Re: [RFA] Work around binutils/15021

Jan Kratochvil
2013-06-08T16:04:30Z

Hello Doug,

On Thu, 17 Jan 2013 00:21:29 +0100, Doug Evans wrote:
> + This is also used to work around a difference between the way gold
> + generates .gdb_index version <=7 and the way gdb does. Arguably this
> + is a gold bug. For symbols coming from TUs, gold records in the index
> + the CU that includes the TU instead of the TU itself. This breaks
> + dw2_lookup_symbol: It assumes that if the index says symbol X lives
> + in CU/TU Y, then one need only expand Y and a subsequent lookup in Y
> + will find X. Alas TUs live in their own symtab, so after expanding CU Y
> + we need to look in TU Z to find X. Fortunately, this is akin to
> + DW_TAG_imported_unit, so we just use the same mechanism: For
> + .gdb_index version <=7 this also records the TUs that the CU referred
> + to. Concurrently with this change gdb was modified to emit version 8
> + indices so we only pay a price for gold generated indices. */

what is the gold bug?

According to the description I would expect from gold a line
[773] C: 1 [no symbol information]

but there is a line similar to what contains a gdb-produced index 7/8.
[773] C: T0 [no symbol information]

dwz currently supports at most version 7 so I am not sure if there is any
change needed in dwz for version 8.

Thanks,
Jan

echo 'void f() {}' >tu0.C;echo 'class C {} c; int main() {}' >tu.C;for i in tu0 tu;do g++ -c -o $i.o $i.C -Wall -g -fdebug-types-section;done;g++ -o tu tu0.o tu.o -Wall;ld.gold -o tugold tu0.o tu.o --gdb-index;cp tu tu7;~/redhat/gdb-test-gdbindex7/gdb-add-index ./tu7;cp tu tu8;gdb-add-index tu8

==> tu0.C <== --- CU 0
void f() {}

==> tu.C <== --- CU 1
class C {} c; int main() {}

readelf --debug-dump=gdb_index tu{7,8} # There is no difference in these
------------------------------------------------------------------------------
Contents of the .gdb_index section:
Version {7,8}

CU table:
[ 0] 0x0 - 0x48
[ 1] 0x49 - 0xb1

TU table:
[ 0] 0x0 0x25 e5fcb7a195b2f213

Address table:
00000000004005b0 00000000004005b6 0
00000000004005b8 00000000004005c3 1

Symbol table:
[489] main: 1 [global function]
[754] int: 1 [static type]
[773] C: T0 [global type]
[1010] c: 1 [global variable]
[1013] f: 0 [global function]
------------------------------------------------------------------------------

readelf --debug-dump=gdb_index tugold
------------------------------------------------------------------------------
Contents of the .gdb_index section:
Version 5

CU table:
[ 0] 0x0 - 0x48
[ 1] 0x49 - 0xb1

TU table:
[ 0] 0x0 0x25 e5fcb7a195b2f213

Address table:
00000000004000e8 00000000004000ee 0
00000000004000f0 00000000004000fb 1

Symbol table:
[489] main: 1 [no symbol information]
[754] int: 1 [no symbol information]
[773] C: T0 [no symbol information]
[1010] c: 1 [no symbol information]
[1013] f: 0 [no symbol information]
------------------------------------------------------------------------------