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Re: Question about extra debug (DWARF) info generated by gcc6/7
On Sat, Jun 17, 2017 at 9:09 AM, Siva Narayanan <sinaraya@arista.com> wrote:
> Hello all,
>
> My colleague Peter managed to use "git bisect" on the git mirror of
> gcc source code. The commit from a branch called "debug-early" seems
> like a possible point when the new behavior got introduced.
>
> We have also looked at this conversation:
> https://gcc.gnu.org/ml/gcc/2015-05/msg00046.html
>
> We would like to know if this work:
>
>>Shortly after the merge I'll work on a pass to prune unused decl DIEs as >we're presently creating more DIEs than mainline. This was expected, >and if I understood Jason correctly, it is ok to work on this post-merge. >However, even without such a pass, the .debug_info size difference is >reasonable:
>
> happened along with a future commit or if there is still some work
> pending. As I wrote, in our environment the debug_info size difference
> (from earlier 5.4 version of gcc) is significant that the object file
> size change is non-trivial with gcc 6.3.1.
To my knowledge this hasn't happened yet.
Richard.
> We appreciate any input and help.
>
> Thanks.
>
> On Thu, Jun 15, 2017 at 2:45 PM, Siva Narayanan <sinaraya@arista.com> wrote:
>> Hi,
>>
>> With a test code like so:
>>
>> struct foo {
>> int fooMember;
>> };
>>
>> extern struct foo fooDeclared;
>>
>> void asymbol() { }
>>
>> We find that gcc6 (or even gcc7) generates a lot more debug
>> information. "readelf -wi <.o file>" shows the following.
>>
>> <1><2d>: Abbrev Number: 2 (DW_TAG_structure_type)
>> <2e> DW_AT_name : foo
>> <32> DW_AT_byte_size : 4
>> <33> DW_AT_decl_file : 1
>> <34> DW_AT_decl_line : 1
>> <35> DW_AT_sibling : <0x46>
>> <2><39>: Abbrev Number: 3 (DW_TAG_member)
>> <3a> DW_AT_name : (indirect string, offset: 0xc): fooMember
>> <3e> DW_AT_decl_file : 1
>> <3f> DW_AT_decl_line : 2
>> <40> DW_AT_type : <0x46>
>> <44> DW_AT_data_member_location: 0
>> <2><45>: Abbrev Number: 0
>> <1><46>: Abbrev Number: 4 (DW_TAG_base_type)
>> <47> DW_AT_byte_size : 4
>> <48> DW_AT_encoding : 5 (signed)
>> <49> DW_AT_name : int
>> <1><4d>: Abbrev Number: 5 (DW_TAG_variable)
>> <4e> DW_AT_name : (indirect string, offset: 0x0): fooDeclared
>> <52> DW_AT_decl_file : 1
>> <53> DW_AT_decl_line : 5
>> <54> DW_AT_type : <0x2d>
>> <58> DW_AT_external : 1
>> <58> DW_AT_declaration : 1
>>
>> The compilation line is: "gcc -o t.o -c -g t.c" in all attempts.
>>
>> It is not at all obvious why there is debug information being
>> generated for an unused "extern" (struct foo, "fooDeclared")
>>
>> We don't observe this with gcc4 or gcc5. In our environment this seems
>> to result in the non-trivial increase of object file size (on disk),
>> resulting in other downstream issues.
>>
>> So we are trying to understand if some config option has some how
>> become default.
>>
>> gcc 5.4 version, built from Fedora based sources: gcc version 5.4.0
>> 20160603 (Red Hat 5.4.0-6) (GCC) - does not produce this behavior.
>>
>> gcc 6.3 version which exhibits this new behavior: gcc version 6.3.1
>> 20170216 (Red Hat 6.3.1-3) (GCC). We picked the source rpm for this
>> from F25.
>>
>> We also tested on gcc 6.2 (Ubuntu Yakkety, and observed the same new
>> behavior): gcc version 6.2.0 20161005 (Ubuntu 6.2.0-5ubuntu12)
>>
>> Many thanks.