On Sat, Jun 17, 2017 at 9:09 AM, Siva Narayanan <sinaraya@xxxxxxxxxx> 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@xxxxxxxxxx> 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.
