Patch to do reorder text and data sections according to a user specified sequence.
Taras Glek
tglek@mozilla.com
Mon May 24 19:27:00 GMT 2010
On 05/21/2010 06:41 PM, Sriraman Tallam wrote:
> Hi Taras,
>
> Did you get a chance to test the timings with the new patch ?
>
I've only tested it briefly on Friday. I'm having some problems with the
new patch. It ranges from working perfectly to occasionally taking
longer than the previous one and/or crashing. I'll let you know more
once I have more time to play with it
Taras
> Thanks,
> -Sri.
>
> On Thu, May 13, 2010 at 5:58 PM, Sriraman Tallam<tmsriram@google.com> wrote:
>
>> Hi Ian and Taras,
>>
>> Sorry for taking this long to get back. I have modified the patch
>> to address Taras' timing issue. Basically, I split patterns specified
>> into glob and non-glob by just looking for wild-card characters in the
>> pattern. Non-glob look-ups are through a hash table and fast. Glob
>> patterns are be searched linearly. So, if you specify non-glob symbol
>> names in the section ordering file, the link should be faster than
>> before.
>>
>> Further, I have prioritized non-glob patters over glob patterns.
>> Let me explain with an example :
>>
>> If object main.o has sections .text._foov, .text._barv, and
>> .text._zapv and the seection ordering file is :
>> ===============================
>> *bar*
>> .text.*
>> .text._barv
>> ===============================
>>
>> Then, .text._barv will be the last section in .text even though it
>> matches the first glob pattern.
>>
>>
>> The changes are :
>>
>> * gold.h (is_wildcard_string): New function.
>> * layout.cc (Layout::layout): Pass this pointer to add_input_section.
>> (Layout::layout_eh_frame): Ditto.
>> (Layout::find_section_order_index): New method.
>> (Layout::read_layout_from_file): New method.
>> * layout.h (Layout::find_section_order_index): New method.
>> (Layout::read_layout_from_file): New method.
>> (Layout::input_section_position_): New private member.
>> (Layout::input_section_glob_): New private member.
>> * main.cc (main): Call read_layout_from_file here.
>> * options.h (--section-ordering-file): New option.
>> * output.cc (Output_section::input_section_order_specified_): New
>> member.
>> (Output_section::Output_section): Initialize new member.
>> (Output_section::add_input_section): Add new parameter.
>> Keep input sections when --section-ordering-file is used.
>> (Output_section::set_final_data_size): Sort input sections when
>> section ordering file is specified.
>> (Output_section::Input_section_sort_entry): Add new parameter.
>> Check sorting type.
>> (Output_section::Input_section_sort_entry::is_before_in_sequence):
>> New method.
>> (Output_section::Input_section_sort_compare::operator()): Change to
>> consider section_order_index.
>> (Output_section::Input_section_sort_init_fini_compare::operator()):
>> Change to consider section_order_index.
>> (Output_section::Input_section_sort_section_order_index_compare
>> ::operator()): New method.
>> (Output_section::sort_attached_input_sections): Change to sort
>> according to section order when specified.
>> (Output_section::add_input_section<32, true>): Add new parameter.
>> (Output_section::add_input_section<64, true>): Add new parameter.
>> (Output_section::add_input_section<32, false>): Add new parameter.
>> (Output_section::add_input_section<64, false>): Add new parameter.
>> * output.h (Output_section::add_input_section): Add new parameter.
>> (Output_section::input_section_order_specified): New
>> method.
>> (Output_section::set_input_section_order_specified): New method.
>> (Input_section::Input_section): Initialize section_order_index_.
>> (Input_section::section_order_index): New method.
>> (Input_section::set_section_order_index): New method.
>> (Input_section::section_order_index_): New member.
>> (Input_section::Input_section_sort_section_order_index_compare): New
>> struct.
>> (Output_section::input_section_order_specified_): New member.
>> * script-sections.cc (is_wildcard_string): Delete and move modified
>> method to gold.h.
>> (Output_section_element_input::Output_section_element_input): Modify
>> call to is_wildcard_string.
>> (Output_section_element_input::Input_section_pattern
>> ::Input_section_pattern): Ditto.
>> (Output_section_element_input::Output_section_element_input): Ditto.
>> * testsuite/Makefile.am (final_layout): New test case.
>> * testsuite/Makefile.in: Regenerate.
>> * testsuite/final_layout.cc: New file.
>> * testsuite/final_layout.sh: New file.
>>
>> Please let me know what you think.
>>
>> Thanks,
>> -Sri.
>>
>>
>>
>> On Tue, Mar 2, 2010 at 7:43 PM, Sriraman Tallam<tmsriram@google.com> wrote:
>>
>>> Hi Ian,
>>>
>>> I finally got around to making the changes you specified. Please
>>> take a look when you get a chance.
>>>
>>> * layout.cc (Layout::read_layout_from_file): New method.
>>> (Layout::layout): Pass this pointer to add_input_section.
>>> (Layout::layout_eh_frame): Ditto.
>>> * layout.h (Layout::read_layout_from_file): New method.
>>> (Layout::input_section_order): New method.
>>> (Layout::input_section_order_): New private member.
>>> * main.cc (main): Call read_layout_from_file here.
>>> * options.h (--section-ordering-file): New option.
>>> * output.cc (Output_section::input_section_order_specified_): New
>>> member.
>>> (Output_section::add_input_section): Add new parameter.
>>> Keep input sections when --section-ordering-file is used.
>>> (Output_section::set_final_data_size): Sort input sections when
>>> section ordering file is specified.
>>> (Output_section::Input_section_sort_entry::section_order_index_): New
>>> member.
>>> (Output_section::Input_section_sort_entry::section_order_index): New
>>> method.
>>> (Output_section::Input_section_sort_compare::operator()): Change to
>>> consider section_order_index.
>>> (Output_section::Input_section_sort_init_fini_compare::operator()):
>>> Change to consider section_order_index.
>>> (Output_section::Input_section_sort_section_order_index_compare
>>> ::operator()): New method.
>>> (Output_section::sort_attached_input_sections): Change to sort
>>> according to section order when specified.
>>> * output.h (Output_section::input_section_order_specified): New
>>> method.
>>> (Output_section::set_input_section_order_specified): New method.
>>> (Input_section::glob_pattern_number): New method.
>>> (Input_section::set_glob_pattern_number): New method.
>>> (Input_section::glob_pattern_number_): New member.
>>> (Input_section::Input_section_sort_section_order_index_compare): New
>>> struct.
>>> (Output_section::input_section_order_specified_): New member.
>>> * testsuite/Makefile.am (final_layout): New test case.
>>> * testsuite/Makefile.in: Regenerate.
>>> * testsuite/final_layout.cc: New file.
>>> * testsuite/final_layout.sh: New file
>>>
>>>
>>> Thanks,
>>> -Sriraman.
>>>
>>>
>>> On Thu, Feb 11, 2010 at 9:29 PM, Ian Lance Taylor<iant@google.com> wrote:
>>>
>>>> Sriraman Tallam<tmsriram@google.com> writes:
>>>>
>>>>
>>>>> I have attached a patch to reorder text and data sections in the
>>>>> linker according to a user specified sequence. I have added a new
>>>>> option --final-layout to do this. Currently, this could be done using
>>>>> linker scripts but this patch makes it really easy to do this. Let me
>>>>> explain using a simple example.
>>>>>
>>>>> test.cc
>>>>>
>>>>> void foo()
>>>>> { }
>>>>>
>>>>> void bar()
>>>>> { }
>>>>>
>>>>> int main()
>>>>> {
>>>>> return 0;
>>>>> }
>>>>>
>>>>> $ g++ -ffunction-sections test.cc
>>>>> $ nm -n a.out
>>>>> ...
>>>>> 000000000040038c T _Z3foov
>>>>> 0000000000400392 T _Z3barv
>>>>> ....
>>>>>
>>>>> foo is laid to be before bar. Now, I can change this order as follows.
>>>>>
>>>>> $ (echo "_Z3barv"&& echo "_Z3foov")> sequence.txt
>>>>> $ g++ -ffunction-sections test.cc -Wl,--final-layout,sequence.txt
>>>>> $ nm -n a.out
>>>>> ...
>>>>> 0000000000400658 T _Z3barv
>>>>> 000000000040065e T _Z3foov
>>>>> ...
>>>>>
>>>>> The order is changed.
>>>>>
>>>>> This can be done for text or data sections.
>>>>>
>>>>
>>>> As I understand it, in linker terms, you are sorting the sections by
>>>> suffixes. When two input sections are in the same output section, and
>>>> both input sections have suffixes which appear in the file, then the
>>>> input sections are sorted in the order in which the suffixes appear in
>>>> the file.
>>>>
>>>> I think it would be more natural to sort the input sections by name
>>>> rather than by suffix. Since you don't want to fuss with writing
>>>> ".text." all the time, suppose we say that we sort the input sections
>>>> by name, and we match the input section names using glob patterns. We
>>>> already use glob patterns in linker scripts, so that is not a big
>>>> stretch.
>>>>
>>>> Just a few comments on the rest of the patch.
>>>>
>>>>
>>>>
>>>>
>>>>> +// Read the sequence of input sections from the file specified with
>>>>> +// --final-layout.
>>>>> +
>>>>> +bool
>>>>> +Layout::read_layout_from_file()
>>>>> +{
>>>>> + const char* filename = parameters->options().final_layout();
>>>>> + char *buf = NULL;
>>>>> + size_t len = 0;
>>>>> + FILE* fp = fopen(filename, "r");
>>>>> +
>>>>> + if (fp == NULL)
>>>>> + {
>>>>> + gold_error(_("Error opening layout file : %s\n"), filename);
>>>>> + gold_exit(false);
>>>>> + }
>>>>> +
>>>>> + while (getline(&buf,&len, fp) != -1)
>>>>> + {
>>>>> + buf[strlen(buf) - 1] = 0;
>>>>> + this->input_section_order_.push_back(std::string(buf));
>>>>> + }
>>>>> +
>>>>> + if (buf != NULL)
>>>>> + free(buf);
>>>>> +
>>>>> + fclose(fp);
>>>>> + return true;
>>>>> +}
>>>>>
>>>> The getline function is insufficient portable for use in gold. Search
>>>> for std::getline in options.cc for an alternate approach you can use.
>>>> Emulate the error message style you see there too--no capital letter,
>>>> name the file, no space before colon, no \n. And if you really want
>>>> to exit on failure, call gold_fatal.
>>>>
>>>>
>>>>
>>>>> +// If --final-layout option is used, reorder the input sections in
>>>>> +// .text, .data, .bss and .rodata according to the specified sequence.
>>>>> +
>>>>> +void
>>>>> +Layout::section_reorder()
>>>>> +{
>>>>> + this->read_layout_from_file();
>>>>> +
>>>>> + for (Section_list::iterator p = this->section_list_.begin();
>>>>> + p != this->section_list_.end();
>>>>> + ++p)
>>>>> + {
>>>>> + if (strcmp(".text", (*p)->name()) == 0
>>>>> + || strcmp(".data", (*p)->name()) == 0
>>>>> + || strcmp(".bss", (*p)->name()) == 0
>>>>> + || strcmp(".rodata", (*p)->name()) == 0)
>>>>> + (*p)->reorder_layout(this);
>>>>> + }
>>>>> +}
>>>>>
>>>> Why restrict this to those output sections? Why not sort input
>>>> sections in any output section?
>>>>
>>>>
>>>>
>>>>> + DEFINE_string(final_layout, options::TWO_DASHES, '\0', NULL,
>>>>> + N_("Layout functions and data in the order specified."),
>>>>> + N_("FILENAME"));
>>>>>
>>>> I'm not sure I care for --final-layout as the option name. Perhaps
>>>> --section-ordering-file? Perhaps somebody else has a better idea.
>>>>
>>>>
>>>>
>>>>> + // the future, we keep track of the sections. If the --final-layout
>>>>> + // option is used to specify the order of sections, we need to keep
>>>>> + // track of sections.
>>>>> if (have_sections_script
>>>>> || !this->input_sections_.empty()
>>>>> || this->may_sort_attached_input_sections()
>>>>> || this->must_sort_attached_input_sections()
>>>>> || parameters->options().user_set_Map()
>>>>> - || parameters->target().may_relax())
>>>>> - this->input_sections_.push_back(Input_section(object, shndx,
>>>>> - shdr.get_sh_size(),
>>>>> - addralign));
>>>>> + || parameters->target().may_relax()
>>>>> + || parameters->options().final_layout())
>>>>> + {
>>>>> + Input_section isecn(object, shndx, shdr.get_sh_size(), addralign);
>>>>> + isecn.set_section_name(secname);
>>>>> + this->input_sections_.push_back(isecn);
>>>>> + }
>>>>>
>>>> Don't save the string here, that's just going to bloat memory usage.
>>>> Instead, when you read the file, give each line a number. Then match
>>>> the section name which you have here against the list of patterns. If
>>>> you find a match, store the number in the Input_section structure.
>>>> Also, if you find a match, set a flag in the output section. Then
>>>> sort the sections, by number, in a function called from
>>>> set_final_data_size.
>>>>
>>>> You will see that there is already some section ordering in that
>>>> function, which is used to implement constructor/destructor priority
>>>> ordering. I guess the priority ordering should take precedence.
>>>> Maybe.
>>>>
>>>> Ian
>>>>
>>>>
>>>
>>
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