[llvm] [AArch64][SME] Support saving/restoring ZT0 in the MachineSMEABIPass (PR #166362)

[Sander de Smalen via llvm-commits]

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@@ -221,18 +248,39 @@ static bool isZAorZTRegOp(const TargetRegisterInfo &TRI,
 /// Returns the required ZA state needed before \p MI and an iterator pointing
 /// to where any code required to change the ZA state should be inserted.
 static std::pair<ZAState, MachineBasicBlock::iterator>
-getZAStateBeforeInst(const TargetRegisterInfo &TRI, MachineInstr &MI,
-                     bool ZAOffAtReturn) {
+getInstNeededZAState(const TargetRegisterInfo &TRI, MachineInstr &MI,
+                     SMEAttrs SMEFnAttrs) {
   MachineBasicBlock::iterator InsertPt(MI);
 
+  // Note: InOutZAUsePseudo, RequiresZASavePseudo, and RequiresZT0SavePseudo are
+  // intended to mark the position immediately before a call. Due to
+  // SelectionDAG constraints, these markers occur after the ADJCALLSTACKDOWN,
+  // so we use std::prev(InsertPt) to get the position before the call.
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sdesmalen-arm wrote:

nit: is it worth adding an assert that `std::prev(InsertPt) == ADJCALLSTACKDOWN` ?

https://github.com/llvm/llvm-project/pull/166362