[PATCH 3/7] btrace: Use binary search to find instruction.

Tim Wiederhake
2016-10-27T06:29:44Z

Currently, btrace_find_insn_by_number will iterate over all function call
segments to find the one that contains the needed instruction. This linear
search is too slow for the upcoming Python bindings that will use this
function to access instructions. This patch introduces a vector in struct
btrace_thread_info that holds pointers to all recorded function segments and
allows to use binary search.

The proper solution is to turn the underlying tree into a vector of objects
and use indices for access. This requires more work. A patch set is
currently being worked on and will be published later.

2016-10-26 Tim Wiederhake <tim.wiederhake@intel.com>

gdb/ChangeLog:
* btrace.c (btrace_fetch): Copy function call segments pointer
into a vector.
(btrace_clear): Clear the vector.
(btrace_find_insn_by_number): Use binary search to find the correct
function call segment.
* btrace.h (brace_fun_p): New typedef.
(struct btrace_thread_info) <functions>: New field.

---
gdb/btrace.c | 39 +++++++++++++++++++++++++++++++++------
gdb/btrace.h | 7 +++++++
2 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/gdb/btrace.c b/gdb/btrace.c
index 0d1ba39..84cf55a 100644
--- a/gdb/btrace.c
+++ b/gdb/btrace.c
@@ -1389,6 +1389,8 @@ btrace_fetch (struct thread_info *tp)
/* Compute the trace, provided we have any. */
if (!btrace_data_empty (&btrace))
{
+ struct btrace_function *bfun;
+
/* Store the raw trace data. The stored data will be cleared in
btrace_clear, so we always append the new trace. */
btrace_data_append (&btinfo->data, &btrace);
@@ -1396,6 +1398,11 @@ btrace_fetch (struct thread_info *tp)

btrace_clear_history (btinfo);
btrace_compute_ftrace (tp, &btrace);
+
+ VEC_truncate (btrace_fun_p, btinfo->functions, 0);
+
+ for (bfun = btinfo->begin; bfun != NULL; bfun = bfun->flow.next)
+ VEC_safe_push (btrace_fun_p, btinfo->functions, bfun);
}

do_cleanups (cleanup);
@@ -1418,6 +1425,8 @@ btrace_clear (struct thread_info *tp)

btinfo = &tp->btrace;

+ VEC_free (btrace_fun_p, btinfo->functions);
+
it = btinfo->begin;
while (it != NULL)
{
@@ -2008,20 +2017,38 @@ btrace_find_insn_by_number (struct btrace_insn_iterator *it,
unsigned int number)
{
const struct btrace_function *bfun;
+ unsigned int upper, lower, average;

- for (bfun = btinfo->end; bfun != NULL; bfun = bfun->flow.prev)
- if (bfun->insn_offset <= number)
- break;
+ if (VEC_empty (btrace_fun_p, btinfo->functions))
+ return 0;

- if (bfun == NULL)
+ lower = 0;
+ bfun = VEC_index (btrace_fun_p, btinfo->functions, lower);
+ if (number < bfun->insn_offset)
return 0;

- if (bfun->insn_offset + ftrace_call_num_insn (bfun) <= number)
+ upper = VEC_length (btrace_fun_p, btinfo->functions) - 1;
+ bfun = VEC_index (btrace_fun_p, btinfo->functions, upper);
+ if (number >= bfun->insn_offset + ftrace_call_num_insn (bfun))
return 0;

+ average = lower + (upper - lower) / 2;
+ bfun = VEC_index (btrace_fun_p, btinfo->functions, average);
+
+ while ((number >= bfun->insn_offset + ftrace_call_num_insn (bfun))
+ || (number < bfun->insn_offset))
+ {
+ if (number < bfun->insn_offset)
+ upper = average - 1;
+ else
+ lower = average + 1;
+
+ average = lower + (upper - lower) / 2;
+ bfun = VEC_index (btrace_fun_p, btinfo->functions, average);
+ }
+
it->function = bfun;
it->index = number - bfun->insn_offset;
-
return 1;
}

diff --git a/gdb/btrace.h b/gdb/btrace.h
index d0497b4..6de2307 100644
--- a/gdb/btrace.h
+++ b/gdb/btrace.h
@@ -187,6 +187,9 @@ struct btrace_function
btrace_function_flags flags;
};

+typedef struct btrace_function *btrace_fun_p;
+DEF_VEC_P (btrace_fun_p);
+
/* A branch trace instruction iterator. */
struct btrace_insn_iterator
{
@@ -337,6 +340,10 @@ struct btrace_thread_info
struct btrace_function *begin;
struct btrace_function *end;

+ /* Vector of pointer to decoded function segments. These are in execution
+ order with the first element == BEGIN and the last element == END. */
+ VEC (btrace_fun_p) *functions;
+
/* The function level offset. When added to each function's LEVEL,
this normalizes the function levels such that the smallest level
becomes zero. */
--
2.7.4