[draft patch] unwinder for btrace [Re: [rfc 3/5] record: make it build again]

Jan Kratochvil
2013-02-11T17:13:46Z

On Mon, 11 Feb 2013 15:14:51 +0100, Jan Kratochvil wrote:
> FYI I have some draft <unavailable>-limitation unwinder for btrace here today,
> to post it later.

./gdb -q ./gdb -ex start -ex record -ex stepi -ex reverse-stepi -ex bt -ex q

No more reverse-execution history.
main (argc=<unavailable>, argv=<unavailable>) at gdb.c:29
29 memset (&args, 0, sizeof args);
#0 main (argc=<unavailable>, argv=<unavailable>) at gdb.c:29
Backtrace stopped: not enough registers or memory available to unwind further

That get_frame_unwind_stop_reason "optimization" to avoid calling an unwinder
was present already in the initial post:
[rfc, frame] Add backtrace stop reasons
http://sourceware.org/ml/gdb-patches/2006-08/msg00129.html
Message-ID: <20060819154646.GA25238@nevyn.them.org>
I don't know its real reason but it is needed for this patch otherwise:
main (argc=<error reading variable: can't compute CFA for this frame>, argv=<error reading variable: can't compute CFA for this frame>) at gdb.c:29

For btrace you will need ptid_t of the unwound thread as the RECORD_IS_REPLAY
flag is apparently irrelevant there. I find inferior_ptid to use there
acceptable despite it is discouraged now, passing there ptid_t from the
unwinder framework needs some larger changes.

No real regressions on {x86_64,x86_64-m32,i686}-fedora19pre-linux-gnu and in
gdbserver mode, with the patch as is the various *reverse* tests apparently
FAIL due to the <unavailable> values (both displayed and implicitly missing).

There should be a testcase but that will be dependent upon the btrace
integration.

Regards,
Jan

gdb/
2013-02-11 Jan Kratochvil <jan.kratochvil@redhat.com>

New record unwinder reporting <unavailable>.
* dwarf2-frame.c (dwarf2_frame_cfa): Move UNWIND_UNAVAILABLE check
earlier.
* frame-unwind.c: Include target.h.
(frame_unwind_try_unwinder): New function with code from ...
(frame_unwind_find_by_frame): ... here. New variable
unwinder_from_target, call also target_get_unwinder and
frame_unwind_try_unwinder for it.
* frame.c (get_frame_unwind_stop_reason): Unconditionally call
get_prev_frame_1.
* record.c: Include frame-unwind.h.
(record_frame_unwind_stop_reason, record_frame_this_id)
(record_frame_prev_register, record_frame_sniffer, record_frame_unwind):
New.
(init_record_ops, init_record_core_ops): Install it.
* target.c (target_get_unwinder): New.
* target.h (struct target_ops): New field to_get_unwinder.
(target_get_unwinder): New declaration.

diff --git a/gdb/dwarf2-frame.c b/gdb/dwarf2-frame.c
index ec4edfa..45be107 100644
--- a/gdb/dwarf2-frame.c
+++ b/gdb/dwarf2-frame.c
@@ -1497,16 +1497,16 @@ dwarf2_frame_cfa (struct frame_info *this_frame)
{
while (get_frame_type (this_frame) == INLINE_FRAME)
this_frame = get_prev_frame (this_frame);
+ if (get_frame_unwind_stop_reason (this_frame) == UNWIND_UNAVAILABLE)
+ throw_error (NOT_AVAILABLE_ERROR,
+ _("can't compute CFA for this frame: "
+ "required registers or memory are unavailable"));
/* This restriction could be lifted if other unwinders are known to
compute the frame base in a way compatible with the DWARF
unwinder. */
if (!frame_unwinder_is (this_frame, &dwarf2_frame_unwind)
&& !frame_unwinder_is (this_frame, &dwarf2_tailcall_frame_unwind))
error (_("can't compute CFA for this frame"));
- if (get_frame_unwind_stop_reason (this_frame) == UNWIND_UNAVAILABLE)
- throw_error (NOT_AVAILABLE_ERROR,
- _("can't compute CFA for this frame: "
- "required registers or memory are unavailable"));
return get_frame_base (this_frame);
}
\f
diff --git a/gdb/frame-unwind.c b/gdb/frame-unwind.c
index b66febf..cf33147 100644
--- a/gdb/frame-unwind.c
+++ b/gdb/frame-unwind.c
@@ -27,6 +27,7 @@
#include "exceptions.h"
#include "gdb_assert.h"
#include "gdb_obstack.h"
+#include "target.h"

static struct gdbarch_data *frame_unwind_data;

@@ -88,6 +89,48 @@ frame_unwind_append_unwinder (struct gdbarch *gdbarch,
(*ip)->unwinder = unwinder;
}

+/* Call SNIFFER from UNWINDER. If it succeeded set UNWINDER for
+ THIS_FRAME and return 1. Otherwise the function keeps THIS_FRAME
+ unchanged and returns 0. */
+
+static int
+frame_unwind_try_unwinder (struct frame_info *this_frame, void **this_cache,
+ const struct frame_unwind *unwinder)
+{
+ struct cleanup *old_cleanup;
+ volatile struct gdb_exception ex;
+ int res = 0;
+
+ old_cleanup = frame_prepare_for_sniffer (this_frame, unwinder);
+
+ TRY_CATCH (ex, RETURN_MASK_ERROR)
+ {
+ res = unwinder->sniffer (unwinder, this_frame, this_cache);
+ }
+ if (ex.reason < 0 && ex.error == NOT_AVAILABLE_ERROR)
+ {
+ /* This usually means that not even the PC is available,
+ thus most unwinders aren't able to determine if they're
+ the best fit. Keep trying. Fallback prologue unwinders
+ should always accept the frame. */
+ do_cleanups (old_cleanup);
+ return 0;
+ }
+ else if (ex.reason < 0)
+ throw_exception (ex);
+ else if (res)
+ {
+ discard_cleanups (old_cleanup);
+ return 1;
+ }
+ else
+ {
+ do_cleanups (old_cleanup);
+ return 0;
+ }
+ gdb_assert_not_reached ("frame_unwind_try_unwinder");
+}
+
/* Iterate through sniffers for THIS_FRAME frame until one returns with an
unwinder implementation. THIS_FRAME->UNWIND must be NULL, it will get set
by this function. Possibly initialize THIS_CACHE. */
@@ -98,37 +141,18 @@ frame_unwind_find_by_frame (struct frame_info *this_frame, void **this_cache)
struct gdbarch *gdbarch = get_frame_arch (this_frame);
struct frame_unwind_table *table = gdbarch_data (gdbarch, frame_unwind_data);
struct frame_unwind_table_entry *entry;
+ const struct frame_unwind *unwinder_from_target;
+
+ unwinder_from_target = target_get_unwinder ();
+ if (unwinder_from_target != NULL
+ && frame_unwind_try_unwinder (this_frame, this_cache,
+ unwinder_from_target))
+ return;

for (entry = table->list; entry != NULL; entry = entry->next)
- {
- struct cleanup *old_cleanup;
- volatile struct gdb_exception ex;
- int res = 0;
-
- old_cleanup = frame_prepare_for_sniffer (this_frame, entry->unwinder);
-
- TRY_CATCH (ex, RETURN_MASK_ERROR)
- {
- res = entry->unwinder->sniffer (entry->unwinder, this_frame,
- this_cache);
- }
- if (ex.reason < 0 && ex.error == NOT_AVAILABLE_ERROR)
- {
- /* This usually means that not even the PC is available,
- thus most unwinders aren't able to determine if they're
- the best fit. Keep trying. Fallback prologue unwinders
- should always accept the frame. */
- }
- else if (ex.reason < 0)
- throw_exception (ex);
- else if (res)
- {
- discard_cleanups (old_cleanup);
- return;
- }
+ if (frame_unwind_try_unwinder (this_frame, this_cache, entry->unwinder))
+ return;

- do_cleanups (old_cleanup);
- }
internal_error (__FILE__, __LINE__, _("frame_unwind_find_by_frame failed"));
}

diff --git a/gdb/frame.c b/gdb/frame.c
index 0821b6e..8fc1ca9 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -2357,13 +2357,10 @@ get_frame_sp (struct frame_info *this_frame)
enum unwind_stop_reason
get_frame_unwind_stop_reason (struct frame_info *frame)
{
- /* If we haven't tried to unwind past this point yet, then assume
- that unwinding would succeed. */
- if (frame->prev_p == 0)
- return UNWIND_NO_REASON;
+ /* Fill-in STOP_REASON. */
+ get_prev_frame_1 (frame);
+ gdb_assert (frame->prev_p);

- /* Otherwise, we set a reason when we succeeded (or failed) to
- unwind. */
return frame->stop_reason;
}

diff --git a/gdb/record.c b/gdb/record.c
index 1a68738..dffce98 100644
--- a/gdb/record.c
+++ b/gdb/record.c
@@ -34,6 +34,7 @@
#include "inf-loop.h"
#include "gdb_bfd.h"
#include "observer.h"
+#include "frame-unwind.h"

#include <signal.h>

@@ -1953,6 +1954,59 @@ record_execution_direction (void)
return record_execution_dir;
}

+/* Implement stop_reason method for record_frame_unwind. */
+
+static enum unwind_stop_reason
+record_frame_unwind_stop_reason (struct frame_info *this_frame,
+ void **this_cache)
+{
+ return UNWIND_UNAVAILABLE;
+}
+
+/* Implement this_id method for record_frame_unwind. */
+
+static void
+record_frame_this_id (struct frame_info *this_frame, void **this_cache,
+ struct frame_id *this_id)
+{
+ /* Leave there the outer_frame_id value. */
+}
+
+/* Implement prev_register method for record_frame_unwind. */
+
+static struct value *
+record_frame_prev_register (struct frame_info *this_frame, void **this_cache,
+ int regnum)
+{
+ throw_error (NOT_AVAILABLE_ERROR,
+ _("Registers are not available in btrace record history"));
+}
+
+/* Implement sniffer method for record_frame_unwind. */
+
+static int
+record_frame_sniffer (const struct frame_unwind *self,
+ struct frame_info *this_frame, void **this_cache)
+{
+ return RECORD_IS_REPLAY;
+}
+
+/* btrace recording does not store previous memory content, neither the stack
+ frames content. Any unwinding would return errorneous results as the stack
+ contents no longer matches the changed PC value restored from history.
+ Therefore this unwinder reports any possibly unwound registers as
+ <unavailable>. */
+
+static const struct frame_unwind record_frame_unwind =
+{
+ NORMAL_FRAME,
+ record_frame_unwind_stop_reason,
+ record_frame_this_id,
+ record_frame_prev_register,
+ NULL,
+ record_frame_sniffer
+};
+
static void
init_record_ops (void)
{
@@ -1984,6 +2038,7 @@ init_record_ops (void)
record_ops.to_can_async_p = record_can_async_p;
record_ops.to_is_async_p = record_is_async_p;
record_ops.to_execution_direction = record_execution_direction;
+ record_ops.to_get_unwinder = &record_frame_unwind;
record_ops.to_magic = OPS_MAGIC;
}

@@ -2209,6 +2264,7 @@ init_record_core_ops (void)
record_core_ops.to_can_async_p = record_can_async_p;
record_core_ops.to_is_async_p = record_is_async_p;
record_core_ops.to_execution_direction = record_execution_direction;
+ record_core_ops.to_get_unwinder = &record_frame_unwind;
record_core_ops.to_magic = OPS_MAGIC;
}

diff --git a/gdb/target.c b/gdb/target.c
index 9d8bf6e..28f00af 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -4147,6 +4147,20 @@ target_ranged_break_num_registers (void)
return -1;
}

+/* See target.h. */
+
+const struct frame_unwind *
+target_get_unwinder (void)
+{
+ struct target_ops *t;
+
+ for (t = current_target.beneath; t != NULL; t = t->beneath)
+ if (t->to_get_unwinder != NULL)
+ return t->to_get_unwinder;
+
+ return NULL;
+}
+
static void
debug_to_prepare_to_store (struct regcache *regcache)
{
diff --git a/gdb/target.h b/gdb/target.h
index 1971265..95bfb47 100644
--- a/gdb/target.h
+++ b/gdb/target.h
@@ -857,6 +857,10 @@ struct target_ops
/* Is the target able to use agent in current state? */
int (*to_can_use_agent) (void);

+ /* This unwinder is tried before any other arch unwinders. Use NULL if it
+ is not used. */
+ const struct frame_unwind *to_get_unwinder;
+
int to_magic;
/* Need sub-structure for target machine related rather than comm related?
*/
@@ -1735,6 +1739,9 @@ extern char *target_fileio_read_stralloc (const char *filename);

extern int target_core_of_thread (ptid_t ptid);

+/* See to_get_unwinder in struct target_ops. */
+extern const struct frame_unwind *target_get_unwinder (void);
+
/* Verify that the memory in the [MEMADDR, MEMADDR+SIZE) range matches
the contents of [DATA,DATA+SIZE). Returns 1 if there's a match, 0
if there's a mismatch, and -1 if an error is encountered while