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

[Jeff Law]

On 11/6/20 4:45 PM, H.J. Lu wrote:
> 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.

I'm not sure that's a good idea either.  Moving symbols into a section
other than they'd normally live doesn't seem all that wise.


Let's face it, there's not a great solution here.  If we mark its
existing section, then everything in that section gets kept.  If we put
the object into a different section than it would normally live, then
that opens a whole new can of worms.


jeff