Hi Fangrui,
on 2023/7/19 14:33, Fangrui Song wrote:
> 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" <li...@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) ?
I just had a check and confirmed that it did fix the wrong
sh_link, in the past it always uses the decl saved in
named.decl (always f for the test case in PR110729),
with this patch, it switches to use the label in its
corresponding .text* (function section).
> If yes, it may be useful to include some assembly tests... Right now
>
> rg '\.section.*__patchable' gcc/testsuite/
>
> returns nothing.
It's a good idea to add some testing coverage, I'm going
to make a test case by checking the given
".section.*__patchable_function_entries.*,\.LPFE[012]".
Thanks for the suggestion!
BR,
Kewen
