[Bug debug/78839] [6/7 Regression] DWARF output different between GCC 5 and 6

[toconnor at forcepoint dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78839

--- Comment #10 from Tom O'Connor  ---
In regards to GDB, I noticed that when using a macro to get the offset for
these bitfields, with objects built by both GCC 5 and earlier as well as GCC 6,
I always get 0.  For example:

(gdb) macro define offsetof(t, f) &((t *) 0)->f)
(gdb) p offsetof(struct s, type)
$1 = (unsigned int *) 0x0

Historically, prior to GCC 6, this always matched up with DWARF's
DW_AT_data_member_location value for bitfields.  Yes, I see how GCC 6's
combination of data_member_location and bit_offset can be combined to reach the
true location.  Historically, my use case has used the data_member_location
value to get me the offset into say a raw memory image, for where to start
reading data from a field of a structure in memory; I'd do bitfield shifts
after reading from the aligned offset as reported in the DWARF structure.

If using DECL_BIT_FIELD_REPRESENTATIVE as data_member_location is nicer and
doesn't break the Ada use case, I'd be all for that.  But if what's currently
being emitted by GCC 6 is considered "more correct" than what previous versions
had emitted, then I'll have to adjust.  I only noticed the change between 5 and
6 once I started seeing odd results with the same code built by different GCCs.

[Bug other/78839] DWARF output different between GCC 5 and 6

[toconnor at forcepoint dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78839

--- Comment #6 from Tom O'Connor  ---
(In reply to Andrew Pinski from comment #5)
> (In reply to Tom O'Connor from comment #4)
> > Bitfields seemed to work fine in all GCC prior to 6, FWIW.  The same
> > attached source code when built with other GCCs prior to 6 generates
> > DW_AT_data_member_location = 0 for all the fields as expected.  Only 6 gets
> > some of the fields "wrong".
> 
> Maybe I am thinking big-endian :).

Ah, I've only observed this on little-endian, as that's the only place I've
looked.

[Bug other/78839] DWARF output different between GCC 5 and 6

[toconnor at forcepoint dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78839

--- Comment #4 from Tom O'Connor  ---
Bitfields seemed to work fine in all GCC prior to 6, FWIW.  The same attached
source code when built with other GCCs prior to 6 generates
DW_AT_data_member_location = 0 for all the fields as expected.  Only 6 gets
some of the fields "wrong".

[Bug other/78839] DWARF output different between GCC 5 and 6

[toconnor at forcepoint dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78839

--- Comment #2 from Tom O'Connor  ---
Created attachment 40355
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40355=edit
debuginfo from a GCC 6.1.0 build of s.c

[Bug other/78839] DWARF output different between GCC 5 and 6

[toconnor at forcepoint dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78839

--- Comment #1 from Tom O'Connor  ---
Created attachment 40354
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40354=edit
debuginfo from a GCC 5.4.0 build of s.c

[Bug other/78839] New: DWARF output different between GCC 5 and 6

[toconnor at forcepoint dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78839

Bug ID: 78839
   Summary: DWARF output different between GCC 5 and 6
   Product: gcc
   Version: 6.1.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: other
  Assignee: unassigned at gcc dot gnu.org
  Reporter: toconnor at forcepoint dot com
  Target Milestone: ---

Created attachment 40353
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40353=edit
sample c code

I've observed a difference in DWARF output for the same input source code
between GCC 5 and GCC 6, specifically in the DW_AT_data_member_location values
for bitfield members of structs. For example,

s.c:
struct s {
  unsigned int shutdown:2,
no_check_tx:1,
no_check_rx:1,
userlocks:4,
protocol:8,
type:16;
};

int main() {
  struct s s;
  return 0;
}


GCC 5 reports DW_AT_data_member_location for all the fields as 0, which is
expected.
GCC 6 reports DW_AT_data_member_location for the "type" and "protocol" fields
as 2 and 1 respectively, all other fields 0 as expected. The DW_AT_bt_offset
fields for "type" and "protocol" change as well.

According to a mailing list posting, this is related to this change: 
http://gcc.gnu.org/r231762