record-btrace: indicate gaps

Indicate gaps in the trace due to decode errors.  Internally, a gap is
represented as a btrace function segment without instructions and with a
non-zero format-specific error code.

Show the gap when traversing the instruction or function call history.
Also indicate gaps in "info record".

It looks like this:

  (gdb) info record
  Active record target: record-btrace
  Recording format: Branch Trace Store.
  Buffer size: 64KB.
  Recorded 32 instructions in 5 functions (1 gaps) for thread 1 (process 7182).
  (gdb) record function-call-history /cli
  1	fib	inst 1,9	at src/fib.c:9,14
  2	  fib	inst 10,20	at src/fib.c:6,14
  3	[decode error (1): instruction overflow]
  4	fib	inst 21,28	at src/fib.c:11,14
  5	  fib	inst 29,33	at src/fib.c:6,9
  (gdb) record instruction-history 20,22
  20	   0x000000000040062f <fib+47>:	sub    $0x1,%rax
  [decode error (1): instruction overflow]
  21	   0x0000000000400613 <fib+19>:	add    $0x1,%rax
  22	   0x0000000000400617 <fib+23>:	mov    %rax,0x200a3a(%rip)
  (gdb)

Gaps are ignored during reverse execution and replay.

2015-02-09  Markus Metzger  <markus.t.metzger@intel.com>

	* btrace.c (ftrace_find_call): Skip gaps.
	(ftrace_new_function): Initialize level.
	(ftrace_new_call, ftrace_new_tailcall, ftrace_new_return)
	(ftrace_new_switch): Update
	level computation.
	(ftrace_new_gap): New.
	(ftrace_update_function): Create new function after gap.
	(btrace_compute_ftrace_bts): Create gap on error.
	(btrace_stitch_bts): Update parameters.  Clear trace if it
	becomes empty.
	(btrace_stitch_trace): Update parameters.  Update callers.
	(btrace_clear): Reset the number of gaps.
	(btrace_insn_get): Return NULL if the iterator points to a gap.
	(btrace_insn_number): Return zero if the iterator points to a gap.
	(btrace_insn_end): Allow gaps at the end.
	(btrace_insn_next, btrace_insn_prev, btrace_insn_cmp): Handle gaps.
	(btrace_find_insn_by_number): Assert that the found iterator does
	not point to a gap.
	(btrace_call_next, btrace_call_prev): Assert that the last function
	is not a gap.
	* btrace.h (btrace_bts_error): New.
	(btrace_function): Update comment.
	(btrace_function) <insn, insn_offset, number>: Update comment.
	(btrace_function) <errcode>: New.
	(btrace_thread_info) <ngaps>: New.
	(btrace_thread_info) <replay>: Update comment.
	(btrace_insn_get): Update comment.
	* record-btrace.c (btrace_ui_out_decode_error): New.
	(record_btrace_info): Print number of gaps.
	(btrace_insn_history, btrace_call_history): Call
	btrace_ui_out_decode_error for gaps.
	(record_btrace_step_thread, record_btrace_start_replaying): Skip gaps.

testsuite/
	* gdb.btrace/buffer-size.exp: Update "info record" output.
	* gdb.btrace/delta.exp: Update "info record" output.
	* gdb.btrace/enable.exp: Update "info record" output.
	* gdb.btrace/finish.exp: Update "info record" output.
	* gdb.btrace/instruction_history.exp: Update "info record" output.
	* gdb.btrace/next.exp: Update "info record" output.
	* gdb.btrace/nexti.exp: Update "info record" output.
	* gdb.btrace/step.exp: Update "info record" output.
	* gdb.btrace/stepi.exp: Update "info record" output.
	* gdb.btrace/nohist.exp: Update "info record" output.

1 files changed, 34 insertions, 6 deletions

diff --git a/gdb/btrace.h b/gdb/btrace.h
index 694d504..0ddd4c1 100644
--- a/gdb/btrace.h
+++ b/gdb/btrace.h
@@ -86,12 +86,25 @@ enum btrace_function_flag
   BFUN_UP_LINKS_TO_TAILCALL = (1 << 1)
 };
 
+/* Decode errors for the BTS recording format.  */
+enum btrace_bts_error
+{
+  /* The instruction trace overflowed the end of the trace block.  */
+  BDE_BTS_OVERFLOW = 1,
+
+  /* The instruction size could not be determined.  */
+  BDE_BTS_INSN_SIZE
+};
+
 /* A branch trace function segment.
 
    This represents a function segment in a branch trace, i.e. a consecutive
    number of instructions belonging to the same function.
 
-   We do not allow function segments without any instructions.  */
+   In case of decode errors, we add an empty function segment to indicate
+   the gap in the trace.
+
+   We do not allow function segments without instructions otherwise.  */
 struct btrace_function
 {
   /* The full and minimal symbol for the function.  Both may be NULL.  */
@@ -110,14 +123,23 @@ struct btrace_function
   struct btrace_function *up;
 
   /* The instructions in this function segment.
-     The instruction vector will never be empty.  */
+     The instruction vector will be empty if the function segment
+     represents a decode error.  */
   VEC (btrace_insn_s) *insn;
 
+  /* The error code of a decode error that led to a gap.
+     Must be zero unless INSN is empty; non-zero otherwise.  */
+  int errcode;
+
   /* The instruction number offset for the first instruction in this
-     function segment.  */
+     function segment.
+     If INSN is empty this is the insn_offset of the succeding function
+     segment in control-flow order.  */
   unsigned int insn_offset;
 
-  /* The function number in control-flow order.  */
+  /* The function number in control-flow order.
+     If INSN is empty indicating a gap in the trace due to a decode error,
+     we still count the gap as a function.  */
   unsigned int number;
 
   /* The function level in a back trace across the entire branch trace.
@@ -223,6 +245,9 @@ struct btrace_thread_info
      becomes zero.  */
   int level;
 
+  /* The number of gaps in the trace.  */
+  unsigned int ngaps;
+
   /* A bit-vector of btrace_thread_flag.  */
   enum btrace_thread_flag flags;
 
@@ -232,7 +257,9 @@ struct btrace_thread_info
   /* The function call history iterator.  */
   struct btrace_call_history *call_history;
 
-  /* The current replay position.  NULL if not replaying.  */
+  /* The current replay position.  NULL if not replaying.
+     Gaps are skipped during replay, so REPLAY always points to a valid
+     instruction.  */
   struct btrace_insn_iterator *replay;
 };
 
@@ -270,7 +297,8 @@ extern void parse_xml_btrace (struct btrace_data *data, const char *xml);
 extern void parse_xml_btrace_conf (struct btrace_config *conf, const char *xml);
 
 /* Dereference a branch trace instruction iterator.  Return a pointer to the
-   instruction the iterator points to.  */
+   instruction the iterator points to.
+   May return NULL if the iterator points to a gap in the trace.  */
 extern const struct btrace_insn *
   btrace_insn_get (const struct btrace_insn_iterator *);