[RFC][PATCH] Check ELF relocs after opening all all input files

[H.J. Lu]

On Tue, Apr 19, 2016 at 4:24 PM, Alan Modra <amodra@gmail.com> wrote:
> On Tue, Apr 19, 2016 at 11:03:15AM -0700, H.J. Lu wrote:
>> On Tue, Apr 19, 2016 at 9:01 AM, Nick Clifton <nickc@redhat.com> wrote:
>> > Hi H.J.
>> >
>> >> Delaying checking ELF relocations until opening all input files so
>> >> that symbol information is final when relocations are checked.  This
>> >> is only enabled for x86 targets.
>> >
>> >> Any comments, feedbacks?
>> >
>> > What benefit is gained by doing this ?  I would guess that it is connected
>> > with symbols changing type and/or visibility and/or protected status, but
>> > it would be nice to know what you are hoping to achieve.
>>
>> Yes,  this change is motivated by that discussion.  Currently backend
>> check_relocs doesn't have the final information on symbols.  A better
>> linker can provide better diagnostics in check_relocs if symbol info is
>> final.
>
> There are other benefits too.  As it is the backend check_relocs
> functions do things like "maybe this symbol remains undefined so
> perhaps it needs a plt entry", and "maybe this symbol remains dynamic
> so perhaps it needs dynamic relocations".  This complexity can
> disappear if all of those "maybe" cases are resolved.

The first phase of x86 backend is done.  I will take a look to see if I can
remove some checks in x86 check_relocs when I have time.

> Now, if you run check_relocs even later, say at
> ldemul_before_allocation, after gc-sections has run, then all the
> reference counting code for plt and got relocs can be removed too.

I  checked in:

commit e66cdd681f47dc51beaeee3d813f1c9cba27dedf
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Wed Apr 20 17:12:46 2016 -0700

    Remove x86 gc_sweep_hook

    Since x86 backends never see the removed sections, there is no need
    for gc_sweep_hook.

      * elf32-i386.c (elf_i386_gc_sweep_hook): Removed.
      (elf_backend_gc_sweep_hook): Likewise.
      * elf64-x86-64.c (elf_x86_64_gc_sweep_hook): Likewise.
      (elf_backend_gc_sweep_hook): Likewise.

Other reference counting code for plt and got relocs in x86 backend
has to stay for TLS optimization and GOT/PLT optimization.

-- 
H.J.