[DAG] Add SDPatternMatch::m_Result to match a specific SDValue result (llvm#145775)

`m_Result<N>` matches a SDValue that is the N-th result of the defining
SDNode. This is useful for creating a more fine-grained matching on
SDNode with multiple results.

-----
Inspired by llvm#145481

Author: mshockwave

llvm/include/llvm/CodeGen/SDPatternMatch.h

@@ -108,6 +108,23 @@ inline Value_match m_Specific(SDValue N) {
   return Value_match(N);
 }
 
+template <unsigned ResNo, typename Pattern> struct Result_match {
+  Pattern P;
+
+  explicit Result_match(const Pattern &P) : P(P) {}
+
+  template <typename MatchContext>
+  bool match(const MatchContext &Ctx, SDValue N) {
+    return N.getResNo() == ResNo && P.match(Ctx, N);
+  }
+};
+
+/// Match only if the SDValue is a certain result at ResNo.
+template <unsigned ResNo, typename Pattern>
+inline Result_match<ResNo, Pattern> m_Result(const Pattern &P) {
+  return Result_match<ResNo, Pattern>(P);
+}
+
 struct DeferredValue_match {
   SDValue &MatchVal;
 

llvm/unittests/CodeGen/SelectionDAGPatternMatchTest.cpp

@@ -264,6 +264,11 @@ TEST_F(SelectionDAGPatternMatchTest, matchBinaryOp) {
   SDValue Rotl = DAG->getNode(ISD::ROTL, DL, Int32VT, Op0, Op1);
   SDValue Rotr = DAG->getNode(ISD::ROTR, DL, Int32VT, Op1, Op0);
 
+  SDValue SMulLoHi = DAG->getNode(ISD::SMUL_LOHI, DL,
+                                  DAG->getVTList(Int32VT, Int32VT), Op0, Op1);
+  SDValue PartsDiff =
+      DAG->getNode(ISD::SUB, DL, Int32VT, SMulLoHi, SMulLoHi.getValue(1));
+
   SDValue ICMP_GT = DAG->getSetCC(DL, MVT::i1, Op0, Op1, ISD::SETGT);
   SDValue ICMP_GE = DAG->getSetCC(DL, MVT::i1, Op0, Op1, ISD::SETGE);
   SDValue ICMP_UGT = DAG->getSetCC(DL, MVT::i1, Op0, Op1, ISD::SETUGT);
@@ -347,6 +352,15 @@ TEST_F(SelectionDAGPatternMatchTest, matchBinaryOp) {
   EXPECT_TRUE(sd_match(UMinLikeULT, m_UMinLike(m_Value(), m_Value())));
   EXPECT_TRUE(sd_match(UMinLikeULE, m_UMinLike(m_Value(), m_Value())));
 
+  // By default, it matches any of the results.
+  EXPECT_TRUE(
+      sd_match(PartsDiff, m_Sub(m_Opc(ISD::SMUL_LOHI), m_Opc(ISD::SMUL_LOHI))));
+  // Matching a specific result.
+  EXPECT_TRUE(sd_match(PartsDiff, m_Sub(m_Opc(ISD::SMUL_LOHI),
+                                        m_Result<1>(m_Opc(ISD::SMUL_LOHI)))));
+  EXPECT_FALSE(sd_match(PartsDiff, m_Sub(m_Opc(ISD::SMUL_LOHI),
+                                         m_Result<0>(m_Opc(ISD::SMUL_LOHI)))));
+
   SDValue BindVal;
   EXPECT_TRUE(sd_match(SFAdd, m_ChainedBinOp(ISD::STRICT_FADD, m_Value(BindVal),
                                              m_Deferred(BindVal))));