Re: [PATCH v4] eliminate mutex in fast path of __register_frame

[Dimitar Dimitrov <dimitar@dinux.eu>]

On Fri, Sep 16, 2022 at 12:19:36PM +0200, Thomas Neumann via Gcc-patches wrote:
> 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.
> 
> Changes since v3:
> - Avoid code duplication by adding query mode to classify_object_over_fdes
> - Adjust all comments as requested
> 
> 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   | 195 ++++++--
>  libgcc/unwind-dw2-fde.h   |   2 +-
>  3 files changed, 1098 insertions(+), 52 deletions(-)
>  create mode 100644 libgcc/unwind-dw2-btree.h
> 

Hi Thomas,

This patch broke avr and pru-elf cross builds:
  gcc/libgcc/unwind-dw2-fde.c:680:28: error: unknown type name ‘uintptr_t’
  680 |                            uintptr_t *range)

Should uintptr_t be replaced with __UINTPTR_TYPE__? Such change fixes the
above broken builds for me. But I'm not sure how valid it is for that
part of libgcc.

Other embedded targets like arm-none-eabi are not broken because they
overwrite LIB2ADDEH, and consequently unwind-dw2-fde.c is not built for
them.

Regards,
Dimitar