[PATCH v2 3/4] gdb: dwarf2 generic implementation for caching function data

Torbjörn SVENSSON via Gdb-patches
2022-11-18T15:54:23Z

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 unqiue 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 | 48 ++++++++++++++++++++++++++++++++++++++++++++++
gdb/dwarf2/frame.h | 20 +++++++++++++++++--
2 files changed, 66 insertions(+), 2 deletions(-)

diff --git a/gdb/dwarf2/frame.c b/gdb/dwarf2/frame.c
index 3f884abe1d5..bff3b706e7e 100644
--- a/gdb/dwarf2/frame.c
+++ b/gdb/dwarf2/frame.c
@@ -831,6 +831,14 @@ dwarf2_fetch_cfa_info (struct gdbarch *gdbarch, CORE_ADDR pc,
}

\f
+struct dwarf2_frame_fn_data
+{
+ struct value *(*fn) (frame_info_ptr this_frame, void **this_cache,
+ int regnum);
+ void *data;
+ struct dwarf2_frame_fn_data* next;
+};
+
struct dwarf2_frame_cache
{
/* DWARF Call Frame Address. */
@@ -862,6 +870,8 @@ 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 +1231,44 @@ dwarf2_frame_prev_register (frame_info_ptr this_frame, void **this_cache,
}
}

+void *dwarf2_frame_get_fn_data (frame_info_ptr this_frame, void **this_cache,
+ fn_prev_register fn)
+{
+ 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->fn == fn)
+ return fn_data->data;
+
+ return nullptr;
+}
+
+void *dwarf2_frame_allocate_fn_data (frame_info_ptr this_frame,
+ void **this_cache,
+ fn_prev_register fn, 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, fn);
+ if (data)
+ return data;
+
+ /* No object found, lets create a new instance. */
+ fn_data = FRAME_OBSTACK_ZALLOC (struct dwarf2_frame_fn_data);
+ fn_data->fn = fn;
+ 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. */

diff --git a/gdb/dwarf2/frame.h b/gdb/dwarf2/frame.h
index 06c8a10c178..444afd9f8eb 100644
--- a/gdb/dwarf2/frame.h
+++ b/gdb/dwarf2/frame.h
@@ -66,6 +66,9 @@ enum dwarf2_frame_reg_rule

/* Register state. */

+typedef struct value *(*fn_prev_register) (frame_info_ptr this_frame,
+ void **this_cache, int regnum);
+
struct dwarf2_frame_state_reg
{
/* Each register save state can be described in terms of a CFA slot,
@@ -78,8 +81,7 @@ struct dwarf2_frame_state_reg
const gdb_byte *start;
ULONGEST len;
} exp;
- struct value *(*fn) (frame_info_ptr this_frame, void **this_cache,
- int regnum);
+ fn_prev_register fn;
} loc;
enum dwarf2_frame_reg_rule how;
};
@@ -262,4 +264,18 @@ extern int dwarf2_fetch_cfa_info (struct gdbarch *gdbarch, CORE_ADDR pc,
const gdb_byte **cfa_start_out,
const gdb_byte **cfa_end_out);

+
+/* Allocate a new instance of the function unique data. */
+
+extern void *dwarf2_frame_allocate_fn_data (frame_info_ptr this_frame,
+ void **this_cache,
+ fn_prev_register fn,
+ unsigned long size);
+
+/* Retrieve the function unique data for this frame. */
+
+extern void *dwarf2_frame_get_fn_data (frame_info_ptr this_frame,
+ void **this_cache,
+ fn_prev_register fn);
+
#endif /* dwarf2-frame.h */
--
2.25.1