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From: "Kewen.Lin" <linkw@linux.ibm.com>
To: Fangrui Song <maskray@google.com>
Cc: richard.sandiford@arm.com, Jakub Jelinek <jakub@redhat.com>,
	Segher Boessenkool <segher@kernel.crashing.org>,
	GCC Patches <gcc-patches@gcc.gnu.org>,
	Peter Bergner <bergner@linux.ibm.com>,
	David Edelsohn <dje.gcc@gmail.com>,
	Richard Biener <richard.guenther@gmail.com>,
	"H.J. Lu" <hjl.tools@gmail.com>, AlanM <amodra@gmail.com>,
	jlaw@ventanamicro.com
Subject: Re: PING^2 [PATCH] Adjust the symbol for SECTION_LINK_ORDER linked_to section [PR99889]
Date: Wed, 19 Jul 2023 16:49:39 +0800	[thread overview]
Message-ID: <cf0183a5-1e2a-8e6f-fdb0-471d09a2de29@linux.ibm.com> (raw)
In-Reply-To: <CAFP8O3+j8KvsiBtL+vPTSKmsns8iC3uFP-StuEc2C=LaF=d80A@mail.gmail.com>

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" <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) ?

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

  reply	other threads:[~2023-07-19  8:49 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-08-24  8:17 Kewen.Lin
2022-09-28  5:41 ` PING^1 " Kewen.Lin
2022-11-10  8:15   ` PING^2 " Kewen.Lin
2022-11-21 14:20     ` Richard Sandiford
2022-11-22  2:58       ` Kewen.Lin
2022-11-22 16:08         ` Richard Sandiford
2022-11-25  3:26           ` Kewen.Lin
2023-07-19  6:33             ` Fangrui Song
2023-07-19  8:49               ` Kewen.Lin [this message]
2022-09-29 20:31 ` Segher Boessenkool
2022-09-30 12:47   ` Kewen.Lin
2022-09-30 17:47     ` Segher Boessenkool

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