[committed][gdb/ada] Fix gdb.ada/dynamic-iface.exp with gcc 7

[Tom de Vries <tdevries@suse.de>]

On 5/13/22 10:36, Tom de Vries wrote:
> Hi,
> 
> This test in test-case gdb.ada/dynamic-iface.exp passes with gcc 8:
> ...
> (gdb) print obj^M
> $1 = (n => 3, a => "ABC", value => 93)^M
> (gdb) PASS: gdb.ada/dynamic-iface.exp: print local as interface
> ...
> but fails with gcc 7:
> ...
> (gdb) print obj^M
> $1 = ()^M
> (gdb) FAIL: gdb.ada/dynamic-iface.exp: print local as interface
> ...
> 
> More concretely, we have trouble finding the type of obj.  With gcc 8:
> ...
> $ gdb -q -batch main -ex "b concrete.adb:20" -ex run -ex "ptype obj"
>    ...
> type = <ref> new concrete.intermediate with record
>      value: integer;
> end record
> ...
> and with gcc 7:
> ...
> type = <ref> tagged record null; end record
> ...
> 
> The translation from tagged type to "full view" type happens in
> ada_tag_value_at_base_address, where we hit this code:
> ...
>    /* Storage_Offset'Last is used to indicate that a dynamic offset to
>       top is used.  In this situation the offset is stored just after
>       the tag, in the object itself.  */
>    if (offset_to_top == last)
>      {
>        struct value *tem = value_addr (tag);
>        tem = value_ptradd (tem, 1);
>        tem = value_cast (ptr_type, tem);
>        offset_to_top = value_as_long (value_ind (tem));
>      }
> ...
> resulting in an offset_to_top for gcc 8:
> ...
> (gdb) p offset_to_top
> $1 = -16
> ...
> and for gcc 7:
> ...
> (gdb) p offset_to_top
> $1 = 16
> ...
> 
> The difference is expected, it bisects to gcc commit d0567dc0dbf ("[multiple
> changes]") which mentions this change.
> 
> There's some code right after the code quoted above that deals with this
> change:
> ...
>    else if (offset_to_top > 0)
>      {
>        /* OFFSET_TO_TOP used to be a positive value to be subtracted
> 	 from the base address.  This was however incompatible with
> 	 C++ dispatch table: C++ uses a *negative* value to *add*
> 	 to the base address.  Ada's convention has therefore been
> 	 changed in GNAT 19.0w 20171023: since then, C++ and Ada
> 	 use the same convention.  Here, we support both cases by
> 	 checking the sign of OFFSET_TO_TOP.  */
>        offset_to_top = -offset_to_top;
>      }
> ...
> but it's not activated because of the 'else'.
> 
> Fix this by removing the 'else'.
> 
> Tested on x86_64-linux, with gcc 7.5.0.
> 
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29057
> 
> Any comments?
> 

Committed.

Thanks,
- Tom

> [gdb/ada] Fix gdb.ada/dynamic-iface.exp with gcc 7
> 
> ---
>   gdb/ada-lang.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/gdb/ada-lang.c b/gdb/ada-lang.c
> index 8333671c48b..e0a7a302e3a 100644
> --- a/gdb/ada-lang.c
> +++ b/gdb/ada-lang.c
> @@ -6492,7 +6492,8 @@ ada_tag_value_at_base_address (struct value *obj)
>         tem = value_cast (ptr_type, tem);
>         offset_to_top = value_as_long (value_ind (tem));
>       }
> -  else if (offset_to_top > 0)
> +
> +  if (offset_to_top > 0)
>       {
>         /* OFFSET_TO_TOP used to be a positive value to be subtracted
>   	 from the base address.  This was however incompatible with