[PATCH] "used" attribute saves decl from linker garbage collection

H.J. Lu hjl.tools@gmail.com
Fri Nov 6 23:45:07 GMT 2020


On Fri, Nov 6, 2020 at 3:37 PM Jeff Law <law@redhat.com> wrote:
>
>
> On 11/6/20 4:29 PM, H.J. Lu wrote:
> > On Fri, Nov 6, 2020 at 3:22 PM Jeff Law <law@redhat.com> wrote:
> >>
> >> On 11/5/20 7:34 AM, H.J. Lu via Gcc-patches wrote:
> >>> On Thu, Nov 5, 2020 at 3:37 AM Jozef Lawrynowicz
> >>> <jozef.l@mittosystems.com> wrote:
> >>>> On Thu, Nov 05, 2020 at 06:21:21AM -0500, Hans-Peter Nilsson wrote:
> >>>>> On Wed, 4 Nov 2020, H.J. Lu wrote:
> >>>>>> .retain is ill-defined.   For example,
> >>>>>>
> >>>>>> [hjl@gnu-cfl-2 gcc]$ cat /tmp/x.c
> >>>>>> static int xyzzy __attribute__((__used__));
> >>>>>> [hjl@gnu-cfl-2 gcc]$ ./xgcc -B./ -S /tmp/x.c -fcommon
> >>>>>> [hjl@gnu-cfl-2 gcc]$ cat x.s
> >>>>>> .file "x.c"
> >>>>>> .text
> >>>>>> .retain xyzzy  <<<<<<<<< What does it do?
> >>>>>> .local xyzzy
> >>>>>> .comm xyzzy,4,4
> >>>>>> .ident "GCC: (GNU) 11.0.0 20201103 (experimental)"
> >>>>>> .section .note.GNU-stack,"",@progbits
> >>>>>> [hjl@gnu-cfl-2 gcc]$
> >>>>> To answer that question: it's up to the assembler, but for ELF
> >>>>> and SHF_GNU_RETAIN, it seems obvious it'd tell the assembler to
> >>>>> set SHF_GNU_RETAIN for the section where the symbol ends up.
> >>>>> We both know this isn't rocket science with binutils.
> >>>> Indeed, and my patch handles it trivially:
> >>>> https://sourceware.org/pipermail/binutils/2020-November/113993.html
> >>>>
> >>>>   +void
> >>>>   +obj_elf_retain (int arg ATTRIBUTE_UNUSED)
> >>>>   .... snip ....
> >>>>   +  sym = get_sym_from_input_line_and_check ();
> >>>>   +  symbol_get_obj (sym)->retain = 1;
> >>>>
> >>>>   @@ -2624,6 +2704,9 @@ elf_frob_symbol (symbolS *symp, int *puntp)
> >>>>     }
> >>>>        }
> >>>>
> >>>>   +  if (symbol_get_obj (symp)->retain)
> >>>>   +    elf_section_flags (S_GET_SEGMENT (symp)) |= SHF_GNU_RETAIN;
> >>>>   +
> >>>>      /* Double check weak symbols.  */
> >>>>      if (S_IS_WEAK (symp))
> >>>>        {
> >>>>
> >>>> We could check that the symbol named in the .retain directive has
> >>>> already been defined, however this isn't compatible with GCC
> >>>> mark_decl_preserved handling, since mark_decl_preserved is called
> >>>> emitted before the local symbols are defined in the assembly output
> >>>> file.
> >>>>
> >>>> GAS should at least validate that the symbol named in the .retain
> >>>> directive does end up as a symbol though.
> >>>>
> >>> Don't add .retain.
> >> Why?  I don't see why you find it so objectionable.
> >>
> > An ELF symbol directive should operate on symbol table:
> >
> > http://www.sco.com/developers/gabi/latest/ch4.symtab.html
> >
> > not the section flags where the symbol is defined.
>
> I agree in general, but I think this is one of those cases where it's
> not so clear.  And what you're talking about is an implementation detail.

There is no need for such a hack.  The proper thing to do in ELF is
to place such a symbol in a section with SHF_GNU_RETAIN flag.   This
also avoids the question what to do with SHN_COMMON.

-- 
H.J.


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