Re: PING^2 [PATCH] Adjust the symbol for SECTION_LINK_ORDER linked_to section [PR99889]

[Fangrui Song <maskray@google.com>]

On Thu, Nov 24, 2022 at 7:26 PM Kewen.Lin via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi Richard,
>
> on 2022/11/23 00:08, Richard Sandiford wrote:
> > "Kewen.Lin" <linkw@linux.ibm.com> writes:
> >> Hi Richard,
> >>
> >> Many thanks for your review comments!
> >>
> >>>>> on 2022/8/24 16:17, Kewen.Lin via Gcc-patches wrote:
> >>>>>> Hi,
> >>>>>>
> >>>>>> As discussed in PR98125, -fpatchable-function-entry with
> >>>>>> SECTION_LINK_ORDER support doesn't work well on powerpc64
> >>>>>> ELFv1 because the filled "Symbol" in
> >>>>>>
> >>>>>>   .section name,"flags"o,@type,Symbol
> >>>>>>
> >>>>>> sits in .opd section instead of in the function_section
> >>>>>> like .text or named .text*.
> >>>>>>
> >>>>>> Since we already generates one label LPFE* which sits in
> >>>>>> function_section of current_function_decl, this patch is
> >>>>>> to reuse it as the symbol for the linked_to section.  It
> >>>>>> avoids the above ABI specific issue when using the symbol
> >>>>>> concluded from current_function_decl.
> >>>>>>
> >>>>>> Besides, with this support some previous workarounds for
> >>>>>> powerpc64 ELFv1 can be reverted.
> >>>>>>
> >>>>>> btw, rs6000_print_patchable_function_entry can be dropped
> >>>>>> but there is another rs6000 patch which needs this rs6000
> >>>>>> specific hook rs6000_print_patchable_function_entry, not
> >>>>>> sure which one gets landed first, so just leave it here.
> >>>>>>
> >>>>>> Bootstrapped and regtested on below:
> >>>>>>
> >>>>>>   1) powerpc64-linux-gnu P8 with default binutils 2.27
> >>>>>>      and latest binutils 2.39.
> >>>>>>   2) powerpc64le-linux-gnu P9 (default binutils 2.30).
> >>>>>>   3) powerpc64le-linux-gnu P10 (default binutils 2.30).
> >>>>>>   4) x86_64-redhat-linux with default binutils 2.30
> >>>>>>      and latest binutils 2.39.
> >>>>>>   5) aarch64-linux-gnu  with default binutils 2.30
> >>>>>>      and latest binutils 2.39.
> >>>>>>
> >>
> >> [snip...]
> >>
> >>>>>> diff --git a/gcc/varasm.cc b/gcc/varasm.cc
> >>>>>> index 4db8506b106..d4de6e164ee 100644
> >>>>>> --- a/gcc/varasm.cc
> >>>>>> +++ b/gcc/varasm.cc
> >>>>>> @@ -6906,11 +6906,16 @@ default_elf_asm_named_section (const char *name, unsigned int flags,
> >>>>>>          fprintf (asm_out_file, ",%d", flags & SECTION_ENTSIZE);
> >>>>>>        if (flags & SECTION_LINK_ORDER)
> >>>>>>          {
> >>>>>> -          tree id = DECL_ASSEMBLER_NAME (decl);
> >>>>>> -          ultimate_transparent_alias_target (&id);
> >>>>>> -          const char *name = IDENTIFIER_POINTER (id);
> >>>>>> -          name = targetm.strip_name_encoding (name);
> >>>>>> -          fprintf (asm_out_file, ",%s", name);
> >>>>>> +          /* For now, only section "__patchable_function_entries"
> >>>>>> +             adopts flag SECTION_LINK_ORDER, internal label LPFE*
> >>>>>> +             was emitted in default_print_patchable_function_entry,
> >>>>>> +             just place it here for linked_to section.  */
> >>>>>> +          gcc_assert (!strcmp (name, "__patchable_function_entries"));
> >>>
> >>> I like the idea of removing the rs600 workaround in favour of making the
> >>> target-independent more robust.  But this seems a bit hackish.  What
> >>> would we do if SECTION_LINK_ORDER was used for something else in future?
> >>>
> >>
> >> Good question!  I think it depends on how we can get the symbol for the
> >> linked_to section, if adopting the name of the decl will suffer the
> >> similar issue which this patch wants to fix, we have to reuse the label
> >> LPFE* or some kind of new artificial label in the related section; or
> >> we can just go with the name of the given decl, or something related to
> >> that decl.  Since we can't predict any future uses, I just placed an
> >> assertion here to ensure that we would revisit and adjust this part at
> >> that time.  Does it sound reasonable to you?
> >
> > Yeah, I guess that's good enough.  If the old scheme ends up being
> > correct for some future use, we can make the new behaviour conditional
> > on __patchable_function_entries.
>
> Yes, we can check if the given section name is
> "__patchable_function_entries".
>
> >
> > So yeah, the patch LGTM to me, thanks.
>
> Thanks again!  I rebased and re-tested it on x86/aarch64/powerpc64{,le},
> just committed in r13-4294-gf120196382ac5a.
>
> BR,
> Kewen

Hi, Kewen, do you think whether your patch fixed
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110729
(__patchable_function_entries has wrong sh_link) ?
If yes, it may be useful to include some assembly tests... Right now

    rg '\.section.*__patchable' gcc/testsuite/

returns nothing.


-- 
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