[compiler-rt] [libcxxabi] [libunwind] [runtimes][PAC] Harden unwinding when possible (#138571) (PR #143230)

[Oliver Hunt via llvm-commits]

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@@ -807,7 +812,12 @@ DEFINE_LIBUNWIND_FUNCTION(__unw_getcontext)
   str    d31,     [x0, #0x208]
 #endif
   mov    x0, #0                   // return UNW_ESUCCESS
+
+#if __has_feature(ptrauth_calls)
+  retab
+#else
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ojhunt wrote:

This is what we have downstream, it's possible we've just developed an assumption that they're equivalent at this point.

That said I'm actually thinking this should also be around an `if __APPLE__` as well pending adoption of/a more generic gate on the ABI.

https://github.com/llvm/llvm-project/pull/143230