Revert "InstCombine: Improvement to check if signed addition overflows."

This reverts commit r209746.

It looks it is causing a crash while building libcxx. I am trying to get a
reduced testcase.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209762 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 44 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index eca4e4a787..c37a9cf2ef 100644
--- a/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -889,34 +889,11 @@ static inline Value *dyn_castFoldableMul(Value *V, Constant *&CST) {
   return nullptr;
 }
 
-// If one of the operands only has one non-zero bit, and if the other
-// operand has a known-zero bit in a more significant place than it (not
-// including the sign bit) the ripple may go up to and fill the zero, but
-// won't change the sign. For example, (X & ~4) + 1.
-// FIXME: Handle case where LHS has a zero before the 1 in the RHS, but also
-// has one after.
-static bool CheckRippleForAdd(APInt Op0KnownZero, APInt Op0KnownOne,
-                              APInt Op1KnownZero, APInt Op1KnownOne) {
-  // Make sure that one of the operand has only one bit set to 1 and all other
-  // bit set to 0.
-  if ((~Op1KnownZero).countPopulation() == 1) {
-    int BitWidth = Op0KnownZero.getBitWidth();
-    // Ignore Sign Bit.
-    Op0KnownZero.clearBit(BitWidth - 1);
-    int Op1OnePosition = BitWidth - Op1KnownOne.countLeadingZeros() - 1;
-    int Op0ZeroPosition = BitWidth - Op0KnownZero.countLeadingZeros() - 1;
-    if ((Op0ZeroPosition != (BitWidth - 1)) &&
-        (Op0ZeroPosition >= Op1OnePosition))
-      return true;
-  }
-  return false;
-}
 
 /// WillNotOverflowSignedAdd - Return true if we can prove that:
 ///    (sext (add LHS, RHS))  === (add (sext LHS), (sext RHS))
 /// This basically requires proving that the add in the original type would not
 /// overflow to change the sign bit or have a carry out.
-/// TODO: Handle this for Vectors.
 bool InstCombiner::WillNotOverflowSignedAdd(Value *LHS, Value *RHS) {
   // There are different heuristics we can use for this.  Here are some simple
   // ones.
@@ -928,29 +905,14 @@ bool InstCombiner::WillNotOverflowSignedAdd(Value *LHS, Value *RHS) {
   if (ComputeNumSignBits(LHS) > 1 && ComputeNumSignBits(RHS) > 1)
     return true;
 
-  if (IntegerType *IT = dyn_cast<IntegerType>(LHS->getType())) {
 
-    int BitWidth = IT->getBitWidth();
-    APInt LHSKnownZero(BitWidth, 0, /*isSigned*/ true);
-    APInt LHSKnownOne(BitWidth, 0, /*isSigned*/ true);
-    computeKnownBits(LHS, LHSKnownZero, LHSKnownOne);
+  // If one of the operands only has one non-zero bit, and if the other operand
+  // has a known-zero bit in a more significant place than it (not including the
+  // sign bit) the ripple may go up to and fill the zero, but won't change the
+  // sign.  For example, (X & ~4) + 1.
+
+  // TODO: Implement.
 
-    APInt RHSKnownZero(BitWidth, 0, /*isSigned*/ true);
-    APInt RHSKnownOne(BitWidth, 0, /*isSigned*/ true);
-    computeKnownBits(RHS, RHSKnownZero, RHSKnownOne);
-
-    // Addition of two 2's compliment numbers having opposite signs will never
-    // overflow.
-    if ((LHSKnownOne[BitWidth - 1] && RHSKnownZero[BitWidth - 1]) ||
-        (LHSKnownZero[BitWidth - 1] && RHSKnownOne[BitWidth - 1]))
-      return true;
-
-    // Check if carry bit of addition will not cause overflow.
-    if (CheckRippleForAdd(LHSKnownZero, LHSKnownOne, RHSKnownZero, RHSKnownOne))
-      return true;
-    if (CheckRippleForAdd(RHSKnownZero, RHSKnownOne, LHSKnownZero, LHSKnownOne))
-      return true;
-  }
   return false;
 }