dwarf2 - multiple DW_TAG_variable for global variable

dwarf2 - multiple DW_TAG_variable for global variable

[Nenad Vukicevic]

I installed gcc-4.5-20091224 snapshot and noticed that for simple 
variable declaration
I get two DW_TAG_variable dies in the object file. For example, the 
following
code

int xxxxx;
main()
{xxxxx=1;}

generates (with -g -gdwarf2 -O0 switches):

<1><54>: Abbrev Number: 4 (DW_TAG_variable)
<55>   DW_AT_name        : (indirect string, offset: 0x36): xxxxx
<59>   DW_AT_decl_file   : 1
<5a>   DW_AT_decl_line   : 1
<5b>   DW_AT_type        : <0x4d>
<5f>   DW_AT_external    : 1
<60>   DW_AT_declaration : 1
<1><61>: Abbrev Number: 5 (DW_TAG_variable)
<62>   DW_AT_name        : (indirect string, offset: 0x36): xxxxx
<66>   DW_AT_decl_file   : 1
<67>   DW_AT_decl_line   : 1
<68>   DW_AT_type        : <0x4d>
<6c>   DW_AT_external    : 1
<6d>   DW_AT_location    : 9 byte block: 3 0 0 0 0 0 0 0 0  (DW_OP_addr: 0)

Is the above normal? 4.3.2 compiler generates only one die, the second 
one with
DW_AT_location attribute, which is correct.

I also noticed that this example (were variable is not used):

int xxxxx;
main()
{}

generates only one DW_TAG_variable, the one  with DW_AT_location, which 
again
should be correct.

I ran into this problem by porting some GDB code that uses DWARF2 and 
got surprised
to see this change from the previous version of gcc (4.3).

Thanks,
Nenad

Re: dwarf2 - multiple DW_TAG_variable for global variable

[Nenad Vukicevic]

This dwarf code started appearing since this patch:

------------------------------------------------------------------------
r145293 | jakub | 2009-03-30 14:35:03 +0000 (Mon, 30 Mar 2009) | 11 lines

         PR debug/39563
         * c-decl.c (struct c_binding): Add locus field.
         (bind): Add locus argument, set locus field from it.
         (pop_scope): For b->nested VAR_DECL or FUNCTION_DECL,
         add a DECL_EXTERNAL copy of b->decl to current BLOCK_VARS.
         (push_file_scope, pushtag, pushdecl, pushdecl_top_level,
         implicitly_declare, undeclared_variable, lookup_label,
         declare_label, c_make_fname_decl, c_builtin_function,
         c_builtin_function_ext_scope, store_parm_decls_newstyle): Adjust
         bind callers.

Jan, can you confirm that this is indeed the correct DWARF that is being generated.

Thank you,
Nenad



On 1/4/10 11:34 PM, Nenad Vukicevic wrote:
> I installed gcc-4.5-20091224 snapshot and noticed that for simple 
> variable declaration
> I get two DW_TAG_variable dies in the object file. For example, the 
> following
> code
>
> int xxxxx;
> main()
> {xxxxx=1;}
>
> generates (with -g -gdwarf2 -O0 switches):
>
> <1><54>: Abbrev Number: 4 (DW_TAG_variable)
> <55>   DW_AT_name        : (indirect string, offset: 0x36): xxxxx
> <59>   DW_AT_decl_file   : 1
> <5a>   DW_AT_decl_line   : 1
> <5b>   DW_AT_type        : <0x4d>
> <5f>   DW_AT_external    : 1
> <60>   DW_AT_declaration : 1
> <1><61>: Abbrev Number: 5 (DW_TAG_variable)
> <62>   DW_AT_name        : (indirect string, offset: 0x36): xxxxx
> <66>   DW_AT_decl_file   : 1
> <67>   DW_AT_decl_line   : 1
> <68>   DW_AT_type        : <0x4d>
> <6c>   DW_AT_external    : 1
> <6d>   DW_AT_location    : 9 byte block: 3 0 0 0 0 0 0 0 0  
> (DW_OP_addr: 0)
>
> Is the above normal? 4.3.2 compiler generates only one die, the second 
> one with
> DW_AT_location attribute, which is correct.
>
> I also noticed that this example (were variable is not used):
>
> int xxxxx;
> main()
> {}
>
> generates only one DW_TAG_variable, the one  with DW_AT_location, 
> which again
> should be correct.
>
> I ran into this problem by porting some GDB code that uses DWARF2 and 
> got surprised
> to see this change from the previous version of gcc (4.3).
>
> Thanks,
> Nenad
>

Re: dwarf2 - multiple DW_TAG_variable for global variable

[Gary Funck]

On 01/09/10 12:39:55, Nenad Vukicevic wrote:
> This dwarf code started appearing since this patch:

Here's the GCC bug report that led to this patch:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39563

It references a GDB fix as well:
http://sourceware.org/ml/gdb-patches/2009-03/threads.html#00595
http://sourceware.org/ml/gdb-patches/2009-04/threads.html#00040
http://sourceware.org/ml/gdb-cvs/2009-04/msg00021.html

Re: dwarf2 - multiple DW_TAG_variable for global variable

[Jan Kratochvil]

On Sat, 09 Jan 2010 22:01:54 +0100, Gary Funck wrote:
> On 01/09/10 12:39:55, Nenad Vukicevic wrote:
> > This dwarf code started appearing since this patch:
> 
> Here's the GCC bug report that led to this patch:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39563

Such DIEs duplicities are being tracked at:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39524

(Unaware how/if any were caused by the PR 39563.)


Regards,
Jan

Re: dwarf2 - multiple DW_TAG_variable for global variable

[Nenad Vukicevic]

We used GCC regression testing to pin point PR39563 when
multiple (but not equal) definitions started appearing in
dwarf code. We used the head version of GCC, gcc-4.5.20091224
to be precise, for testing this abnormally.

I also saw appearance of DIEs duplicates you mention in PR 39524
in the following example:

extern int xxxxx;
int main(){xxxxx=1;}

gcc 4.3.2 - does NOT have duplicates
gcc 4..4.1 20090725 (REDHAT 4.4.1-2) - does have duplicates
gcc 4.4.2 - does NOT have duplicates
gcc 4.5.20091224 - does have duplicates

Duplicates are in the form described in PR39524.

In the case of this code:

int xxxxx;
int main(){xxxxx=1;}

I see duplicates in the form of:

<1><54>: Abbrev Number: 4 (DW_TAG_variable)
<55>   DW_AT_name        : (indirect string, offset: 0x38): xxxxx
<59>   DW_AT_decl_file   : 1
<5a>   DW_AT_decl_line   : 1
<5b>   DW_AT_type        : <0x4d>
<5f>   DW_AT_external    : 1
<60>   DW_AT_declaration : 1
<1><61>: Abbrev Number: 5 (DW_TAG_variable)
<62>   DW_AT_name        : (indirect string, offset: 0x38): xxxxx
<66>   DW_AT_decl_file   : 1
<67>   DW_AT_decl_line   : 1
<68>   DW_AT_type        : <0x4d>
<6c>   DW_AT_external    : 1
<6d>   DW_AT_location    : 9 byte block: 3 0 0 0 0 0 0 0 0  (DW_OP_addr: 0)

in 4.4.1 and 4.5 releases.

Any idea if this is a correct dwarf? Or must be treated as a duplicate 
somehow?

Thanks,
Nenad

On 1/9/10 1:18 PM, Jan Kratochvil wrote:
> On Sat, 09 Jan 2010 22:01:54 +0100, Gary Funck wrote:
>    
>> On 01/09/10 12:39:55, Nenad Vukicevic wrote:
>>      
>>> This dwarf code started appearing since this patch:
>>>        
>> Here's the GCC bug report that led to this patch:
>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39563
>>      
> Such DIEs duplicities are being tracked at:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39524
>
> (Unaware how/if any were caused by the PR 39563.)
>
>
> Regards,
> Jan
>    

Re: dwarf2 - multiple DW_TAG_variable for global variable

[Jan Kratochvil]

On Sun, 10 Jan 2010 00:31:56 +0100, Nenad Vukicevic wrote:
...
> <1><54>: Abbrev Number: 4 (DW_TAG_variable)
> <55>   DW_AT_name        : (indirect string, offset: 0x38): xxxxx
> <59>   DW_AT_decl_file   : 1
> <5a>   DW_AT_decl_line   : 1
> <5b>   DW_AT_type        : <0x4d>
> <5f>   DW_AT_external    : 1
> <60>   DW_AT_declaration : 1
> <1><61>: Abbrev Number: 5 (DW_TAG_variable)
> <62>   DW_AT_name        : (indirect string, offset: 0x38): xxxxx
> <66>   DW_AT_decl_file   : 1
> <67>   DW_AT_decl_line   : 1
> <68>   DW_AT_type        : <0x4d>
> <6c>   DW_AT_external    : 1
> <6d>   DW_AT_location    : 9 byte block: 3 0 0 0 0 0 0 0 0  (DW_OP_addr: 0)
> 
> in 4.4.1 and 4.5 releases.
> 
> Any idea if this is a correct dwarf? Or must be treated as a
> duplicate somehow?

It does not hurt much GDB - if the debugger ignores the second definition it
looks as a declaration of external symbol and it gets correctly looked up
through the ELF (not DWARF) .symtab symbol without using DW_AT_location.

PR 39524 mentions a special scope case which gets broken by it but in common
cases it works fine.


Regards,
Jan