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

["Kewen.Lin" <linkw@linux.ibm.com>]

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