[llvm] [AArch64][NEON][SVE] Lower mixed sign/zero extended partial reductions to usdot (PR #107566)

[Paul Walker via llvm-commits]

================
@@ -21824,37 +21830,52 @@ SDValue tryLowerPartialReductionToDot(SDNode *N,
 
   auto ExtA = MulOp->getOperand(0);
   auto ExtB = MulOp->getOperand(1);
-  bool IsSExt = ExtA->getOpcode() == ISD::SIGN_EXTEND;
-  bool IsZExt = ExtA->getOpcode() == ISD::ZERO_EXTEND;
-  if (ExtA->getOpcode() != ExtB->getOpcode() || (!IsSExt && !IsZExt))
+
+  if (!ISD::isExtOpcode(ExtA->getOpcode()) ||
+      !ISD::isExtOpcode(ExtB->getOpcode()))
     return SDValue();
+  bool AIsSigned = ExtA->getOpcode() == ISD::SIGN_EXTEND;
+  bool BIsSigned = ExtB->getOpcode() == ISD::SIGN_EXTEND;
 
   auto A = ExtA->getOperand(0);
   auto B = ExtB->getOperand(0);
   if (A.getValueType() != B.getValueType())
     return SDValue();
 
+  EVT ReducedType = N->getValueType(0);
+  EVT MulSrcType = A.getValueType();
+
+  // Dot products operate on chunks of four elements so there must be four times
+  // as many elements in the wide type
+  if (!(ReducedType == MVT::nxv4i32 && MulSrcType == MVT::nxv16i8) &&
+      !(ReducedType == MVT::nxv2i64 && MulSrcType == MVT::nxv8i16) &&
+      !(ReducedType == MVT::v4i32 && MulSrcType == MVT::v16i8) &&
+      !(ReducedType == MVT::v2i32 && MulSrcType == MVT::v8i8))
+    return SDValue();
+
+  // If the extensions are mixed, we should lower it to a usdot instead
   unsigned Opcode = 0;
+  if (AIsSigned != BIsSigned) {
+    if (!Subtarget->hasMatMulInt8())
+      return SDValue();
+
+    bool Scalable = N->getValueType(0).isScalableVT();
+    // There's no nxv2i64 version of usdot
+    if (Scalable && ReducedType != MVT::nxv4i32)
+      return SDValue();
 
-  if (IsSExt)
+    Opcode = AArch64ISD::USDOT;
+    // USDOT expects the signed operand to be last
+    if (!BIsSigned)
+      std::swap(A, B);
+  } else if (AIsSigned)
     Opcode = AArch64ISD::SDOT;
-  else if (IsZExt)
+  else if (!AIsSigned)
----------------
paulwalker-arm wrote:

This should be just `else` given by this point we know `AIsSigned` has to be false.

https://github.com/llvm/llvm-project/pull/107566