[Bug libstdc++/107675] [13 Regression] GCC-13 is significantly slower to startup on C++ programs

["tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107675

--- Comment #10 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
I've bisected this to:

commit 6e80a1d164d1f996ad08a512c000025a7c2ca893
Author: Thomas Neumann <tneumann@users.sourceforge.net>
Date:   Tue Mar 1 21:57:35 2022 +0100

    eliminate mutex in fast path of __register_frame

    The __register_frame/__deregister_frame functions are used to register
    unwinding frames from JITed code in a sorted list. That list itself
    is protected by object_mutex, which leads to terrible performance
    in multi-threaded code and is somewhat expensive even if single-threaded.
    There was already a fast-path that avoided taking the mutex if no
    frame was registered at all.

    This commit eliminates both the mutex and the sorted list from
    the atomic fast path, and replaces it with a btree that uses
    optimistic lock coupling during lookup. This allows for fully parallel
    unwinding and is essential to scale exception handling to large
    core counts.

    libgcc/ChangeLog:

            * unwind-dw2-fde.c (release_registered_frames): Cleanup at
shutdown.
            (__register_frame_info_table_bases): Use btree in atomic fast path.
            (__deregister_frame_info_bases): Likewise.
            (_Unwind_Find_FDE): Likewise.
            (base_from_object): Make parameter const.
            (classify_object_over_fdes): Add query-only mode.
            (get_pc_range): Compute PC range for lookup.
            * unwind-dw2-fde.h (last_fde): Make parameter const.
            * unwind-dw2-btree.h: New file.

 libgcc/unwind-dw2-btree.h | 953 ++++++++++++++++++++++++++++++++++++++++++++++
 libgcc/unwind-dw2-fde.c   | 194 +++++++---
 libgcc/unwind-dw2-fde.h   |   2 +-
 3 files changed, 1098 insertions(+), 51 deletions(-)
 create mode 100644 libgcc/unwind-dw2-btree.h

Looking at the patch it looks like it now forces an eager registration of
objects during frame registration vs a lazy initialization that was there
before.