[libunwind] [Libunwind] Try to fix msan failures (PR #120013)

[Alexander Richardson via cfe-commits]

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@@ -133,6 +133,10 @@ _LIBUNWIND_EXPORT _Unwind_Reason_Code
 _Unwind_Backtrace(_Unwind_Trace_Fn callback, void *ref) {
   unw_cursor_t cursor;
   unw_context_t uc;
+#if __has_feature(memory_sanitizer)
+  __builtin_memset(&cursor, 0, sizeof(cursor));
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arichardson wrote:

I'm not keen on working around the missing msan annotations on every call. This seems fragile.

https://github.com/llvm/llvm-project/pull/120013