Re: [PATCH 14/14] Share DWARF partial symtabs

Luis Machado
2020-02-18T12:26:23Z

On 2/15/20 1:54 PM, Tom Tromey wrote:
> This changes the DWARF reader to share partial symtabs (or indices if
> they are available) across objfiles. This has a few parts.
>
> * The dwarf2_unshareable object is created per-objfile and is stored
> on the objfile using a registry key. dwarf2_enter_objfile is
> modified to save and restore this pointer in the dwarf2_per_objfile.
>
> * objfile::partial_symtabs is changed to be a shared_ptr again. This
> lets us stash a second reference in dwarf2_per_objfile; if the DWARF
> data is being shared, we can simply copy this value to the new
> objfile.
>
> * The dwarf2_per_objfile itself may be shared via the BFD -- using a
> new per-BFD registry key -- or not. This depends both on whether
> some other symbol reader has found symbols, and whether any BFD
> sections require relocation.
>
> 2020-02-15 Tom Tromey <tom@tromey.com>
>
> * objfiles.h (struct objfile) <partial_symtabs>: Now a
> shared_ptr.
> * dwarf2/read.h (struct dwarf2_per_objfile) <partial_symtabs>: New
> member.
> (class dwarf2_enter_objfile): Constructor no longer inline.
> <m_restore_unshared>: New member.
> * dwarf2/read.c (dwarf2_unshared_data_key, dwarf2_bfd_data_key):
> New globals.
> (dwarf2_objfile_data_key): Add comment.
> (get_dwarf2_per_objfile): Rewrite.
> (dwarf2_enter_objfile::dwarf2_enter_objfile): Move from read.h.
> Initialize m_restore_unshared.
> (dwarf2_per_objfile::dwarf2_per_objfile): Add "unshared"
> parameter. Don't initialize objfile member.
> (dwarf2_per_objfile::~dwarf2_per_objfile): Don't deleted
> "unshareable".

"Don't delete..."

> (dwarf2_has_info): Check dwarf2_bfd_data_key and
> dwarf2_unshared_data_key.
> (dwarf2_get_section_info): Use get_dwarf2_per_objfile.
> (dwarf2_build_psymtabs): Set objfile::partial_symtabs and
> short-circuit when sharing.
> (dwarf2_build_psymtabs): Set dwarf2_per_objfile::partial_symtabs.
> (dwarf2_psymtab::expand_psymtab): Use free_cached_comp_units.
> ---
> gdb/ChangeLog | 26 ++++++++++++++++++
> gdb/dwarf2/read.c | 70 +++++++++++++++++++++++++++++++++++++----------
> gdb/dwarf2/read.h | 12 ++++----
> gdb/objfiles.h | 2 +-
> 4 files changed, 89 insertions(+), 21 deletions(-)
>
> diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
> index 15813b72005..7be57661296 100644
> --- a/gdb/dwarf2/read.c
> +++ b/gdb/dwarf2/read.c
> @@ -100,7 +100,15 @@ static bool check_physname = false;
> /* When true, do not reject deprecated .gdb_index sections. */
> static bool use_deprecated_index_sections = false;
>
> -static const struct objfile_key<dwarf2_per_objfile> dwarf2_objfile_data_key;
> +/* This is used to store the data that is always per objfile. */
> +static const objfile_key<dwarf2_unshareable> dwarf2_unshared_data_key;
> +
> +/* These are used to store the dwarf2_per_objfile; either on the
> + objfile or on the BFD. */
> +static const struct objfile_key<dwarf2_per_objfile>
> + dwarf2_objfile_data_key;
> +static const struct bfd_key<dwarf2_per_objfile>
> + dwarf2_bfd_data_key;
>
> /* The "aclass" indices for various kinds of computed DWARF symbols. */
>
> @@ -274,7 +282,18 @@ struct mapped_debug_names final : public mapped_index_base
> dwarf2_per_objfile *
> get_dwarf2_per_objfile (struct objfile *objfile)
> {
> - return dwarf2_objfile_data_key.get (objfile);
> + dwarf2_per_objfile *result = dwarf2_bfd_data_key.get (objfile->obfd);
> + if (result == nullptr)
> + result = dwarf2_objfile_data_key.get (objfile);
> + return result;
> +}
> +
> +dwarf2_enter_objfile::dwarf2_enter_objfile (struct objfile *objfile)
> + : m_per_objfile (get_dwarf2_per_objfile (objfile)),
> + m_restore_objfile (&m_per_objfile->objfile, objfile),
> + m_restore_unshared (&m_per_objfile->unshareable,
> + dwarf2_unshared_data_key.get (objfile))
> +{
> }
>
> /* Default names of the debugging sections. */
> @@ -1754,16 +1773,12 @@ line_header_eq_voidp (const void *item_lhs, const void *item_rhs)
> dwarf2_per_objfile::dwarf2_per_objfile (struct objfile *objfile_,
> const dwarf2_debug_sections *names,
> bool can_copy_)
> - : objfile (objfile_),
> - can_copy (can_copy_),
> - /* Temporarily just allocate one per objfile -- we don't have
> - sharing yet. */
> - unshareable (new dwarf2_unshareable)
> + : can_copy (can_copy_)
> {
> if (names == NULL)
> names = &dwarf2_elf_names;
>
> - bfd *obfd = objfile->obfd;
> + bfd *obfd = objfile_->obfd;
>
> for (asection *sec = obfd->sections; sec != NULL; sec = sec->next)
> locate_sections (obfd, sec, *names);
> @@ -1780,8 +1795,6 @@ dwarf2_per_objfile::~dwarf2_per_objfile ()
> for (signatured_type *sig_type : all_type_units)
> sig_type->per_cu.imported_symtabs_free ();
>
> - delete unshareable;
> -
> /* Everything else should be on the objfile obstack. */
> }
>
> @@ -1841,13 +1854,26 @@ dwarf2_has_info (struct objfile *objfile,
> if (objfile->flags & OBJF_READNEVER)
> return 0;
>
> + struct dwarf2_unshareable *unshared = dwarf2_unshared_data_key.get (objfile);
> + if (unshared == nullptr)
> + unshared = dwarf2_unshared_data_key.emplace (objfile);
> +
> struct dwarf2_per_objfile *dwarf2_per_objfile
> = get_dwarf2_per_objfile (objfile);
>
> if (dwarf2_per_objfile == NULL)
> - dwarf2_per_objfile = dwarf2_objfile_data_key.emplace (objfile, objfile,
> - names,
> - can_copy);
> + {
> + dwarf2_per_objfile = new ::dwarf2_per_objfile (objfile, names,
> + can_copy);
> + /* We can share this if the objfile doesn't require relocations
> + and if there aren't partial symbols from some other
> + reader. */
> + if (!objfile_has_partial_symbols (objfile)
> + && !gdb_bfd_requires_relocations (objfile->obfd))
> + dwarf2_bfd_data_key.set (objfile->obfd, dwarf2_per_objfile);
> + else
> + dwarf2_objfile_data_key.set (objfile, dwarf2_per_objfile);
> + }
>
> return (!dwarf2_per_objfile->info.is_virtual
> && dwarf2_per_objfile->info.s.section != NULL
> @@ -2006,7 +2032,7 @@ dwarf2_get_section_info (struct objfile *objfile,
> asection **sectp, const gdb_byte **bufp,
> bfd_size_type *sizep)
> {
> - struct dwarf2_per_objfile *data = dwarf2_objfile_data_key.get (objfile);
> + struct dwarf2_per_objfile *data = get_dwarf2_per_objfile (objfile);
> struct dwarf2_section_info *info;
>
> /* We may see an objfile without any DWARF, in which case we just
> @@ -5862,6 +5888,15 @@ dwarf2_build_psymtabs (struct objfile *objfile)
> struct dwarf2_per_objfile *dwarf2_per_objfile
> = get_dwarf2_per_objfile (objfile);
>
> + if (dwarf2_per_objfile->partial_symtabs != nullptr)
> + {
> + /* Partial symbols were already read, so now we can simply
> + attach them. */
> + objfile->partial_symtabs = dwarf2_per_objfile->partial_symtabs;
> + dwarf2_resize_unshareable (dwarf2_per_objfile);
> + return;
> + }
> +
> init_psymbol_list (objfile, 1024);
>
> try
> @@ -5882,6 +5917,11 @@ dwarf2_build_psymtabs (struct objfile *objfile)
> {
> exception_print (gdb_stderr, except);
> }
> +
> + /* Finish by setting the local reference to partial symtabs, so that
> + we don't try to read again. If we can't in fact share, this
> + won't make a difference anyway. */
> + dwarf2_per_objfile->partial_symtabs = objfile->partial_symtabs;
> }
>
> /* Find the base address of the compilation unit for range lists and
> @@ -8915,8 +8955,8 @@ dwarf2_psymtab::expand_psymtab (struct objfile *objfile)
> if (symtab.has_value ())
> return;
>
> + free_cached_comp_units freer (dwarf2_per_objfile);
> read_dependencies (objfile);
> -
> dw2_do_instantiate_symtab (per_cu_data, false);
> }
>

Spurious line removal, but it doesn't matter much.

> diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
> index 241f3c8e71e..891a844a673 100644
> --- a/gdb/dwarf2/read.h
> +++ b/gdb/dwarf2/read.h
> @@ -299,6 +299,11 @@ public:
> /* CUs that are queued to be read. */
> std::queue<dwarf2_queue_item> queue;
>
> + /* We keep a separate reference to the partial symtabs, in case we
> + are sharing them between objfiles. This is only set after
> + partial symbols have been read the first time. */
> + std::shared_ptr<psymtab_storage> partial_symtabs;
> +
> /* The total number of per_cu and signatured_type objects that have
> been created for this reader. */
> size_t num_psymtabs = 0;
> @@ -321,11 +326,7 @@ class dwarf2_enter_objfile
> {
> public:
>
> - dwarf2_enter_objfile (struct objfile *objfile)
> - : m_per_objfile (get_dwarf2_per_objfile (objfile)),
> - m_restore_objfile (&m_per_objfile->objfile, objfile)
> - {
> - }
> + dwarf2_enter_objfile (struct objfile *objfile);
>
> ~dwarf2_enter_objfile () = default;
>
> @@ -335,6 +336,7 @@ private:
>
> dwarf2_per_objfile *m_per_objfile;
> scoped_restore_tmpl<struct objfile *> m_restore_objfile;
> + scoped_restore_tmpl<dwarf2_unshareable *> m_restore_unshared;
> };
>
> /* A partial symtab specialized for DWARF. */
> diff --git a/gdb/objfiles.h b/gdb/objfiles.h
> index b71a8a9edb8..12892f80f71 100644
> --- a/gdb/objfiles.h
> +++ b/gdb/objfiles.h
> @@ -558,7 +558,7 @@ public:
>
> /* The partial symbol tables. */
>
> - std::unique_ptr<psymtab_storage> partial_symtabs;
> + std::shared_ptr<psymtab_storage> partial_symtabs;
>
> /* The object file's BFD. Can be null if the objfile contains only
> minimal symbols, e.g. the run time common symbols for SunOS4. */
>

I've gone through the series and didn't see anything obviously wrong. I
suppose this will get more thoroughly exercised when running in
multi-process/multi-target mode with quite a few objfiles and libraries.

Is it currently possible to exercise such a scenario automatically? I
suppose manually loading something like a web browser will be a good
test as well.