[llvm] [X86] Resolve FIXME: Accept live flag if no overflow occurs (PR #86836)

[Simon Pilgrim via llvm-commits]

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@@ -54522,14 +54522,27 @@ static SDValue combineADC(SDNode *N, SelectionDAG &DAG,
   }
 
   // Fold ADC(C1,C2,Carry) -> ADC(0,C1+C2,Carry)
-  // iff the flag result is dead.
-  // TODO: Allow flag result if C1+C2 doesn't signed/unsigned overflow.
-  if (LHSC && RHSC && !LHSC->isZero() && !N->hasAnyUseOfValue(1)) {
-    SDLoc DL(N);
-    APInt Sum = LHSC->getAPIntValue() + RHSC->getAPIntValue();
-    return DAG.getNode(X86ISD::ADC, DL, N->getVTList(),
-                       DAG.getConstant(0, DL, LHS.getValueType()),
-                       DAG.getConstant(Sum, DL, LHS.getValueType()), CarryIn);
+  // if the flag result is dead, or if C1+C2 doesn't signed/unsigned overflow.
+  if (LHSC && RHSC && !LHSC->isZero()) {
+    // Calculate the sum and check for overflow.
+    bool Overflow;
+
+    // Check for signed overflow first. Because it is more involved, let sadd_ov
+    // do the work
+    APInt Sum = LHSC->getAPIntValue().sadd_ov(RHSC->getAPIntValue(), Overflow);
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RKSimon wrote:

Looks like we'd benefit from a `APInt::add_ov(const APInt &RHS, bool &SignedOverflow, bool &UnsignedOverflow)` wtapper helper

https://github.com/llvm/llvm-project/pull/86836