[llvm] [DAGCombiner] Add generic DAG combine for ISD::PARTIAL_REDUCE_MLA (PR #127083)

[James Chesterman via llvm-commits]

================
@@ -12497,6 +12501,54 @@ SDValue DAGCombiner::visitMHISTOGRAM(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitPARTIAL_REDUCE_MLA(SDNode *N) {
+  // Makes PARTIAL_REDUCE_*MLA(Acc, MUL(ZEXT(MulOpLHS), ZEXT(MulOpRHS)),
+  // Splat(1)) into
+  // PARTIAL_REDUCE_UMLA(Acc, MulOpLHS, MulOpRHS).
+  // Makes PARTIAL_REDUCE_*MLA(Acc, MUL(SEXT(MulOpLHS), SEXT(MulOpRHS)),
+  // Splat(1)) into
+  // PARTIAL_REDUCE_SMLA(Acc, MulOpLHS, MulOpRHS).
+  SDLoc DL(N);
+
+  SDValue Op0 = N->getOperand(0);
+  SDValue Op1 = N->getOperand(1);
+
+  if (Op1->getOpcode() != ISD::MUL)
+    return SDValue();
+
+  APInt ConstantOne;
+  if (!ISD::isConstantSplatVector(N->getOperand(2).getNode(), ConstantOne) ||
+      !ConstantOne.isOne())
+    return SDValue();
+
+  SDValue ExtMulOpLHS = Op1->getOperand(0);
+  SDValue ExtMulOpRHS = Op1->getOperand(1);
+  unsigned ExtMulOpLHSOpcode = ExtMulOpLHS->getOpcode();
+  unsigned ExtMulOpRHSOpcode = ExtMulOpRHS->getOpcode();
+  if (!ISD::isExtOpcode(ExtMulOpLHSOpcode) ||
+      !ISD::isExtOpcode(ExtMulOpRHSOpcode))
+    return SDValue();
+
+  SDValue MulOpLHS = ExtMulOpLHS->getOperand(0);
+  SDValue MulOpRHS = ExtMulOpRHS->getOperand(0);
+  EVT MulOpLHSVT = MulOpLHS.getValueType();
+  if (MulOpLHSVT != MulOpRHS.getValueType())
+    return SDValue();
+
+  // FIXME: Add a check to only perform the DAG combine if there is lowering
+  // provided by the target
+
+  bool LHSIsSigned = ExtMulOpLHSOpcode == ISD::SIGN_EXTEND;
+  bool RHSIsSigned = ExtMulOpRHSOpcode == ISD::SIGN_EXTEND;
+  if (LHSIsSigned != RHSIsSigned)
----------------
JamesChesterman wrote:

Done, sorry about that, I misunderstood.

https://github.com/llvm/llvm-project/pull/127083