btrace: update tail call heuristic

An unconditional jump to the start of a function typically indicates a tail
call.

If we can't determine the start of the function at the destination address, we
used to treat it as a tail call, as well.  This results in lots of tail calls
for code for which we don't have symbol information.

Restrict the heuristic to only consider jumps as tail calls that switch
functions in the case where we can't determine the start of a function.  This
effectively disables tail call detection for code without symbol information.

gdb/
	* btrace.c (ftrace_update_function): Update tail call heuristic.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 350e5a6..98e13ea 100644 (file)
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,9 @@
 2016-10-28  Markus Metzger  <markus.t.metzger@intel.com>
 
+       * btrace.c (ftrace_update_function): Update tail call heuristic.
+
+2016-10-28  Markus Metzger  <markus.t.metzger@intel.com>
+
        * btrace.c (btrace_compute_ftrace_bts, ftrace_add_pt): Allow
        leading gaps.
        * record-btrace.c (record_btrace_single_step_forward)

diff --git a/gdb/btrace.c b/gdb/btrace.c
index 648d495..445f0a4 100644 (file)
--- a/gdb/btrace.c
+++ b/gdb/btrace.c
@@ -529,10 +529,17 @@ ftrace_update_function (struct btrace_function *bfun, CORE_ADDR pc)
 
            start = get_pc_function_start (pc);
 
+           /* A jump to the start of a function is (typically) a tail call.  */
+           if (start == pc)
+             return ftrace_new_tailcall (bfun, mfun, fun);
+
            /* If we can't determine the function for PC, we treat a jump at
-              the end of the block as tail call.  */
-           if (start == 0 || start == pc)
+              the end of the block as tail call if we're switching functions
+              and as an intra-function branch if we don't.  */
+           if (start == 0 && ftrace_function_switched (bfun, mfun, fun))
              return ftrace_new_tailcall (bfun, mfun, fun);
+
+           break;
          }
        }
     }