[libunwind] [libunwind] Call `__arm_za_disable` before entering EH (PR #160905)

[Sander de Smalen via cfe-commits]

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@@ -198,6 +202,29 @@ unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor,
   _LIBUNWIND_TRACE_UNWINDING("unwind_phase2(ex_obj=%p)",
                              (void *)exception_object);
 
+#if defined(__aarch64__)
+  // The platform must ensure that all the following conditions are true on
+  // entry to EH:
+  //
+  // - PSTATE.SM is 0.
+  // - PSTATE.ZA is 0.
+  // - TPIDR2_EL0 is null.
+  //
+  // The first point is ensured by routines for throwing exceptions having a
+  // non-streaming interface. TPIDR2_EL0 is set to null and ZA disabled by
+  // calling __arm_za_disable.
+  //
+  // See:
+  // https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#exceptions
+  if (__arm_za_disable) {
+    // FIXME: Is SME is available and `__arm_za_disable` is not, this should
+    // abort.
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sdesmalen-arm wrote:

nit: s/Is/If/
nit: this belongs in the `else` branch?

https://github.com/llvm/llvm-project/pull/160905