[llvm] [TargetLowering] Change subtraction to do (LHS < RHS) XOR (RESULT < 0) (PR #150872)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: AZero13 (AZero13)

<details>
<summary>Changes</summary>

This folds better.

---
Full diff: https://github.com/llvm/llvm-project/pull/150872.diff


1 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp (+19-7) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 1764910861df4..cbd42251bc841 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -11447,13 +11447,25 @@ void TargetLowering::expandSADDSUBO(
   // For a subtraction, the result should be less than one of the operands
   // (LHS) if and only if the other operand (RHS) is (non-zero) positive,
   // otherwise there will be overflow.
-  SDValue ResultLowerThanLHS = DAG.getSetCC(dl, OType, Result, LHS, ISD::SETLT);
-  SDValue ConditionRHS =
-      DAG.getSetCC(dl, OType, RHS, Zero, IsAdd ? ISD::SETLT : ISD::SETGT);
-
-  Overflow = DAG.getBoolExtOrTrunc(
-      DAG.getNode(ISD::XOR, dl, OType, ConditionRHS, ResultLowerThanLHS), dl,
-      ResultType, ResultType);
+  
+  if (IsAdd) {
+    // For addition, the result should be less than one of the operands (LHS)
+    // if and only if the other operand (RHS) is negative, otherwise there will
+    // be overflow.
+    SDValue ResultLowerThanLHS = DAG.getSetCC(dl, OType, Result, LHS, ISD::SETLT);
+    SDValue ConditionRHS = DAG.getSetCC(dl, OType, RHS, Zero, ISD::SETLT);
+    Overflow = DAG.getBoolExtOrTrunc(
+        DAG.getNode(ISD::XOR, dl, OType, ConditionRHS, ResultLowerThanLHS), dl,
+        ResultType, ResultType);
+  } else {
+    // For subtraction, overflow occurs when the signed comparison of operands
+    // doesn't match the sign of the result
+    SDValue LHSLessThanRHS = DAG.getSetCC(dl, OType, LHS, RHS, ISD::SETLT);
+    SDValue ResultNegative = DAG.getSetCC(dl, OType, Result, Zero, ISD::SETLT);
+    Overflow = DAG.getBoolExtOrTrunc(
+        DAG.getNode(ISD::XOR, dl, OType, LHSLessThanRHS, ResultNegative), dl,
+        ResultType, ResultType);
+  }
 }
 
 bool TargetLowering::expandMULO(SDNode *Node, SDValue &Result,

``````````

</details>


https://github.com/llvm/llvm-project/pull/150872