[PATCH v2 00/42] Share DWARF partial symtabs between objfiles

[Simon Marchi]

On 2020-05-12 5:08 p.m., Simon Marchi via Gdb-patches wrote:
> This is version 2 of this patchset originally written by Tom:
> 
>     https://sourceware.org/legacy-ml/gdb-patches/2020-02/msg00594.html
> 
> The current patchset therefore contains a mix of Tom's patches that I modified,
> plus a few new patches.  I won't repeat everything that is said in the cover
> letter of that patchset, please read that to understand the motivation behind
> this change..
> 
> The big difference between v1 and v2 is how the split is done.  In v1, the
> object structure looks like this:
> 
>     objfile -> dwarf2_per_objfile -> dwarf2_unshareable
> 
> where `dwarf2_per_objfile` is actually shared between objfiles (there is one
> copy per BFD), and `dwarf2_unshareable` is not (there is one copy per objfile).
> The dwarf2_per_objfile -> dwarf2_unshareable link is updated at all entry
> points of the DWARF code based on what is the current objfile.
> 
> The current patchset (v2) proposes this structure instead, which in my opinion
> is more natural:
> 
>     objfile -> dwarf2_per_objfile -> dwarf2_per_bfd
> 
> Where `dwarf2_per_objfile` is really per-objfile (there is one copy per
> objfile) and `dwarf2_per_bfd` is per-bfd (there is one copy per-bfd).  There is
> no link to update, whatever is shared between objfiles is put in
> `dwarf2_per_bfd` and each `dwarf2_per_objfile` always points to one
> `dwarf2_per_bfd`.
> 
> Since there were performance numbers with the original patchset, I made my own
> here to see if the speedups were still there.  Like in the original patchset, I
> set "maintenance time 1" and did "add-inferior -exec ./gdb" after "./gdb gdb".
> I disabled any auto-loading to avoid loading gdb-gdb.{gdb,py}.  GDB is built
> with -O2 and gcc 9, and it is running on an AMD EPYC 7351P.
> 
> Before: Command execution time: 1.213480 (cpu), 1.214268 (wall)
> After: Command execution time: 0.001697 (cpu), 0.001694 (wall)
> 
> These are the times of one run, but all runs I did are very close.  The "after"
> time is surpringly low compared to the original numbers, I hope it doesn't mean
> I messed up something and we are skipping something important...
> 
> At some point I tested with all these boards and did not see any regression
> (they helped very much to find some issues in the process though):
> 
>     unix cc-with-debug-names cc-with-dwz cc-with-dwz-m cc-with-gdb-index dwarf4-gdb-index fission-dwp fission readnow
> 
> I have since rebased the patchset and resolved conflicts, it's therefore
> possible I messed something up.
> 
> Simon Marchi (35):
>   Split dwarf2_per_objfile into dwarf2_per_objfile and dwarf2_per_bfd
>   Remove symtab links from dwarf2_psymtab and dwarf2_per_cu_quick_data
>   Move die_type_hash to dwarf2_per_objfile
>   Add dwarf2_per_objfile field to dwarf2_cu
>   Remove reference to dwarf2_per_cu_data::dwarf2_per_objfile in
>     dw2_do_instantiate_symtab
>   Remove dwarf2_cu->per_cu->dwarf2_per_objfile references
>   Add dwarf2_per_bfd field to dwarf2_per_cu_data
>   Make dwarf2_get_dwz_file take a dwarf2_per_bfd
>   Use bfd_get_filename instead of objfile_name in lookup_dwo_unit
>   Add dwarf2_per_objfile parameter to cutu_reader's constructors
>   Make queue_and_load_dwo_tu receive a dwarf2_cu
>   Remove dwarf2_per_cu_data::dwarf2_per_objfile reference in
>     cutu_reader::keep
>   Add dwarf2_per_objfile parameter to create_partial_symtab
>   Add dwarf2_per_objfile parameter to recursively_compute_inclusions
>   Add dwarf2_per_objfile parameter to process_full_{comp,type}_unit
>   Pass dwarf2_cu objects to dwo-related functions, instead of
>     dwarf2_per_cu_data
>   Remove reference to dwarf2_per_cu_data::dwarf2_per_objfile in
>     queue_and_load_all_dwo_tus
>   Move int type methods out of dwarf2_per_cu_data
>   Add dwarf2_per_objfile to dwarf_expr_context and dwarf2_frame_cache
>   Remove dwarf2_per_cu_data::text_offset
>   Add dwarf2_per_objfile parameter to dwarf2_read_addr_index
>   Add dwarf2_per_objfile parameter to allocate_piece_closure
>   Add dwarf2_per_objfile parameters to dwarf2_fetch_* functions
>   Remove dwarf2_per_cu_data::objfile ()
>   Add dwarf2_per_objfile parameter to free_one_cached_comp_unit
>   Add dwarf2_per_objfile parameter to get_die_type_at_offset
>   Remove leftover references to dwarf2_per_cu_data::dwarf2_per_objfile
>   Remove dwarf2_per_cu_data::dwarf2_per_objfile
>   Pass dwarf2_per_bfd instead of dwarf2_per_objfile to some
>     index-related functions
>   Pass dwarf2_cu to process_full_{comp,type}_unit
>   Make load_cu return the loaded dwarf2_cu
>   Add comp_unit_head to dwarf2_per_cu_data
>   Pass existing_cu object to cutu_reader
>   Replace dwarf2_per_cu_data::cu backlink with per-objfile map
>   Make mapped_debug_names independent of objfile
> 
> Tom Tromey (7):
>   Introduce dwarf2_per_objfile::obstack
>   Add "objfile" parameter to two partial_symtab methods
>   Add dwarf2_per_cu_data::index
>   Add dwarf2_per_objfile member to DWARF batons
>   Split type_unit_group
>   Move signatured_type::type to unshareable object
>   Share DWARF partial symtabs
> 
>  gdb/compile/compile-loc2c.c |   20 +-
>  gdb/compile/compile.h       |   13 +-
>  gdb/dwarf2/expr.c           |   11 +-
>  gdb/dwarf2/expr.h           |   11 +-
>  gdb/dwarf2/frame.c          |   67 +-
>  gdb/dwarf2/index-cache.c    |    2 +-
>  gdb/dwarf2/index-write.c    |   51 +-
>  gdb/dwarf2/loc.c            |  289 ++--
>  gdb/dwarf2/loc.h            |   11 +-
>  gdb/dwarf2/macro.c          |   11 +-
>  gdb/dwarf2/read.c           | 2490 ++++++++++++++++++-----------------
>  gdb/dwarf2/read.h           |  298 +++--
>  gdb/gdbtypes.h              |    8 +-
>  gdb/objfiles.h              |    2 +-
>  gdb/psympriv.h              |   19 +-
>  gdb/psymtab.c               |   44 +-
>  16 files changed, 1848 insertions(+), 1499 deletions(-)
> 
> -- 
> 2.26.2
> 

For convenience, I pushed this series on user branch `users/simark/submit/share-dwarf-partial-symtabs-v2`:

  https://sourceware.org/git/?p=binutils-gdb.git;a=shortlog;h=refs/heads/users/simark/submit/share-dwarf-partial-symtabs-v2

Simon