHSKnownZero, RHSKnownOne, 0, II);
-
-    // Get the largest possible values for each operand.
-    APInt LHSMax = ~LHSKnownZero;
-    APInt RHSMax = ~RHSKnownZero;
-
-    // If multiplying the maximum values does not overflow then we can turn
-    // this into a plain NUW mul.
-    bool Overflow;
-    LHSMax.umul_ov(RHSMax, Overflow);
-    if (!Overflow) {
+    OverflowResult OR = computeOverflowForUnsignedMul(LHS, RHS, II);
+    if (OR == OverflowResult::NeverOverflows) {
       return CreateOverflowTuple(II, Builder->CreateNUWMul(LHS, RHS), false);
     }
   } // FALL THROUGH
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index d444d33ca8a..255e5872583 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -165,39 +165,6 @@ bool InstCombiner::WillNotOverflowSignedMul(Value *LHS, Value *RHS,
   return false;
 }
 
-/// \brief Return true if we can prove that:
-///    (mul LHS, RHS)  === (mul nuw LHS, RHS)
-bool InstCombiner::WillNotOverflowUnsignedMul(Value *LHS, Value *RHS,
-                                              Instruction *CxtI) {
-  // Multiplying n * m significant bits yields a result of n + m significant
-  // bits. If the total number of significant bits does not exceed the
-  // result bit width (minus 1), there is no overflow.
-  // This means if we have enough leading zero bits in the operands
-  // we can guarantee that the result does not overflow.
-  // Ref: "Hacker's Delight" by Henry Warren
-  unsigned BitWidth = LHS->getType()->getScalarSizeInBits();
-  APInt LHSKnownZero(BitWidth, 0);
-  APInt RHSKnownZero(BitWidth, 0);
-  APInt TmpKnownOne(BitWidth, 0);
-  computeKnownBits(LHS, LHSKnownZero, TmpKnownOne, 0, CxtI);
-  computeKnownBits(RHS, RHSKnownZero, TmpKnownOne, 0, CxtI);
-  // Note that underestimating the number of zero bits gives a more
-  // conservative answer.
-  unsigned ZeroBits = LHSKnownZero.countLeadingOnes() +
-                      RHSKnownZero.countLeadingOnes();
-  // First handle the easy case: if we have enough zero bits there's
-  // definitely no overflow.
-  if (ZeroBits >= BitWidth)
-    return true;
-
-  // There is an ambiguous cases where there can be no overflow:
-  //   ZeroBits == BitWidth - 1
-  // However, determining overflow requires calculating the sign bit of
-  // LHS * RHS/2.
-
-  return false;
-}
-
 Instruction *InstCombiner::visitMul(BinaryOperator &I) {
   bool Changed = SimplifyAssociativeOrCommutative(I);
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
@@ -413,7 +380,9 @@ Instruction *InstCombiner::visitMul(BinaryOperator &I) {
     I.setHasNoSignedWrap(true);
   }
 
-  if (!I.hasNoUnsignedWrap() && WillNotOverflowUnsignedMul(Op0, Op1, &I)) {
+  if (!I.hasNoUnsignedWrap() &&
+      computeOverflowForUnsignedMul(Op0, Op1, &I) ==
+          OverflowResult::NeverOverflows) {
     Changed = true;
     I.setHasNoUnsignedWrap(true);
   }