Re: [PATCH v8 1/5] gdb: make gdbarch store a vector of frame unwinders

[Andrew Burgess <aburgess@redhat.com>]

Guinevere Larsen <guinevere@redhat.com> writes:

> Before this commit, all frame unwinders would be stored in the obstack
> of a gdbarch and accessed by using the registry system. This made for
> unwieldy code, and unnecessarily complex logic in the frame_unwinder
> implementation, along with making frame_unwind structs be unable to have
> non-trivial destructors.
>
> Seeing as a future patch of this series wants to refactor the
> frame_unwind struct to use inheritance, and we'd like to not restrict
> the future derived classes on what destructors are allowed. In
> preparation for that change, this commit changes the registry in gdbarch
> to instead store an std::vector, which doesn't require using an obstack
> and doesn't rely on a linked list.
>
> There should be no user-visible changes.
>
> Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
> ---
>  gdb/frame-unwind.c | 107 +++++++++++++++------------------------------
>  1 file changed, 36 insertions(+), 71 deletions(-)
>
> diff --git a/gdb/frame-unwind.c b/gdb/frame-unwind.c
> index 352779fcdcc..e61f6244913 100644
> --- a/gdb/frame-unwind.c
> +++ b/gdb/frame-unwind.c
> @@ -31,61 +31,42 @@
>  #include "cli/cli-cmds.h"
>  #include "inferior.h"
>  
> -struct frame_unwind_table_entry
> +/* Default sniffers, that must always be the first in the unwinder list,
> +   no matter the architecture.  */
> +static constexpr auto standard_unwinders =

I'm not a huge fan of `auto` when the type is well known.  My thinking
is write once, read many times.  So I'd rather have the type information
available when I read the code.  For me:

  static constexpr std::initializer_list<const frame_unwind *> standard_unwinders =

tells me what's going on...

>  {
> -  const struct frame_unwind *unwinder;
> -  struct frame_unwind_table_entry *next;
> +  &dummy_frame_unwind,
> +  /* The DWARF tailcall sniffer must come before the inline sniffer.
> +     Otherwise, we can end up in a situation where a DWARF frame finds
> +     tailcall information, but then the inline sniffer claims a frame
> +     before the tailcall sniffer, resulting in confusion.  This is
> +     safe to do always because the tailcall sniffer can only ever be
> +     activated if the newer frame was created using the DWARF
> +     unwinder, and it also found tailcall information.  */
> +  &dwarf2_tailcall_frame_unwind,
> +  &inline_frame_unwind,
>  };
>  
> -struct frame_unwind_table
> -{
> -  struct frame_unwind_table_entry *list = nullptr;
> -  /* The head of the OSABI part of the search list.  */
> -  struct frame_unwind_table_entry **osabi_head = nullptr;
> -};
> +/* If an unwinder should be prepended to the list, this is the
> +   index in which it should be inserted.  */
> +static constexpr int prepend_unwinder_index = standard_unwinders.size ();
>  
> -static const registry<gdbarch>::key<struct frame_unwind_table>
> +static const registry<gdbarch>::key<std::vector<const frame_unwind *>>
>       frame_unwind_data;
>  
> -/* A helper function to add an unwinder to a list.  LINK says where to
> -   install the new unwinder.  The new link is returned.  */
> -
> -static struct frame_unwind_table_entry **
> -add_unwinder (struct obstack *obstack, const struct frame_unwind *unwinder,
> -	      struct frame_unwind_table_entry **link)
> -{
> -  *link = OBSTACK_ZALLOC (obstack, struct frame_unwind_table_entry);
> -  (*link)->unwinder = unwinder;
> -  return &(*link)->next;
> -}
> -
> -static struct frame_unwind_table *
> +/* Retrieve the list of frame unwinders available in GDBARCH.
> +   If this list is empty, it is initialized before being returned.  */
> +static std::vector<const frame_unwind *> *
>  get_frame_unwind_table (struct gdbarch *gdbarch)

Given you're changing this code anyway, how about returning a reference
rather than a pointer, i.e.:

  static std::vector<const frame_unwind *> &
  get_frame_unwind_table (struct gdbarch *gdbarch)
  { ... }

The users of get_frame_unwind_table will need to be updated to match.

>  {
> -  struct frame_unwind_table *table = frame_unwind_data.get (gdbarch);
> +  std::vector<const frame_unwind *> *table = frame_unwind_data.get (gdbarch);
>    if (table != nullptr)
>      return table;
>  
> -  table = new frame_unwind_table;
> -
> -  /* Start the table out with a few default sniffers.  OSABI code
> -     can't override this.  */
> -  struct frame_unwind_table_entry **link = &table->list;
> +  table = new std::vector<const frame_unwind *>;
> +  table->insert (table->begin (), standard_unwinders.begin (),
> +		 standard_unwinders.end ());
>  
> -  struct obstack *obstack = gdbarch_obstack (gdbarch);
> -  link = add_unwinder (obstack, &dummy_frame_unwind, link);
> -  /* The DWARF tailcall sniffer must come before the inline sniffer.
> -     Otherwise, we can end up in a situation where a DWARF frame finds
> -     tailcall information, but then the inline sniffer claims a frame
> -     before the tailcall sniffer, resulting in confusion.  This is
> -     safe to do always because the tailcall sniffer can only ever be
> -     activated if the newer frame was created using the DWARF
> -     unwinder, and it also found tailcall information.  */
> -  link = add_unwinder (obstack, &dwarf2_tailcall_frame_unwind, link);
> -  link = add_unwinder (obstack, &inline_frame_unwind, link);
> -
> -  /* The insertion point for OSABI sniffers.  */
> -  table->osabi_head = link;
>    frame_unwind_data.set (gdbarch, table);
>  
>    return table;
> @@ -95,27 +76,16 @@ void
>  frame_unwind_prepend_unwinder (struct gdbarch *gdbarch,
>  				const struct frame_unwind *unwinder)
>  {
> -  struct frame_unwind_table *table = get_frame_unwind_table (gdbarch);
> -  struct frame_unwind_table_entry *entry;
> -
> -  /* Insert the new entry at the start of the list.  */
> -  entry = GDBARCH_OBSTACK_ZALLOC (gdbarch, struct frame_unwind_table_entry);
> -  entry->unwinder = unwinder;
> -  entry->next = (*table->osabi_head);
> -  (*table->osabi_head) = entry;
> +  std::vector<const frame_unwind *> *table = get_frame_unwind_table (gdbarch);
> +
> +  table->insert (table->begin () + prepend_unwinder_index, unwinder);
>  }
>  
>  void
>  frame_unwind_append_unwinder (struct gdbarch *gdbarch,
>  			      const struct frame_unwind *unwinder)
>  {
> -  struct frame_unwind_table *table = get_frame_unwind_table (gdbarch);
> -  struct frame_unwind_table_entry **ip;
> -
> -  /* Find the end of the list and insert the new entry there.  */
> -  for (ip = table->osabi_head; (*ip) != NULL; ip = &(*ip)->next);
> -  (*ip) = GDBARCH_OBSTACK_ZALLOC (gdbarch, struct frame_unwind_table_entry);
> -  (*ip)->unwinder = unwinder;
> +  get_frame_unwind_table (gdbarch)->push_back (unwinder);
>  }
>  
>  /* Call SNIFFER from UNWINDER.  If it succeeded set UNWINDER for
> @@ -188,9 +158,6 @@ frame_unwind_find_by_frame (const frame_info_ptr &this_frame, void **this_cache)
>    FRAME_SCOPED_DEBUG_ENTER_EXIT;
>    frame_debug_printf ("this_frame=%d", frame_relative_level (this_frame));
>  
> -  struct gdbarch *gdbarch = get_frame_arch (this_frame);
> -  struct frame_unwind_table *table = get_frame_unwind_table (gdbarch);
> -  struct frame_unwind_table_entry *entry;
>    const struct frame_unwind *unwinder_from_target;
>  
>    unwinder_from_target = target_get_unwinder ();
> @@ -205,8 +172,10 @@ frame_unwind_find_by_frame (const frame_info_ptr &this_frame, void **this_cache)
>  				   unwinder_from_target))
>      return;
>  
> -  for (entry = table->list; entry != NULL; entry = entry->next)
> -    if (frame_unwind_try_unwinder (this_frame, this_cache, entry->unwinder))
> +  struct gdbarch *gdbarch = get_frame_arch (this_frame);
> +  std::vector<const frame_unwind *> *table = get_frame_unwind_table (gdbarch);
> +  for (auto unwinder : *table)

I think you can use:

  for (const auto &unwinder : *table)

here.  The 'const' is just good style as you're not going to modify it.
The '&' is not really necessary as the type is actually a pointer.
Better still might be:

  for (const frame_unwind *unwinder : *table)

But I don't mind the 'auto' here too much as the type is on the line
immediately above.  But I do think, if you're sticking with 'auto' here
then throwing the '&' in is a good idea.

> +    if (frame_unwind_try_unwinder (this_frame, this_cache, unwinder))
>        return;
>  
>    internal_error (_("frame_unwind_find_by_frame failed"));
> @@ -347,7 +316,7 @@ static void
>  maintenance_info_frame_unwinders (const char *args, int from_tty)
>  {
>    gdbarch *gdbarch = current_inferior ()->arch ();
> -  struct frame_unwind_table *table = get_frame_unwind_table (gdbarch);
> +  std::vector<const frame_unwind *> *table = get_frame_unwind_table (gdbarch);
>  
>    ui_out *uiout = current_uiout;
>    ui_out_emit_table table_emitter (uiout, 2, -1, "FrameUnwinders");
> @@ -355,15 +324,11 @@ maintenance_info_frame_unwinders (const char *args, int from_tty)
>    uiout->table_header (25, ui_left, "type", "Type");
>    uiout->table_body ();
>  
> -  for (struct frame_unwind_table_entry *entry = table->list; entry != NULL;
> -       entry = entry->next)
> +  for (auto unwinder : *table)

See the comments above.

If you're happy making this changes I suggest then:

Approved-By: Andrew Burgess <aburgess@redhat.com>

Thanks,
Andrew

>      {
> -      const char *name = entry->unwinder->name;
> -      const char *type = frame_type_str (entry->unwinder->type);
> -
>        ui_out_emit_list tuple_emitter (uiout, nullptr);
> -      uiout->field_string ("name", name);
> -      uiout->field_string ("type", type);
> +      uiout->field_string ("name", unwinder->name);
> +      uiout->field_string ("type", frame_type_str (unwinder->type));
>        uiout->text ("\n");
>      }
>  }
> -- 
> 2.47.0