[Cc'ing Eric, as he is the compiler expert - GDB just represents what
the compiler tells us]

> Upstream gnat seems to create different debug data for string parameters
> on s390x than on x86, x86_64 or s390.  On s390x the DW_AT_type of a
> string parameter is a constant /reference/ to the two-element structure
> "string___XUP".  On the other platforms it is the constant structure,
> not a reference.

My first guess is that the variantion in type definition is caused
by differences in ABI. I don't know s390 platforms, so I can't
confirm offhand. But it seems like a plausible way to pass the
argument.

If Eric confirms my guess, the attached patch should fix the problem.

[the rest of your email is kept below - as a reference for Eric, in case
he needs it]

> Is the DW_AT_type output intentional?  Why does it depend on the
> architecture?  Note that for C or C++, gcc correctly declares structure
> parameters as /structures/ (not references) in their DW_AT_type.

> 
> If the output is intentional, I suggest to relax the pattern in the test
> case accordingly.  In any case it would be nice to differentiate the two
> "backtrace" test points, e.g. like this:
> 
> 
> --- a/gdb/testsuite/gdb.ada/lang_switch.exp
> +++ b/gdb/testsuite/gdb.ada/lang_switch.exp
> @@ -43,5 +43,5 @@ gdb_test "bt" \
>  gdb_test "set lang c"
>  gdb_test "bt" \
>           ".*#1.*lang_switch\\.ada_procedure\\s*\\(msg={.*\\).*" \
> -         "backtrace"
> +	 "backtrace (C)"

-- 
Joel