Re: Patch to do reorder text and data sections according to a user specified sequence.

[Sriraman Tallam <tmsriram at google dot com>]

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
>>
>

Attachment: final_layout_patch.txt
Description: Text document