[llvm] [AArch64][SME2] Add FORM_STRIDED_TUPLE pseudo nodes (PR #116399)

[Kerry McLaughlin via llvm-commits]

================
@@ -8666,6 +8671,77 @@ void AArch64TargetLowering::AdjustInstrPostInstrSelection(MachineInstr &MI,
     }
   }
 
+  if (MI.getOpcode() == AArch64::FORM_STRIDED_TUPLE_X2_PSEUDO ||
+      MI.getOpcode() == AArch64::FORM_STRIDED_TUPLE_X4_PSEUDO) {
+    MachineRegisterInfo &MRI = MI.getMF()->getRegInfo();
+    bool UseFormStrided = false;
+    unsigned Size =
+        MI.getOpcode() == AArch64::FORM_STRIDED_TUPLE_X2_PSEUDO ? 2 : 4;
+
+    // The FORM_STRIDED_TUPLE pseudo should only be used if the input operands
+    // are copy nodes where the source register is in a StridedOrContiguous
+    // class. For example:
+    //   %3:zpr2stridedorcontiguous = LD1B_2Z_IMM_PSEUDO ..
+    //   %4:zpr = COPY %3.zsub1:zpr2stridedorcontiguous
+    //   %5:zpr = COPY %3.zsub0:zpr2stridedorcontiguous
+    //   %6:zpr2stridedorcontiguous = LD1B_2Z_PSEUDO ..
+    //   %7:zpr = COPY %6.zsub1:zpr2stridedorcontiguous
+    //   %8:zpr = COPY %6.zsub0:zpr2stridedorcontiguous
+    //   %9:zpr2mul2 = FORM_STRIDED_TUPLE_X2_PSEUDO %5:zpr, %8:zpr
+
+    SmallVector<unsigned, 4> OpSubRegs;
+    for (unsigned I = 1; I < MI.getNumOperands(); ++I) {
+      MachineOperand &MO = MI.getOperand(I);
+      if (!MO.isReg())
+        continue;
+
+      MachineOperand *Def = MRI.getOneDef(MO.getReg());
+      if (!Def || !Def->isReg() || !Def->getParent()->isCopy())
+        continue;
+
+      MachineInstr *Cpy = Def->getParent();
+      MachineOperand CpyOp = Cpy->getOperand(1);
+      if (!CpyOp.isReg())
+        continue;
+
+      MachineOperand *Ld = MRI.getOneDef(CpyOp.getReg());
+      OpSubRegs.push_back(CpyOp.getSubReg());
+      if (!Ld || !Ld->isReg())
+        continue;
+
+      const TargetRegisterClass *RegClass =
+          Size == 2 ? &AArch64::ZPR2StridedOrContiguousRegClass
+                    : &AArch64::ZPR4StridedOrContiguousRegClass;
+
+      if (MRI.getRegClass(Ld->getReg()) == RegClass)
+        UseFormStrided = true;
----------------
kmclaughlin-arm wrote:

I've moved this to a new function called `shouldUseFormStridedPseudo()` & removed the `std::equal` test.

https://github.com/llvm/llvm-project/pull/116399