gdb: dwarf2 generic implementation for caching function data

When there is no dwarf2 data for a register, a function can be called
to provide the value of this register.  In some situations, it might
not be trivial to determine the value to return and it would cause a
performance bottleneck to do the computation each time.

This patch allows the called function to have a "cache" object that it
can use to store some metadata between calls to reduce the performance
impact of the complex logic.

The cache object is unique for each function and frame, so if there are
more than one function pointer stored in the dwarf2_frame_cache->reg
array, then the appropriate pointer will be supplied (the type is not
known by the dwarf2 implementation).

dwarf2_frame_get_fn_data can be used to retrieve the function unique
cache object.
dwarf2_frame_allocate_fn_data can be used to allocate and retrieve the
function unique cache object.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
Signed-off-by: Yvan Roux <yvan.roux@foss.st.com>

gdb/dwarf2/frame.c

@@ -831,6 +831,22 @@ dwarf2_fetch_cfa_info (struct gdbarch *gdbarch, CORE_ADDR pc,
 }
 
 
+/* Custom function data object for architecture specific prev_register
+   implementation.  Main purpose of this object is to allow caching of
+   expensive data lookups in the prev_register handling.  */
+
+struct dwarf2_frame_fn_data
+{
+  /* The cookie to identify the custom function data by.  */
+  fn_prev_register cookie;
+
+  /* The custom function data.  */
+  void *data;
+
+  /* Pointer to the next custom function data object for this frame.  */
+  struct dwarf2_frame_fn_data *next;
+};
+
 struct dwarf2_frame_cache
 {
   /* DWARF Call Frame Address.  */
@@ -862,6 +878,8 @@ struct dwarf2_frame_cache
      dwarf2_tailcall_frame_unwind unwinder so this field does not apply for
      them.  */
   void *tailcall_cache;
+
+  struct dwarf2_frame_fn_data *fn_data;
 };
 
 static struct dwarf2_frame_cache *
@@ -1221,6 +1239,48 @@ dwarf2_frame_prev_register (frame_info_ptr this_frame, void **this_cache,
     }
 }
 
+/* See frame.h.  */
+
+void *
+dwarf2_frame_get_fn_data (frame_info_ptr this_frame, void **this_cache,
+			  fn_prev_register cookie)
+{
+  struct dwarf2_frame_fn_data *fn_data = nullptr;
+  struct dwarf2_frame_cache *cache
+    = dwarf2_frame_cache (this_frame, this_cache);
+
+  /* Find the object for the function.  */
+  for (fn_data = cache->fn_data; fn_data; fn_data = fn_data->next)
+    if (fn_data->cookie == cookie)
+      return fn_data->data;
+
+  return nullptr;
+}
+
+/* See frame.h.  */
+
+void *
+dwarf2_frame_allocate_fn_data (frame_info_ptr this_frame, void **this_cache,
+			       fn_prev_register cookie, unsigned long size)
+{
+  struct dwarf2_frame_fn_data *fn_data = nullptr;
+  struct dwarf2_frame_cache *cache
+    = dwarf2_frame_cache (this_frame, this_cache);
+
+  /* First try to find an existing object.  */
+  void *data = dwarf2_frame_get_fn_data (this_frame, this_cache, cookie);
+  gdb_assert (data == nullptr);
+
+  /* No object found, lets create a new instance.  */
+  fn_data = FRAME_OBSTACK_ZALLOC (struct dwarf2_frame_fn_data);
+  fn_data->cookie = cookie;
+  fn_data->data = frame_obstack_zalloc (size);
+  fn_data->next = cache->fn_data;
+  cache->fn_data = fn_data;
+
+  return fn_data->data;
+}
+
 /* Proxy for tailcall_frame_dealloc_cache for bottom frame of a virtual tail
    call frames chain.  */