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

[Guinevere Larsen <guinevere@redhat.com>]

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 =
 {
-  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)
 {
-  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)
+    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)
     {
-      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