[lldb][debugserver] Save and restore the SVE/SME register state (llvm#134184) (swiftlang#10418)

debugserver isn't saving and restoring the SVE/SME register state around
inferior function calls.

Making arbitrary function calls while in Streaming SVE mode is generally
a poor idea because a NEON instruction can be hit and crash the
expression execution, which is how I missed this, but they should be
handled correctly if the user knows it is safe to do.

Re-landing this change after fixing an incorrect behavior on systems
without SME support.

rdar://146886210
(cherry picked from commit f1c6612)

Author: jasonmolenda

lldb/tools/debugserver/source/MacOSX/arm64/DNBArchImplARM64.cpp

@@ -2952,8 +2952,15 @@ kern_return_t DNBArchMachARM64::SetRegisterState(int set) {
     return err;
 
   switch (set) {
-  case e_regSetALL:
-    return SetGPRState() | SetVFPState() | SetEXCState() | SetDBGState(false);
+  case e_regSetALL: {
+    kern_return_t ret =
+        SetGPRState() | SetVFPState() | SetEXCState() | SetDBGState(false);
+    if (CPUHasSME()) {
+      SetSVEState();
+      SetSMEState();
+    }
+    return ret;
+  }
   case e_regSetGPR:
     return SetGPRState();
   case e_regSetVFP:
@@ -3123,6 +3130,12 @@ uint32_t DNBArchMachARM64::SaveRegisterState() {
                                  "error: %s regs failed to read: %u",
                      "VFP", kret);
   } else {
+    if (CPUHasSME()) {
+      // These can fail when processor is not in streaming SVE mode,
+      // and that failure should be ignored.
+      GetSVEState(force);
+      GetSMEState(force);
+    }
     const uint32_t save_id = GetNextRegisterStateSaveID();
     m_saved_register_states[save_id] = m_state.context;
     return save_id;
@@ -3150,6 +3163,12 @@ bool DNBArchMachARM64::RestoreRegisterState(uint32_t save_id) {
                        save_id, "VFP", kret);
       success = false;
     }
+    if (CPUHasSME()) {
+      // These can fail when processor is not in streaming SVE mode,
+      // and that failure should be ignored.
+      SetSVEState();
+      SetSMEState();
+    }
     m_saved_register_states.erase(pos);
     return success;
   }