On Mon, 28 Sep 2020 12:42:33 +0200, Jakub Jelinek wrote:
> On Mon, Sep 28, 2020 at 12:31:59PM +0200, Mark Wielaard wrote:
> > Finally I am interested in your proposal to implement a different way
> > to reduce the size of DIE trees by eliminating "unused" DIEs. It is
> > hard to predict what effect that would have without seeing an
> > implementation (in theory GCC with LTO would not actually generate
> > debuginfo for unused functions). But I think that can be done separate
> > from your proposal and combined with other size reduction techniques.
>
> And note that GCC already does implement
> -feliminate-unused-debug-{symbols,types} which are enabled by default and
> are (at least in my eyes) sometimes too aggressive, so by eliminating even further
> DIEs the debug experience might be even worse.
git clone git://git.jankratochvil.net/massrebuild
./dwarfredundant lldb-debuginfo-11.0.0-0.2.rc3.fc34.x86_64/usr/lib/debug/usr/lib64/liblldb.so.11.0.0-11.0.0-0.2.rc3.fc34.x86_64.debug
For example:
saved=27:
0x0193b058: DW_TAG_subprogram [47] *
DW_AT_abstract_origin [DW_FORM_ref_addr] (0x000000000515d936 "_ZN12lldb_private17StoppointLocationD2Ev")
DW_AT_low_pc [DW_FORM_addr] (0x0000000000000000)
DW_AT_high_pc [DW_FORM_udata] (1)
DW_AT_frame_base [DW_FORM_exprloc] (DW_OP_call_frame_cfa)
DW_AT_GNU_all_call_sites [DW_FORM_flag_present] (true)
DW_AT_sibling [DW_FORM_ref_udata] (cu + 0x1da4f => {0x0193b073})
0x0193b06b: DW_TAG_formal_parameter [55]
DW_AT_abstract_origin [DW_FORM_ref_addr] (0x000000000515d941)
DW_AT_location [DW_FORM_exprloc] (DW_OP_reg5 RDI)
0x0193b072: NULL
This DIE describes only a concrete function instance at address 0x0.
No function can exist on address 0x0 on x86_64, that is
a discarded/deduplicated function:
[Dwarf-Discuss] DWARF for linker GC'd code
http://lists.dwarfstd.org/pipermail/dwarf-discuss-dwarfstd.org/2020-July/004683.html
I do not see what any DWARF consumer could find out from such a DIE.
And there are many such DIEs, something a bit less than 28% of what DWZ saves
(28% is incl. removal of DW_UT_type declarations).
Jan
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