Re: GDB returns wrong type when traversing optimized-out Fields

[Roman Popov <ripopov at gmail dot com>]

Thanks Simon,

Your patch solves the issue for all of my clients!

-Roman

2018-02-04 21:39 GMT-08:00 Simon Marchi <simark@simark.ca>:

> On 2018-02-04 11:06 PM, Roman Popov wrote:
> > I apologize for code typo in previous email. Here is correct code sample:
> >
> > template <unsigned v1, unsigned v2>
> > struct TRAITS {
> >     static const unsigned val1 = v1;
> >     static const unsigned val2 = v2;
> > };
> > template < class TRAITS >
> > struct foo {
> >     static const unsigned x1 = TRAITS::val1;
> >     static const unsigned x2 = TRAITS::val2;
> > };
> >
> > int main () {
> >     foo<TRAITS<1,2>> f1;
> >     // SET BREAKPOINT HERE
> >     return 0;
> > }
> >
> > -Roman
> >
> >
> > 2018-02-04 20:02 GMT-08:00 Roman Popov <ripopov@gmail.com>:
> >
> >> Hi all,
> >> I've encountered strange GDB behavior when requesting a value of
> >> optimized-away field.
> >> Instead of returning None or raising exception, GDB returns an
> >> optimizied-out value of wrong type.
> >>
> >> Here is a small reproducer *optimize_out.cpp*:
> >>
> >> template <unsigned v1, unsigned v2>
> >> struct TRAITS {
> >>     static const unsigned val1 = v1;
> >>     static const unsigned val2 = v2;
> >> };
> >> template < class TRAITS >
> >> struct foo {
> >>     static const unsigned x1 = TRAITS::v1;
> >>     static const unsigned x2 = TRAITS::v2;
> >> };
> >>
> >> int main () {
> >>     foo<TRAITS<1,2>> f1;
> >>     // SET BREAKPOINT HERE
> >>     return 0;
> >> }
> >>
> >> # Using g++ 7.3
> >> $ g++ -g optimize_out.cpp
> >>
> >> # Using gdb 8.1
> >> $ gdb a.out
> >>
> >> (gdb) break optimize_out.cpp:14
> >> (gdb) r
> >> (gdb) p f1
> >> $1 = {static x1 = <optimized out>, static x2 = <optimized out>}
> >>
> >> Ok, looks good.  Now traverse fields:
> >>
> >> (gdb) python
> >>> f1 = gdb.parse_and_eval("f1")
> >>> for field in f1.type.fields():
> >>>    print ("field name: ", field.name, "field type: ", field.type)
> >>>    field_val = f1[field]
> >>>    print ("optout?: ",field_val.is_optimized_out, "type:
> >> ",field_val.type)
> >>> end
> >> field name:  x1 field type:  const unsigned int
> >> optout?:  True type:  foo<TRAITS<1, 2> >
> >> field name:  x2 field type:  const unsigned int
> >> optout?:  True type:  foo<TRAITS<1, 2> >
> >>
> >>
> >>
> >> So type we get is foo<TRAITS<1, 2> >, not unsigned int.
> >>
> >> Looks like GDB-MI has same behavior. At least this code sample totatlly
> >> confuses GDB GUI I use.
> >>
> >> Thanks,
> >> Roman
> >>
>
> Hi Roman,
>
> It just seems to be that we return a value with the type of the structure
> instead of the type of the field in an error path somewhere.   It causes
> these weird behaviors in the CLI too:
>
> (gdb) p f1
> $1 = {static x1 = <optimized out>, static x2 = <optimized out>}
> (gdb) p f1.x1
> $2 = <optimized out>
> (gdb) ptype f1.x1
> type = struct foo<TRAITS<1, 2> > [with TRAITS = TRAITS<1, 2>] {
>     static const unsigned int x1;
>     static const unsigned int x2;
> }
> (gdb) ptype f1.x1.x1
> type = struct foo<TRAITS<1, 2> > [with TRAITS = TRAITS<1, 2>] {
>     static const unsigned int x1;
>     static const unsigned int x2;
> }
> (gdb) p f1.x1.x1.x1.x1
> $3 = <optimized out>
>
> Can you try the patch below?  I did not run the testsuite on it.
>
>
> From ef7f73557e291ed2d8bc7f175765bdbb91e2c817 Mon Sep 17 00:00:00 2001
> From: Simon Marchi <simon.marchi@polymtl.ca>
> Date: Mon, 5 Feb 2018 00:34:08 -0500
> Subject: [PATCH] Return right type in value_static_field
>
> ---
>  gdb/value.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/gdb/value.c b/gdb/value.c
> index 9a144fb7fb..063f57129a 100644
> --- a/gdb/value.c
> +++ b/gdb/value.c
> @@ -2978,7 +2978,10 @@ value_static_field (struct type *type, int fieldno)
>             = lookup_minimal_symbol (phys_name, NULL, NULL);
>
>           if (!msym.minsym)
> -           return allocate_optimized_out_value (type);
> +           {
> +             struct type *field_type = TYPE_FIELD_TYPE (type, fieldno);
> +             return allocate_optimized_out_value (field_type);
> +           }
>           else
>             {
>               retval = value_at_lazy (TYPE_FIELD_TYPE (type, fieldno),
> --
> 2.16.1
>
>
>