[libunwind] [libunwind][AArch64] Disable ZA before resuming from unwinding (on Linux) (PR #165451)

[Sander de Smalen via cfe-commits]

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@@ -827,6 +827,53 @@ DEFINE_LIBUNWIND_FUNCTION(__unw_getcontext)
   ret
 #endif
 
+//
+// extern "C" bool __libunwind_Registers_arm64_za_disable()
+//
+// On return:
+//  success (true/false) is returned in x0
+//
+  .p2align 2
+DEFINE_LIBUNWIND_FUNCTION(__libunwind_Registers_arm64_za_disable)
+  // If TPIDR2_EL0 is null, the subroutine does nothing.
+  .inst 0xd53bd0b0 // mrs x16, TPIDR2_EL0
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sdesmalen-arm wrote:

>From the [docs](https://developer.arm.com/documentation/ddi0601/2025-09/AArch64-Registers/TPIDR2-EL0--EL0-Read-Write-Software-Thread-ID-Register-2):

> This register is present only when FEAT_SME is implemented and FEAT_AA64 is implemented. Otherwise, direct accesses to TPIDR2_EL0 are UNDEFINED

So this function assumes that SME is available, which is fine given how it's used (because it's only called when `_misc_registers.__has_sme` is non-zero).

Can you add a comment to the description of the function that this assumption is made and that this function should not be called if SME may not be available?

https://github.com/llvm/llvm-project/pull/165451