[PatternMatch] Match XOR variant of unsigned-add overflow check.

Instcombine folds (a + b <u a) to (a ^ -1 <u b) and that does not match
the expected pattern in CodeGenPerpare via UAddWithOverflow.

This causes a regression over Clang 7 on both X86 and AArch64:
https://gcc.godbolt.org/z/juhXYV

This patch extends UAddWithOverflow to also catch the XOR case, if the
XOR is only used in the ICMP. This covers just a single case, but I'd
like to make sure I am not missing anything before tackling the other
cases.

Reviewers: nikic, RKSimon, lebedev.ri, spatel

Reviewed By: nikic, lebedev.ri

Differential Revision: https://reviews.llvm.org/D74228

1 files changed, 19 insertions, 9 deletions

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index cf5f4c7..4d6777c 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -399,7 +399,8 @@ class TypePromotionTransaction;
     bool simplifyOffsetableRelocate(Instruction &I);
 
     bool tryToSinkFreeOperands(Instruction *I);
-    bool replaceMathCmpWithIntrinsic(BinaryOperator *BO, CmpInst *Cmp,
+    bool replaceMathCmpWithIntrinsic(BinaryOperator *BO, Value *Arg0,
+                                     Value *Arg1, CmpInst *Cmp,
                                      Intrinsic::ID IID);
     bool optimizeCmp(CmpInst *Cmp, bool &ModifiedDT);
     bool combineToUSubWithOverflow(CmpInst *Cmp, bool &ModifiedDT);
@@ -1185,6 +1186,7 @@ static bool OptimizeNoopCopyExpression(CastInst *CI, const TargetLowering &TLI,
 }
 
 bool CodeGenPrepare::replaceMathCmpWithIntrinsic(BinaryOperator *BO,
+                                                 Value *Arg0, Value *Arg1,
                                                  CmpInst *Cmp,
                                                  Intrinsic::ID IID) {
   if (BO->getParent() != Cmp->getParent()) {
@@ -1202,8 +1204,6 @@ bool CodeGenPrepare::replaceMathCmpWithIntrinsic(BinaryOperator *BO,
   }
 
   // We allow matching the canonical IR (add X, C) back to (usubo X, -C).
-  Value *Arg0 = BO->getOperand(0);
-  Value *Arg1 = BO->getOperand(1);
   if (BO->getOpcode() == Instruction::Add &&
       IID == Intrinsic::usub_with_overflow) {
     assert(isa<Constant>(Arg1) && "Unexpected input for usubo");
@@ -1222,12 +1222,16 @@ bool CodeGenPrepare::replaceMathCmpWithIntrinsic(BinaryOperator *BO,
 
   IRBuilder<> Builder(InsertPt);
   Value *MathOV = Builder.CreateBinaryIntrinsic(IID, Arg0, Arg1);
-  Value *Math = Builder.CreateExtractValue(MathOV, 0, "math");
+  if (BO->getOpcode() != Instruction::Xor) {
+    Value *Math = Builder.CreateExtractValue(MathOV, 0, "math");
+    BO->replaceAllUsesWith(Math);
+  } else
+    assert(BO->hasOneUse() &&
+           "Patterns with XOr should use the BO only in the compare");
   Value *OV = Builder.CreateExtractValue(MathOV, 1, "ov");
-  BO->replaceAllUsesWith(Math);
   Cmp->replaceAllUsesWith(OV);
-  BO->eraseFromParent();
   Cmp->eraseFromParent();
+  BO->eraseFromParent();
   return true;
 }
 
@@ -1267,9 +1271,13 @@ bool CodeGenPrepare::combineToUAddWithOverflow(CmpInst *Cmp,
                                                bool &ModifiedDT) {
   Value *A, *B;
   BinaryOperator *Add;
-  if (!match(Cmp, m_UAddWithOverflow(m_Value(A), m_Value(B), m_BinOp(Add))))
+  if (!match(Cmp, m_UAddWithOverflow(m_Value(A), m_Value(B), m_BinOp(Add)))) {
     if (!matchUAddWithOverflowConstantEdgeCases(Cmp, Add))
       return false;
+    // Set A and B in case we match matchUAddWithOverflowConstantEdgeCases.
+    A = Add->getOperand(0);
+    B = Add->getOperand(1);
+  }
 
   if (!TLI->shouldFormOverflowOp(ISD::UADDO,
                                  TLI->getValueType(*DL, Add->getType()),
@@ -1282,7 +1290,8 @@ bool CodeGenPrepare::combineToUAddWithOverflow(CmpInst *Cmp,
   if (Add->getParent() != Cmp->getParent() && !Add->hasOneUse())
     return false;
 
-  if (!replaceMathCmpWithIntrinsic(Add, Cmp, Intrinsic::uadd_with_overflow))
+  if (!replaceMathCmpWithIntrinsic(Add, A, B, Cmp,
+                                   Intrinsic::uadd_with_overflow))
     return false;
 
   // Reset callers - do not crash by iterating over a dead instruction.
@@ -1344,7 +1353,8 @@ bool CodeGenPrepare::combineToUSubWithOverflow(CmpInst *Cmp,
                                  Sub->hasNUsesOrMore(2)))
     return false;
 
-  if (!replaceMathCmpWithIntrinsic(Sub, Cmp, Intrinsic::usub_with_overflow))
+  if (!replaceMathCmpWithIntrinsic(Sub, Sub->getOperand(0), Sub->getOperand(1),
+                                   Cmp, Intrinsic::usub_with_overflow))
     return false;
 
   // Reset callers - do not crash by iterating over a dead instruction.