[llvm] [AArch64][SME] Use entry pstate.sm for conditional streaming-mode changes (PR #152169)

[Sander de Smalen via llvm-commits]

================
@@ -3101,6 +3101,32 @@ AArch64TargetLowering::EmitGetSMESaveSize(MachineInstr &MI,
   return BB;
 }
 
+MachineBasicBlock *
+AArch64TargetLowering::EmitEntryPStateSM(MachineInstr &MI,
+                                         MachineBasicBlock *BB) const {
+  MachineFunction *MF = BB->getParent();
+  AArch64FunctionInfo *FuncInfo = MF->getInfo<AArch64FunctionInfo>();
+  const TargetInstrInfo *TII = Subtarget->getInstrInfo();
+  if (FuncInfo->IsPStateSMRegUsed()) {
+    const AArch64RegisterInfo *TRI = Subtarget->getRegisterInfo();
+    BuildMI(*BB, MI, MI.getDebugLoc(), TII->get(AArch64::BL))
+        .addExternalSymbol("__arm_sme_state")
+        .addReg(AArch64::X0, RegState::ImplicitDefine)
+        .addRegMask(TRI->getCallPreservedMask(
+            *MF, CallingConv::
+                     AArch64_SME_ABI_Support_Routines_PreserveMost_From_X2));
+    BuildMI(*BB, MI, MI.getDebugLoc(), TII->get(TargetOpcode::COPY),
+            MI.getOperand(0).getReg())
+        .addReg(AArch64::X0);
+  } else {
+    BuildMI(*BB, MI, MI.getDebugLoc(), TII->get(TargetOpcode::COPY),
+            MI.getOperand(0).getReg())
+        .addReg(AArch64::XZR);
----------------
sdesmalen-arm wrote:

If the register isn't used, could be defined with `IMPLICIT_DEF` instead? And does it actually need to be defined at all, if it has no uses?

https://github.com/llvm/llvm-project/pull/152169