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


Ian, could you please take a look? I rebased the patch..

thank you,
Alexander

2013/3/12 Alexander Ivchenko <aivchenk@gmail.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

Attachment: enable_sorting_sections_by_name_8.patch
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