Speed up Python (according to pybench and 2to3-on-itself) by 1-2% by caching

whether any thread has tracing turned on, which saves one load instruction in
the fast_next_opcode path in PyEval_EvalFrameEx().  See issue 4477.

diff --git a/Python/ceval.c b/Python/ceval.c
index 6eef7efa84b11ab40a4692548c670b8e00ed3898..cdcf9f6f32abffd3b7b5a07e5e5e7ee98c116a0a 100644 (file)
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -504,6 +504,13 @@ enum why_code {
 static enum why_code do_raise(PyObject *, PyObject *, PyObject *);
 static int unpack_iterable(PyObject *, int, PyObject **);
 
+/* Records whether tracing is on for any thread.  Counts the number of
+   threads for which tstate->c_tracefunc is non-NULL, so if the value
+   is 0, we know we don't have to check this thread's c_tracefunc.
+   This speeds up the if statement in PyEval_EvalFrameEx() after
+   fast_next_opcode*/
+static int _Py_TracingPossible = 0;
+
 /* for manipulating the thread switch and periodic "stuff" - used to be
    per thread, now just a pair o' globals */
 int _Py_CheckInterval = 100;
@@ -886,7 +893,8 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 
                /* line-by-line tracing support */
 
-               if (tstate->c_tracefunc != NULL && !tstate->tracing) {
+               if (_Py_TracingPossible &&
+                   tstate->c_tracefunc != NULL && !tstate->tracing) {
                        /* see maybe_call_line_trace
                           for expository comments */
                        f->f_stacktop = stack_pointer;
@@ -3414,6 +3422,7 @@ PyEval_SetTrace(Py_tracefunc func, PyObject *arg)
 {
        PyThreadState *tstate = PyThreadState_GET();
        PyObject *temp = tstate->c_traceobj;
+       _Py_TracingPossible += (func != NULL) - (tstate->c_tracefunc != NULL);
        Py_XINCREF(arg);
        tstate->c_tracefunc = NULL;
        tstate->c_traceobj = NULL;