[PATCH] [python] Optimize python_string_to_host_string() for Python 2.

["Yichun Zhang (agentzh)" <agentzh@gmail.com>]

Hi,

I have been writing quite some advanced GDB Python tools for LuaJIT
and NGINX in the nginx-gdb-utils project [1] and have been suffering
from serious performance issues even for relatively small working sets.

I have noticed from a typical on-CPU C-land Flame Graph [2]
for my tools that python_string_to_host_string() is painfully slow
when using Python 2 (2.7 to be more specific) due to the
expensive and meaningless unicode conversions there (involved with
hot function calls like utf_8_decode).

With the following patch, my most complicated Python tools
finally have comparable performance between Python 2 and
Python 3. In the case of Python 2, some real-world Python scripts'
overall speedup can be as big as 34% (for my "lgcpath" command [3])
or even 57% (for my "lgcstat" command [4]). And from the new Flame
Graph [5], we can see that the corresponding function frames are
indeed gone.

Comments are welcome!

[1] https://github.com/openresty/nginx-gdb-utils#readme
[2] http://agentzh.org/misc/flamegraph/gdb-py-lgcstat-2015-0113.svg
[3] https://github.com/openresty/nginx-gdb-utils#lgcpath
[4] https://github.com/openresty/nginx-gdb-utils#lgcstat
[5] http://agentzh.org/misc/flamegraph/patched-gdb-py-lgcstat-2015-0113.svg

gdb/ChangeLog:

    * python/py-utils.c (python_string_to_host_string): use
    xstrdup directly for Python 2.
    * python/py-utils.c (gdbpy_obj_to_string): use
    python_string_to_host_string for both Python 2 and 3.
---
 gdb/python/py-utils.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/gdb/python/py-utils.c b/gdb/python/py-utils.c
index 58a5934..ed7be3b 100644
--- a/gdb/python/py-utils.c
+++ b/gdb/python/py-utils.c
@@ -209,6 +209,7 @@ python_string_to_target_python_string (PyObject *obj)
 char *
 python_string_to_host_string (PyObject *obj)
 {
+#ifdef IS_PY3K
   PyObject *str;
   char *result;
 
@@ -219,6 +220,9 @@ python_string_to_host_string (PyObject *obj)
   result = unicode_to_encoded_string (str, host_charset ());
   Py_DECREF (str);
   return result;
+#else
+  return xstrdup (PyString_AsString (obj));
+#endif
 }
 
 /* Return true if OBJ is a Python string or unicode object, false
@@ -245,12 +249,7 @@ gdbpy_obj_to_string (PyObject *obj)
 
   if (str_obj != NULL)
     {
-#ifdef IS_PY3K
       char *msg = python_string_to_host_string (str_obj);
-#else
-      char *msg = xstrdup (PyString_AsString (str_obj));
-#endif
-
       Py_DECREF (str_obj);
       return msg;
     }
-- 
1.8.5.6