[llvm] [DAG] Fold logic of zero-checks to multiplication for MinSize (PR #171805)

[via llvm-commits]

================
@@ -6648,6 +6649,58 @@ static unsigned getMinMaxOpcodeForFP(SDValue Operand1, SDValue Operand2,
   return ISD::DELETED_NODE;
 }
 
+// Fold the following patterns for small integers in -Oz mode.
+// (X == 0) || (Y == 0) --> (X * Y) == 0
+// (X != 0) && (Y != 0) --> (X * Y) != 0
+SDValue DAGCombiner::foldLogicSetCCToMul(SDNode *N, const SDLoc &DL) {
+  if (!DAG.getMachineFunction().getFunction().hasMinSize())
+    return SDValue();
+
+  unsigned Opcode = N->getOpcode();
+  SDValue N0 = N->getOperand(0);
+  SDValue N1 = N->getOperand(1);
+
+  ISD::CondCode ExpectedCC;
+  if (Opcode == ISD::OR) {
+    ExpectedCC = ISD::SETEQ;
+  } else if (Opcode == ISD::AND) {
+    ExpectedCC = ISD::SETNE;
+  } else {
+    return SDValue();
+  }
+
+  if (N0.getOpcode() != ISD::SETCC || N1.getOpcode() != ISD::SETCC)
+    return SDValue();
+
+  SDValue A = N0.getOperand(0);
+  SDValue B = N1.getOperand(0);
+  SDValue C0 = N0.getOperand(1);
+  SDValue C1 = N1.getOperand(1);
+  ISD::CondCode CC0 = cast<CondCodeSDNode>(N0.getOperand(2))->get();
+  ISD::CondCode CC1 = cast<CondCodeSDNode>(N1.getOperand(2))->get();
+
+  if (CC0 != ExpectedCC || CC1 != ExpectedCC ||
+      !A.getValueType().isScalarInteger() ||
+      A.getValueType() != B.getValueType() || !isNullConstant(C0) ||
+      !isNullConstant(C1))
+    return SDValue();
+
+  unsigned BitWidth = A.getValueSizeInBits();
+  KnownBits KnownA = DAG.computeKnownBits(A);
+  KnownBits KnownB = DAG.computeKnownBits(B);
+
+  if (KnownA.countMaxActiveBits() + KnownB.countMaxActiveBits() > BitWidth)
+    return SDValue();
+
+  SDNodeFlags Flags;
+  Flags.setNoUnsignedWrap(true);
+  Flags.setNoSignedWrap(true);
+
+  SDValue Mul = DAG.getNode(ISD::MUL, DL, A.getValueType(), A, B, Flags);
----------------
TelGome wrote:

I’ve added the legality check and updated the patch.

https://github.com/llvm/llvm-project/pull/171805