[libunwind] [libunwind] Fix problems caused by combining BTI and GCS (PR #102322)

[Daniel Kiss via cfe-commits]

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@@ -680,7 +684,17 @@ DEFINE_LIBUNWIND_FUNCTION(__libunwind_Registers_arm64_jumpto)
   ldr    x16,     [x0, #0x0F8]
   ldp    x0, x1,  [x0, #0x000]  // restore x0,x1
   mov    sp,x16                 // restore sp
-  br     x30                    // jump to pc
+#if defined(__ARM_FEATURE_GCS_DEFAULT)
+  // If GCS is enabled we need to push the address we're returning to onto the
+  // GCS stack. We can't just return using br, as there won't be a BTI landing
+  // pad instruction at the destination.
+  mov      x16, #1
+  chkfeat  x16
+  cbnz     x16, Lnogcs
+  gcspushm x30
+Lnogcs:
+#endif
+  ret                           // jump to pc
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DanielKristofKiss wrote:

I'd keep spelling of x30 for sake of readability. (NFC)
```suggestion
  ret    x30                    // jump to pc
```

https://github.com/llvm/llvm-project/pull/102322