[Bug target/85492] riscv64: endless loop when throwing an exception from a constructor

["aurelien at aurel32 dot net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85492

--- Comment #7 from Aurelien Jarno <aurelien at aurel32 dot net> ---
(In reply to Jim Wilson from comment #3)
Thanks a lot Jim for finding out the issue without using gdb, that was quite a
challenge.

(In reply to Jim Wilson from comment #6)
> I suggest you handle the glibc patch.

Should I just close this bug and open a new one on the glibc side?

> Note that you can probably also fix this by adding unwind direcives to
> _start to say that the return address is in x0.  This would avoid the minor
> code size increase, but takes a little more effort to figure out how to add
> the right unwind directives to assembly code to make this work.  I haven't
> tried that.

I think this should be done with the cfi_undefined directive, like in the patch
below:

--- a/sysdeps/riscv/start.S
+++ b/sysdeps/riscv/start.S
@@ -43,6 +43,8 @@
    __libc_start_main wants this in a5.  */

 ENTRY (ENTRY_POINT)
+       /* Mark ra as undefined in order to stop unwinding here!  */
+       cfi_undefined (ra)
        call  .Lload_gp
        mv    a5, a0  /* rtld_fini.  */
        /* main may be in a shared library.  */