[PATCH v2] Add -fgnu-retain to place used symbols in SHF_GNU_RETAIN section

[Richard Biener]

On Wed, Feb 17, 2021 at 3:33 PM Jakub Jelinek via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Tue, Feb 16, 2021 at 11:59:21AM -0800, H.J. Lu wrote:
> >       PR target/99113
> >       * common.opt: Add -fgnu-retain.
>
> I'm not sure -fgnu-retain as the option name.
> Wouldn't say -fretain-used-vars be better?

-fretain-used-annotated-vars?  Because 'used' has a meaning in user terms
and I would not associate it with only 'used' attribute annotated variables.

> > @@ -1666,6 +1666,10 @@ floop-unroll-and-jam
> >  Common Var(flag_unroll_jam) Optimization
> >  Perform unroll-and-jam on loops.
> >
> > +fgnu-retain
> > +Common Var(flag_gnu_retain)
> > +Use SHF_GNU_RETAIN on used symbols if supported by the assembler and the linker.
>
> on variables with the used attribute?
>
> > diff --git a/gcc/toplev.c b/gcc/toplev.c
> > index d8cc254adef..119cd7c0432 100644
> > --- a/gcc/toplev.c
> > +++ b/gcc/toplev.c
> > @@ -1761,6 +1761,13 @@ process_options (void)
> >    if (flag_large_source_files)
> >      line_table->default_range_bits = 0;
> >
> > +  if (flag_gnu_retain && !SUPPORTS_SHF_GNU_RETAIN)
> > +    {
> > +      warning_at (UNKNOWN_LOCATION, 0, "%qs is not supported for this target",
> > +               "-fgnu-retain");
> > +      flag_gnu_retain = 0;
> > +    }
> > +
> >    /* Please don't change global_options after this point, those changes won't
> >       be reflected in optimization_{default,current}_node.  */
> >  }
> > diff --git a/gcc/varasm.c b/gcc/varasm.c
> > index 29478ab0d8d..4e0e30abee5 100644
> > --- a/gcc/varasm.c
> > +++ b/gcc/varasm.c
> > @@ -297,7 +297,7 @@ get_section (const char *name, unsigned int flags, tree decl,
> >    slot = section_htab->find_slot_with_hash (name, htab_hash_string (name),
> >                                           INSERT);
> >    flags |= SECTION_NAMED;
> > -  if (SUPPORTS_SHF_GNU_RETAIN
> > +  if (flag_gnu_retain
> >        && decl != nullptr
> >        && DECL_P (decl)
> >        && DECL_PRESERVE_P (decl))
> > @@ -487,7 +487,7 @@ resolve_unique_section (tree decl, int reloc ATTRIBUTE_UNUSED,
> >    if (DECL_SECTION_NAME (decl) == NULL
> >        && targetm_common.have_named_sections
> >        && (flag_function_or_data_sections
> > -       || (SUPPORTS_SHF_GNU_RETAIN && DECL_PRESERVE_P (decl))
> > +       || (flag_gnu_retain && DECL_PRESERVE_P (decl))
> >         || DECL_COMDAT_GROUP (decl)))
> >      {
> >        targetm.asm_out.unique_section (decl, reloc);
>
> I'm not convinced this will work properly with LTO, the option from
> -flto -c compilation would be ignored.
> For functions, we usually mark the option Optimization and make it
> saved/restored on function switches, but this is for variables, so it will
> not work for those.
> I'd think better would be to add when seeing "used" attribute some
> artificial attribute (unless we have "retain" attribute to mean that, say
> "retain ") when the flag is on in handle_used_attribute and use that
> attribute in varasm instead of the option?

Yeah, that's very much better and more likely to work.

> > @@ -1227,7 +1227,7 @@ get_variable_section (tree decl, bool prefer_noswitch_p)
> >      vnode->get_constructor ();
> >
> >    if (DECL_COMMON (decl)
> > -      && !(SUPPORTS_SHF_GNU_RETAIN && DECL_PRESERVE_P (decl)))
> > +      && !(flag_gnu_retain && DECL_PRESERVE_P (decl)))
> >      {
> >        /* If the decl has been given an explicit section name, or it resides
> >        in a non-generic address space, then it isn't common, and shouldn't
> > @@ -7761,7 +7761,7 @@ switch_to_section (section *new_section, tree decl)
> >  {
> >    if (in_section == new_section)
> >      {
> > -      if (SUPPORTS_SHF_GNU_RETAIN
> > +      if (flag_gnu_retain
> >         && (new_section->common.flags & SECTION_NAMED)
> >         && decl != nullptr
> >         && DECL_P (decl)
>
> > --- a/gcc/doc/invoke.texi
> > +++ b/gcc/doc/invoke.texi
> > @@ -16168,6 +16168,11 @@ DSOs; if your program relies on reinitialization of a DSO via
> > @code{dlclose} and @code{dlopen}, you can use
> > @option{-fno-gnu-unique}.
> >
> > +@item -fgnu-retain
> > +@opindex fgnu-retain
> > +On systems with recent GNU assembler and linker, the compiler places
> > +used symbols in separate SHF_GNU_RETAIN sections.
>
> Again, variables with the used attribute.
>
>         Jakub
>