[Bug gdb/23710] gdb is slow and memory hungry consuming debug generated with LTO by GCC

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=23710

--- Comment #13 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Tom de Vries <vries@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=589902954da0d1dd140b33e578954746c9bfc374

commit 589902954da0d1dd140b33e578954746c9bfc374
Author: Tom de Vries <tdevries@suse.de>
Date:   Tue Mar 17 08:56:36 2020 +0100

    [gdb] Skip imports of c++ CUs

    The DWARF standard appendix E.1 describes techniques that can be used for
    compression and deduplication: DIEs can be factored out into a new
compilation
    unit, and referenced using DW_FORM_ref_addr.

    Such a new compilation unit can either use a DW_TAG_compile_unit or
    DW_TAG_partial_unit.  If a DW_TAG_compile_unit is used, its contents is
    evaluated by consumers as though it were an ordinary compilation unit.  If
a
    DW_TAG_partial_unit is used, it's only considered by consumers in the
context
    of a DW_TAG_imported_unit.

    An example of when DW_TAG_partial_unit is required is when the factored out
    DIEs are not top-level, f.i. because they were children of a namespace.  In
    such a case the corresponding DW_TAG_imported_unit will occur as child of
the
    namespace.

    In the case of factoring out DIEs from c++ compilation units, we can factor
    out into a new DW_TAG_compile_unit, and no DW_TAG_imported_unit is
required.

    This begs the question how to interpret a top-level DW_TAG_imported_unit of
a
    c++ DW_TAG_compile_unit compilation unit.  The semantics of
    DW_TAG_imported_unit describe that the imported unit logically appears at
the
    point of the DW_TAG_imported_unit entry.  But it's not clear what the
effect
    should be in this case, since all the imported DIEs are already globally
    visible anyway, due to the use of DW_TAG_compile_unit.

    So, skip top-level imports of c++ DW_TAG_compile_unit compilation units in
    process_imported_unit_die.

    Using the cc1 binary from PR23710 comment 1 and setting a breakpoint on
do_rpo_vn:
    ...
    $ gdb \
        -batch \
        -iex "maint set dwarf max-cache-age 316" \
        -iex "set language c++" \
        -ex "b do_rpo_vn" \
        cc1
    ...
    we get a 8.1% reduction in execution time, due to reducing the number of
    partial symtabs expanded into full symtabs from 212 to 175.

    Build and reg-tested on x86_64-linux.

    gdb/ChangeLog:

    2020-03-17  Tom de Vries  <tdevries@suse.de>

            PR gdb/23710
            * dwarf2/read.h (struct dwarf2_per_cu_data): Add unit_type and lang
            fields.
            * dwarf2/read.c (process_psymtab_comp_unit): Initialize unit_type
and lang
            fields.
            (process_imported_unit_die): Skip import of c++ CUs.

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