[RFC,Offloading] Fix PR68463

Hi!

On Tue, Jan 19, 2016 at 16:32:13 +0300, Ilya Verbin wrote:
> On Tue, Jan 19, 2016 at 10:36:28 +0100, Jakub Jelinek wrote:
> > On Tue, Jan 19, 2016 at 09:57:01AM +0100, Richard Biener wrote:
> > > On Mon, 18 Jan 2016, Ilya Verbin wrote:
> > > > On Fri, Jan 15, 2016 at 09:15:01 +0100, Richard Biener wrote:
> > > > > On Fri, 15 Jan 2016, Ilya Verbin wrote:
> > > > > > II) The __offload_func_table, __offload_funcs_end, __offload_var_table,
> > > > > > __offload_vars_end are now provided by the linker script, instead of
> > > > > > crtoffload{begin,end}.o, this allows to surround all offload objects, even
> > > > > > those that are not claimed by lto-plugin.
> > > > > > Unfortunately it works only with ld, but doen't work with gold, because
> > > > > > https://sourceware.org/bugzilla/show_bug.cgi?id=15373
> > > > > > Any thoughts how to enable this linker script for gold?
> > > > > 
> > > > > The easiest way would probably to add this handling to the default
> > > > > "linker script" in gold.  I don't see an easy way around requiring
> > > > > changes to gold here - maybe dumping the default linker script from
> > > > > bfd and injecting the rules with some scripting so you have a complete
> > > > > script.  Though likely gold won't grok that result.
> > > > > 
> > > > > Really a question for Ian though.
> > > > 
> > > > Or the gcc driver can add crtoffload{begin,end}.o, but the problem is that it
> > > > can't determine whether the program contains offloading or not.  So it can add
> > > > them to all -fopenmp/-fopenacc programs, if the compiler was configured with
> > > > --enable-offload-targets=...  The overhead would be about 340 bytes for
> > > > binaries which doesn't use offloading.  Is this acceptable?  (Jakub?)
> > > 
> > > Can lto-wrapper add them as plugin outputs?  Or does that wreck ordering?
> 
> Currently it's implemented this way, but it will not work after my patch,
> because e.g. offload-without-lto.o and offload-with-lto.o will be linked in
> this order:
> offload-without-lto.o, crtoffloadbegin.o, offload-with-lto.o, crtoffloadend.o
> ^^^^^^^^^^^^^^^^^^^^^
> (will be not claimed by the plugin)
> 
> But we need this one:
> crtoffloadbegin.o, offload-without-lto.o, offload-with-lto.o, crtoffloadend.o
> 
> > Yeah, if that would work, it would be certainly appreciated, one thing is
> > wasting .text space and relocations in all -fopenmp programs (for -fopenacc
> > programs one kind of assumes there will be some offloading in there),
> > another one some extra constructor/destructor or what that would be even
> > worse.
> 
> They contain only 5 symbols, without constructors/destructors.

This patch adds crtoffload{begin,end}.o to all -fopenmp programs, if they exist.
I couldn't think of a better solution...
Tested using the testcase from the previous mail, e.g.:

$ gcc -DNUM=1 -c -fopenmp test.c -o obj1.o
$ gcc -DNUM=2 -c -fopenmp test.c -o obj2.o
$ gcc -DNUM=3 -c -fopenmp test.c -o obj3.o
$ gcc -DNUM=4 -c -fopenmp test.c -o obj4.o -flto
$ gcc -DNUM=5 -c -fopenmp test.c -o obj5.o
$ gcc -DNUM=6 -c -fopenmp test.c -o obj6.o -flto
$ gcc -DNUM=7 -c -fopenmp test.c -o obj7.o
$ gcc-ar -cvq libtest.a obj3.o obj4.o obj5.o
$ gcc -fopenmp main.c obj1.o obj2.o libtest.a obj6.o obj7.o

And other combinations.

gcc/
	PR driver/68463
	* config/gnu-user.h (GNU_USER_TARGET_STARTFILE_SPEC): Add
	crtoffloadbegin.o for -fopenacc/-fopenmp if it exists.
	(GNU_USER_TARGET_ENDFILE_SPEC): Add crtoffloadend.o for
	-fopenacc/-fopenmp if it exists.
	* lto-wrapper.c (offloadbegin, offloadend): Remove static vars.
	(offload_objects_file_name): New static var.
	(tool_cleanup): Remove offload_objects_file_name file.
	(copy_file): Remove function.
	(find_offloadbeginend): Remove function.
	(run_gcc): Remove offload_argc and offload_argv.
	Get offload_objects_file_name from -foffload-objects=... option.
	Read names of object files with offload from this file, pass them to
	compile_images_for_offload_targets.  Don't call find_offloadbeginend and
	don't pass offloadbegin and offloadend to the linker.  Don't pass
	offload non-LTO files to the linker, because now they're not claimed.
lto-plugin/
	PR driver/68463
	* lto-plugin.c (struct plugin_offload_file): New.
	(offload_files): Change type.
	(offload_files_last, offload_files_last_obj): New.
	(offload_files_last_lto): New.
	(free_2): Adjust accordingly.
	(all_symbols_read_handler): Don't add offload files to lto_arg_ptr.
	Don't call free_1 for offload_files.  Write names of object files with
	offloading to the temporary file.  Add new option to lto_arg_ptr.
	(claim_file_handler): Don't claim file if it contains offload sections
	without LTO sections.  If it contains offload sections, add to the list.

  -- Ilya

[RFC,Offloading] Fix PR68463

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