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

[Guinevere Larsen <guinevere@redhat.com>]

On 1/14/25 11:28 AM, Andrew Burgess wrote:
> 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.
I decided to go with the "auto" version, since table is always declared 
close to the usage, and that doesn't use auto.
>
>> +    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 for the review, I went along with the changes and added your 
approval!

-- 
Cheers,
Guinevere Larsen
She/Her/Hers

>
> 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