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From: Richard Sandiford <richard.sandiford@arm.com>
To: "Kewen.Lin" <linkw@linux.ibm.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>,
	 Jakub Jelinek <jakub@redhat.com>,
	 Segher Boessenkool <segher@kernel.crashing.org>,
	 Peter Bergner <bergner@linux.ibm.com>,
	 jlaw@ventanamicro.com,  AlanM <amodra@gmail.com>,
	 David Edelsohn <dje.gcc@gmail.com>,
	 Richard Biener <richard.guenther@gmail.com>,
	 "H.J. Lu" <hjl.tools@gmail.com>
Subject: Re: PING^2 [PATCH] Adjust the symbol for SECTION_LINK_ORDER linked_to section [PR99889]
Date: Tue, 22 Nov 2022 16:08:43 +0000	[thread overview]
Message-ID: <mptcz9f7z04.fsf@arm.com> (raw)
In-Reply-To: <e0fabd5e-2965-5a36-320d-6ec357f47032@linux.ibm.com> (Kewen Lin's message of "Tue, 22 Nov 2022 10:58:04 +0800")

"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.

So yeah, the patch LGTM to me, thanks.

Richard

  reply	other threads:[~2022-11-22 16:08 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 [this message]
2022-11-25  3:26           ` Kewen.Lin
2023-07-19  6:33             ` Fangrui Song
2023-07-19  8:49               ` Kewen.Lin
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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