[gold] enable sorting of text sections with the same prefix
Tue Mar 12 08:40:00 GMT 2013
2013/2/13 Alexander Ivchenko <firstname.lastname@example.org>:
>>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.
> 2013/2/12 Sriraman Tallam <email@example.com>:
>> On Tue, Feb 12, 2013 at 10:22 AM, Sriraman Tallam <firstname.lastname@example.org> wrote:
>>> On Tue, Feb 12, 2013 at 9:21 AM, Ian Lance Taylor <email@example.com> wrote:
>>>> On Tue, Feb 12, 2013 at 7:01 AM, Alexander Ivchenko <firstname.lastname@example.org> wrote:
>>>>> 2013/2/9 Ian Lance Taylor <email@example.com>:
>>>>>> On Fri, Feb 8, 2013 at 5:11 PM, Ian Lance Taylor <firstname.lastname@example.org> wrote:
>>>>>>> On Thu, Feb 7, 2013 at 3:56 AM, Alexander Ivchenko <email@example.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?
>>>>> 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
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