[2/2] Early LTO debug -- main part

This is the main part of the early LTO debug support.  The main parts
of the changes are to dwarf2out.c where most of the changes are related
to the fact that we eventually have to output debug info twice, once
for the early LTO part and once for the fat part of the object file.

Bootstrapped and tested on x86_64-unknown-linux-gnu with ASAN and TSAN
extra FAILs (see PR78063, a libbacktrace missing feature or libsanitizer
being too pessimistic).  There's an extra

XPASS: gcc.dg/guality/inline-params.c   -O2 -flto -fuse-linker-plugin 
-fno-fat-lto-objects  execution test

the previously reported extra VLA guality FAILs are gone.

I've compared testresults with -flto -g added for all languages and
only see expected differences (libstdc++ pretty printers now work,
most scan-assembler-times debug testcases fail because we have everything
twice now).

See https://gcc.gnu.org/ml/gcc-patches/2016-08/msg01842.html for
the last posting of this patch which has a high-level overview of
Early LTO debug.  You may want to refer to the slides I presented
at the GNU Cauldron as well.

Thanks,
Richard.

2016-10-21  Richard Biener  <rguenther@suse.de>

        * debug.h (struct gcc_debug_hooks): Add die_ref_for_decl and
        register_external_die hooks.
        (debug_false_tree_charstarstar_uhwistar): Declare.
        (debug_nothing_tree_charstar_uhwi): Likewise.
        * debug.c (do_nothing_debug_hooks): Adjust.
        (debug_false_tree_charstarstar_uhwistar): New do nothing.
        (debug_nothing_tree_charstar_uhwi): Likewise.
        * dbxout.c (dbx_debug_hooks): Adjust.
        (xcoff_debug_hooks): Likewise.
        * sdbout.c (sdb_debug_hooks): Likewise.
        * vmsdbgout.c (vmsdbg_debug_hooks): Likewise.

        * dwarf2out.c (macinfo_label_base): New global.
	(dwarf2out_register_external_die): New function for the
	register_external_die hook.
        (dwarf2out_die_ref_for_decl): Likewise for die_ref_for_decl.
        (dwarf2_debug_hooks): Use them.
        (dwarf2_lineno_debug_hooks): Adjust.
        (struct die_struct): Add with_offset flag.
        (DEBUG_LTO_DWO_INFO_SECTION, DEBUG_LTO_INFO_SECTION,
        DEBUG_LTO_DWO_ABBREV_SECTION, DEBUG_LTO_ABBREV_SECTION,
        DEBUG_LTO_DWO_MACINFO_SECTION, DEBUG_LTO_MACINFO_SECTION,
        DEBUG_LTO_DWO_MACRO_SECTION, DEBUG_LTO_MACRO_SECTION,
        DEBUG_LTO_LINE_SECTION, DEBUG_LTO_DWO_STR_OFFSETS_SECTION,
        DEBUG_LTO_STR_DWO_SECTION, DEBUG_STR_LTO_SECTION): New macros
        defining section names for the early LTO debug variants.
	(reset_indirect_string): New helper.
        (add_AT_external_die_ref): Helper for dwarf2out_register_external_die.
        (print_dw_val): Add support for offsetted symbol references.
        (compute_section_prefix_1): Split out worker to distinguish
        the comdat from the LTO case.
        (compute_section_prefix): Wrap old comdat case here.
        (output_die): Skip DIE symbol output for the LTO added one.
        Handle DIE symbol references with offset.
        (output_comp_unit): Guard section name mangling properly.
        For LTO debug sections emit a symbol at the section beginning
        which we use to refer to its DIEs.
        (add_abstract_origin_attribute): For DIEs registered via
        dwarf2out_register_external_die directly refer to the early
        DIE rather than indirectly through the shadow one we created.
        (gen_array_type_die): When generating early LTO debug do
        not emit DW_AT_string_length.
        (gen_formal_parameter_die): Do not re-create DIEs for PARM_DECLs
        late when in LTO.
        (gen_subprogram_die): Adjust the check for whether we face
        a concrete instance DIE for an inline we can reuse for the
        late LTO case.  Likewise avoid another specification DIE
        for early built declarations/definitions for the late LTO case.
        (gen_variable_die): Add type references for late duplicated VLA dies
        when in late LTO.
        (gen_inlined_subroutine_die): Do not call dwarf2out_abstract_function,
        we have the abstract instance already.
        (process_scope_var): Adjust decl DIE contexts in LTO which
        first puts them in limbo.
        (gen_decl_die): Do not generate type DIEs late apart from
        types for VLAs or for decls we do not yet have a DIE.
        (dwarf2out_early_global_decl): Make sure to create DIEs
        for abstract instances of a decl first.
        (dwarf2out_late_global_decl): Adjust comment.
        (output_macinfo_op): With multiple macro sections use
	macinfo_label_base to distinguish labels.
        (output_macinfo): Likewise.  Update macinfo_label_base.
	Pass in the line info label.
        (init_sections_and_labels): Add early LTO debug flag parameter
        and generate different sections and names if set.  Add generation
        counter for the labels so we can have multiple of them.
        (reset_dies): Helper to allow DIEs to be output multiple times.
        (dwarf2out_finish): When outputting DIEs to the fat part of an
	LTO object first reset DIEs.
        (dwarf2out_early_finish): Output early DIEs when generating LTO.

Cleanups we can do (and need) when removing the "old" LTO path and add
the early LTO path.

        (set_decl_abstract_flags): Remove.
        (set_block_abstract_flags): Likewise.
        (dwarf2out_abstract_function): Treat the early generated DIEs
        as the abstract copy and only add DW_AT_inline and
        DW_AT_artificial here.

        * lto-streamer-in.c: Include debug.h.
        (dref_queue): New global.
        (lto_read_tree_1): Stream in DIE references.
        (lto_input_tree): Register DIE references.
	(input_function): Stream DECL_DEBUG_ARGS.
        * lto-streamer-out.c: Include debug.h.
        (lto_write_tree_1): Output DIE references.
        (DFS::DFS_write_tree_body): Follow DECL_ABSTRACT_ORIGIN.
	(output_function): Stream DECL_DEBUG_ARGS.
        * tree-streamer-in.c (lto_input_ts_decl_common_tree_pointers):
        Stream DECL_ABSTRACT_ORIGIN.
        * tree-streamer-out.c (write_ts_decl_common_tree_pointers): Likewise.
        * lto-streamer.h (struct dref_entry): Declare.
        (dref_queue): Likewise.

        * lto-wrapper.c (debug_obj): New global.
        (tool_cleanup): Unlink it if required.
        (debug_objcopy): New function.
        (run_gcc): Handle early debug sections in the IL files by
        extracting them to separate files, partially linkin them and
        feeding the result back as result to the linker.

        * config/darwin.h (DEBUG_LTO_INFO_SECTION, DEBUG_LTO_ABBREV_SECTION,
        DEBUG_LTO_MACINFO_SECTION, DEBUG_LTO_LINE_SECTION,
        DEBUG_STR_LTO_SECTION, DEBUG_LTO_MACRO_SECTION): Put early debug
        sections into a separate segment.
        * config/darwin.c (darwin_asm_named_section): Handle __GNU_DWARF_LTO
        segments.
        (darwin_asm_dwarf_section): Likewise.
        (darwin_asm_output_dwarf_offset): Likewise.

        lto/
        * lto.c (unify_scc): Truncate DIE reference queue for dropped SCCs.
        (lto_read_decls): Process TRANSLATION_UNIT_DECLs.  Remove
        TYPE_DECL debug processing, register DIE references from
        prevailing SCCs with the debug machinery.
        (lto_section_with_id): Handle LTO debug sections.

[2/2] Early LTO debug -- main part

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