Plugin interfaces to do Pettis Hansen style code layout in the gold linker.

[Sriraman Tallam <tmsriram at google dot com>]

Hi,

  I am working on a linker plugin to use callgraph edge profiles
generated by GCC to guide function layout in the linker. The compiler
passes the callgraph edge profile values to the linker via special
.note sections, I am working on a GCC patch to do this. The linker
plugin reads the .note sections and constructs an annotated callgraph
with the edge profiles. It then uses the Pettis-Hansen algorithm,
"Profile-guided Code Positioning, PLDI 1990, K. Pettis, and R.C.
Hansen.", to determine the order of functions in the final layout.

  In order for my plugin to do the above, I need the interfaces to
read section contents & attributes and reorder them. I have attached a
patch for review that adds these interfaces to the GOLD plugin
infrastructure. I have also made appropriate changes to allow section
ordering from a plugin.

	* layout.cc (Layout::Layout): Initialize section_ordering_specified_,
	input_section_position_, and input_section_glob_.
	(Layout::update_layout_of_sections): New function.
	(read_layout_from_file): Add parameter filename and call function
	section_ordering_specified.
	* layout.h (is_section_ordering_specified): New function.
	(section_ordering_specified): New function.
	(read_layout_from_file): Add parameter filename.
	(update_layout_of_sections): New function.
	(section_ordering_specified_): New boolean member.
	* main.cc(main): Call load_plugins after layout object is defined.
	Call read_layout_from_file when section ordering is specified.
	* output.cc (Output_section::add_input_section): Use
	function section_ordering_specified to check if section ordering is
	needed.
	(Output_section::update_section_layout): New function.
	(Output_section::sort_attached_input_sections): Check if input section
	must be reordered.
	* plugin.cc (get_object_handle): New function.
	(get_section_count): New function.
	(get_section_type): New function.
	(get_section_name): New function.
	(get_section_contents): New function.
	(read_layout_from_file): New function.
	(allow_section_ordering): New function.	
	(Plugin::load): Add the new interfaces to the transfer vector.
	(Plugin_manager::load_plugins): New parameter.
	* plugin.h (input_objects): New function.
	(layout): New function.
	* plugin_test.c (get_object_handle): New function pointer.
	(get_section_count): New function pointer.
	(get_section_type): New function pointer.
	(get_section_name): New function pointer.
	(get_section_contents): New function pointer.
	(read_layout_from_file): New function pointer.
	(allow_section_ordering): New function pointer.
	(onload): Check if the new interfaces exist.

Thanks,
-Sri.

Attachment: gold_patch.txt
Description: Text document