Re: [gold] enable sorting of text sections with the same prefix

[Alexander Ivchenko <aivchenk at gmail dot com>]

*ping*

thanks,
Alexander

2013/2/13 Alexander Ivchenko <aivchenk@gmail.com>:
>>The documentation of --sort-section=name for GNU ld is unfortunately
>>tied to the notion of a default linker script, which gold does not
>>share.  Still, we ought to be able to come up with some plausible
>>meaning for gold.  And restricting the behaviour to .data and .bss
>>does not make sense to me
>
> that's true, e.g for a testcase with .data and .sdata here what ld -M gave me:
>
> .data           0x0000000000600188        0xc
>  *(SORT(.data) SORT(.data.*) SORT(.gnu.linkonce.d.*))
>  .data          0x0000000000600188        0x0 section_sorting_name_3.o
>  .data          0x0000000000600188        0x0 section_sorting_name_1.o
>  .data          0x0000000000600188        0x0 section_sorting_name_2.o
>  .data.0001     0x0000000000600188        0x4 section_sorting_name_1.o
>                 0x0000000000600188                vdata_0001
>  .data.0002     0x000000000060018c        0x4 section_sorting_name_2.o
>                 0x000000000060018c                vdata_0002
>  .data.0003     0x0000000000600190        0x4 section_sorting_name_3.o
>                 0x0000000000600190                vdata_0003
>
> .sdata.0003     0x0000000000600194        0x4
>  .sdata.0003    0x0000000000600194        0x4 section_sorting_name_3.o
>                 0x0000000000600194                vsdata_0003
>
> .sdata.0001     0x0000000000600198        0x4
>  .sdata.0001    0x0000000000600198        0x4 section_sorting_name_1.o
>                 0x0000000000600198                vsdata_0001
>
> .sdata.0002     0x000000000060019c        0x4
>  .sdata.0002    0x000000000060019c        0x4 section_sorting_name_2.o
>                 0x000000000060019c                vsdata_0002
>
> It seems like there is no particular reason for not sorting .sdata
> sections: just BFD script doesn't do it. Therefore I agree with Sri
> when he said:
>
>>Why not sort all output sections when --sort-section=name is
>>specified? However, for special output sections like ctors, dtors,
>>init_array, fini_array, etc. the original sort compare function will
>>be used. For all other sections, use the new sort compare can be used.
>>No need to hard code any names.
>
> that approach looks reasonable. Also thank you very much, Sri, for
> your fix. I attached the patch with your
> changes and also with updated ChangLog.
>
> thanks,
> Alexander
>
> 2013/2/12 Sriraman Tallam <tmsriram@google.com>:
>> On Tue, Feb 12, 2013 at 10:22 AM, Sriraman Tallam <tmsriram@google.com> wrote:
>>> On Tue, Feb 12, 2013 at 9:21 AM, Ian Lance Taylor <iant@google.com> wrote:
>>>> On Tue, Feb 12, 2013 at 7:01 AM, Alexander Ivchenko <aivchenk@gmail.com> wrote:
>>>>> 2013/2/9 Ian Lance Taylor <iant@google.com>:
>>>>>> On Fri, Feb 8, 2013 at 5:11 PM, Ian Lance Taylor <iant@google.com> wrote:
>>>>>>> On Thu, Feb 7, 2013 at 3:56 AM, Alexander Ivchenko <aivchenk@gmail.com> wrote:
>>>>>>>> thank you for your help, Sri. I fixed help string and deleted warning.
>>>>>>>>
>>>>>>>> Ian, could you please take a look at the attached patch?
>>>>>>>
>>>>>>> As far as I can see, with this patch, when you use
>>>>>>> --sort-section=name, gold will only sort sections that start with
>>>>>>> .data. and .bss. by name.  Other sections remain unsorted.  This does
>>>>>>> not appear to be what the GNU linker does.  The GNU linker appears to
>>>>>>> sort all input sections by name when using --sort-section=name.
>>>>>>
>>>>>> Sorry, I guess that's not quite right.  You call the sorting code one
>>>>>> every section.  But you only set must_sort_attached_input_sections on
>>>>>> the .data and .bss sections.  How can you get away with that?
>>>>>>
>>>>>> Ian
>>>>>
>>>>> You mean from hardcoding those names (.bss and .data) there? I'm not
>>>>> sure so far,
>>>>> but I know that BFD sorts them by name when we have -sort-section=name
>>>>> and at the
>>>>> same time, BFD doesn't sort, say,.sdata and .sbss.
>>>>> Do we need to fully mimic the behavior of BFD for this option?
>>>>
>>>> We do not need to fully mimic GNU ld.  However, we need to understand
>>>> how and why GNU ld behaves the way it does.  When I look at the GNU ld
>>>> code, I don't see anything that restricts the effect of
>>>> --sort-section=name to the .data and .bss sections.  Nor is it
>>>> documented to behave that way.
>>>>
>>>> The documentation of --sort-section=name for GNU ld is unfortunately
>>>> tied to the notion of a default linker script, which gold does not
>>>> share.  Still, we ought to be able to come up with some plausible
>>>> meaning for gold.  And restricting the behaviour to .data and .bss
>>>> does not make sense to me.
>>>
>>> Why not sort all output sections when --sort-section=name is
>>> specified? However, for special output sections like ctors, dtors,
>>> init_array, fini_array, etc. the original sort compare function will
>>> be used. For all other sections, use the new sort compare can be used.
>>> No need to hard code any names.
>>>
>>>>
>>>> Also, in order for this to work correctly, you must call
>>>> set_may_sort_attached_input_sections when you create the output
>>>> section.  You aren't doing that, and I'm surprised that your code is
>>>> working reliably.
>>>
>>> I missed this part completely when I was reviewing his code, sorry!. I
>>> am not surprised his patch works for ".text" because the input
>>> sections are retained as he can piggy back on default text sorting.
>>> But, how does his test pass for bss and data? I will apply his patch
>>> and find out.
>>
>> I figured out how input sections are kept for some ".data" sections.
>> When the first object is seen, the isecn entries for its .data and
>> .bss are not kept. But after the first object, Layout::layout is
>> called which sets must_sort for .data and .bss. From then on, .data
>> and .bss are saved. This is definitely wrong. Infact, this patch does
>> not work correctly on the test case included and produces an assert in
>> reloc.cc:830 when I tried it. This is because some input section
>> entries have isecn and some do not. We encountered an instance of this
>> problem earlier with the text reordering patch.
>>
>> This can be fixed by removing the lines which set_must_sort in
>> Layout::layout and simply set_must_sort to all output sections in
>> Layout::make_output_section. Please note that setting may_sort and
>> then must_sort later is not necessary here since we know for sure that
>> we are going to sort this.
>>
>> I have modified this patch accordingly and attached a new patch that
>> sorts all output sections by name when --sort-section=name is passed.
>> For special output sections like .ctors, it will still use the
>> original sort compare function. I have not special cased if for
>> ".data" and ".bss" but I am not sure if ".sdata" and ".sbss" must be
>> ignored.
>>
>> Thanks
>> Sri
>>
>>
>>
>>>
>>> Sri
>>>
>>>>
>>>> Ian