diff -r 82d1c8d15e18 Include/ceval.h
--- a/Include/ceval.h	Mon Feb 06 10:41:46 2017 +0200
+++ b/Include/ceval.h	Mon Feb 06 10:27:12 2017 +0100
@@ -79,8 +79,8 @@ PyAPI_FUNC(void) Py_SetRecursionLimit(in
 PyAPI_FUNC(int) Py_GetRecursionLimit(void);
 
 #define Py_EnterRecursiveCall(where)  \
-            (_Py_MakeRecCheck(PyThreadState_GET()->recursion_depth) &&  \
-             _Py_CheckRecursiveCall(where))
+    (_Py_unlikely(_Py_MakeRecCheck(PyThreadState_GET()->recursion_depth) \
+                  && _Py_CheckRecursiveCall(where)))
 #define Py_LeaveRecursiveCall()                         \
     do{ if(_Py_MakeEndRecCheck(PyThreadState_GET()->recursion_depth))  \
       PyThreadState_GET()->overflowed = 0;  \
diff -r 82d1c8d15e18 Include/modsupport.h
--- a/Include/modsupport.h	Mon Feb 06 10:41:46 2017 +0200
+++ b/Include/modsupport.h	Mon Feb 06 10:27:12 2017 +0100
@@ -63,11 +63,14 @@ PyAPI_FUNC(int) _PyArg_NoKeywords(const 
 PyAPI_FUNC(int) _PyArg_NoStackKeywords(const char *funcname, PyObject *kwnames);
 PyAPI_FUNC(int) _PyArg_NoPositional(const char *funcname, PyObject *args);
 #define _PyArg_NoKeywords(funcname, kwargs) \
-    ((kwargs) == NULL || _PyArg_NoKeywords((funcname), (kwargs)))
+    (_Py_unlikely((kwargs) == NULL) \
+     || _PyArg_NoKeywords((funcname), (kwargs)))
 #define _PyArg_NoStackKeywords(funcname, kwnames) \
-    ((kwnames) == NULL || _PyArg_NoStackKeywords((funcname), (kwnames)))
+    (_Py_unlikely((kwnames) == NULL) \
+     || _PyArg_NoStackKeywords((funcname), (kwnames)))
 #define _PyArg_NoPositional(funcname, args) \
-    ((args) == NULL || _PyArg_NoPositional((funcname), (args)))
+    (_Py_unlikely((args) == NULL) \
+     || _PyArg_NoPositional((funcname), (args)))
 
 #endif
 
diff -r 82d1c8d15e18 Include/object.h
--- a/Include/object.h	Mon Feb 06 10:41:46 2017 +0200
+++ b/Include/object.h	Mon Feb 06 10:27:12 2017 +0100
@@ -788,7 +788,7 @@ PyAPI_FUNC(void) _Py_Dealloc(PyObject *)
     do {                                                \
         PyObject *_py_decref_tmp = (PyObject *)(op);    \
         if (_Py_DEC_REFTOTAL  _Py_REF_DEBUG_COMMA       \
-        --(_py_decref_tmp)->ob_refcnt != 0)             \
+        _Py_unlikely(--(_py_decref_tmp)->ob_refcnt != 0)) \
             _Py_CHECK_REFCNT(_py_decref_tmp)            \
         else                                            \
             _Py_Dealloc(_py_decref_tmp);                \
diff -r 82d1c8d15e18 Include/pyport.h
--- a/Include/pyport.h	Mon Feb 06 10:41:46 2017 +0200
+++ b/Include/pyport.h	Mon Feb 06 10:27:12 2017 +0100
@@ -521,6 +521,28 @@ extern "C" {
 #define _Py_HOT_FUNCTION
 #endif
 
+/* _Py_likely(test) and _Py_unlikely() macros can be used to provide the
+   compiler with branch prediction information. In general, you should prefer
+   to use actual profile feedback for this (PGO compilation, enabled by
+   ./configure --with-optimizations), as programmers are notoriously bad at
+   predicting how their programs actually perform. However, there are
+   applications in which this data is hard to collect.
+
+   Example to indicate that we expect that tp_init is set (is not NULL):
+
+   if (_Py_likely(tp_init != NULL)) { ... use tp_init ... }
+
+   Example to indicate that we expect that kwargs is NULL:
+
+   if (_Py_unlikely(kwargs != NULL)) { ... error ... } */
+#if defined(__GNUC__) || defined(__clang__)
+#  define _Py_likely(test)   __builtin_expect(!!(test), 1)
+#  define _Py_unlikely(test) __builtin_expect(!!(test), 0)
+#else
+#  define _Py_likely(test)   (!!(test))
+#  define _Py_unlikely(test) (!!(test))
+#endif
+
 /* _Py_NO_INLINE
  * Disable inlining on a function. For example, it helps to reduce the C stack
  * consumption.
diff -r 82d1c8d15e18 Objects/abstract.c
--- a/Objects/abstract.c	Mon Feb 06 10:41:46 2017 +0200
+++ b/Objects/abstract.c	Mon Feb 06 10:27:12 2017 +0100
@@ -2186,7 +2186,7 @@ PyObject*
     assert((callable != NULL) ^ (where != NULL));
 
     if (result == NULL) {
-        if (!err_occurred) {
+        if (_Py_unlikely(!err_occurred)) {
             if (callable)
                 PyErr_Format(PyExc_SystemError,
                              "%R returned NULL without setting an error",
@@ -2203,7 +2203,7 @@ PyObject*
         }
     }
     else {
-        if (err_occurred) {
+        if (_Py_unlikely(err_occurred)) {
             Py_DECREF(result);
 
             if (callable) {
@@ -2265,7 +2265,7 @@ PyObject* _Py_NO_INLINE
     Py_ssize_t i;
 
     args = PyTuple_New(nargs);
-    if (args == NULL) {
+    if (_Py_unlikely(args == NULL)) {
         return NULL;
     }
 
@@ -2333,14 +2333,14 @@ PyObject *
 
         /* Slow-path: build a temporary tuple */
         call = callable->ob_type->tp_call;
-        if (call == NULL) {
+        if (_Py_unlikely(call == NULL)) {
             PyErr_Format(PyExc_TypeError, "'%.200s' object is not callable",
                          callable->ob_type->tp_name);
             goto exit;
         }
 
         tuple = _PyStack_AsTuple(args, nargs);
-        if (tuple == NULL) {
+        if (_Py_unlikely(tuple == NULL)) {
             goto exit;
         }
 
@@ -2406,7 +2406,7 @@ PyObject *
     assert(kwnames != NULL);
     nkwargs = PyTuple_GET_SIZE(kwnames);
     kwdict = _PyDict_NewPresized(nkwargs);
-    if (kwdict == NULL) {
+    if (_Py_unlikely(kwdict == NULL)) {
         return NULL;
     }
 
@@ -2414,7 +2414,7 @@ PyObject *
         PyObject *key = PyTuple_GET_ITEM(kwnames, i);
         PyObject *value = *values++;
         /* If key already exists, replace it with the new value */
-        if (PyDict_SetItem(kwdict, key, value)) {
+        if (_Py_unlikely(PyDict_SetItem(kwdict, key, value))) {
             Py_DECREF(kwdict);
             return NULL;
         }
@@ -2519,20 +2519,20 @@ PyObject *
         }
 
         call = callable->ob_type->tp_call;
-        if (call == NULL) {
+        if (_Py_unlikely(call == NULL)) {
             PyErr_Format(PyExc_TypeError, "'%.200s' object is not callable",
                          callable->ob_type->tp_name);
             goto exit;
         }
 
         argtuple = _PyStack_AsTuple(stack, nargs);
-        if (argtuple == NULL) {
+        if (_Py_unlikely(argtuple == NULL)) {
             goto exit;
         }
 
         if (nkwargs > 0) {
             kwdict = _PyStack_AsDict(stack + nargs, kwnames);
-            if (kwdict == NULL) {
+            if (_Py_unlikely(kwdict == NULL)) {
                 Py_DECREF(argtuple);
                 goto exit;
             }
diff -r 82d1c8d15e18 Objects/clinic/unicodeobject.c.h
--- a/Objects/clinic/unicodeobject.c.h	Mon Feb 06 10:41:46 2017 +0200
+++ b/Objects/clinic/unicodeobject.c.h	Mon Feb 06 10:27:12 2017 +0100
@@ -83,8 +83,8 @@ unicode_center(PyObject *self, PyObject 
     Py_ssize_t width;
     Py_UCS4 fillchar = ' ';
 
-    if (!_PyArg_ParseStack(args, nargs, "n|O&:center",
-        &width, convert_uc, &fillchar)) {
+    if (_Py_unlikely(!_PyArg_ParseStack(args, nargs, "n|O&:center",
+        &width, convert_uc, &fillchar))) {
         goto exit;
     }
 
@@ -435,8 +435,8 @@ unicode_ljust(PyObject *self, PyObject *
     Py_ssize_t width;
     Py_UCS4 fillchar = ' ';
 
-    if (!_PyArg_ParseStack(args, nargs, "n|O&:ljust",
-        &width, convert_uc, &fillchar)) {
+    if (_Py_unlikely(!_PyArg_ParseStack(args, nargs, "n|O&:ljust",
+        &width, convert_uc, &fillchar))) {
         goto exit;
     }
 
@@ -487,9 +487,9 @@ unicode_strip(PyObject *self, PyObject *
     PyObject *return_value = NULL;
     PyObject *chars = Py_None;
 
-    if (!_PyArg_UnpackStack(args, nargs, "strip",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "strip",
         0, 1,
-        &chars)) {
+        &chars))) {
         goto exit;
     }
 
@@ -522,9 +522,9 @@ unicode_lstrip(PyObject *self, PyObject 
     PyObject *return_value = NULL;
     PyObject *chars = NULL;
 
-    if (!_PyArg_UnpackStack(args, nargs, "lstrip",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "lstrip",
         0, 1,
-        &chars)) {
+        &chars))) {
         goto exit;
     }
 
@@ -557,9 +557,9 @@ unicode_rstrip(PyObject *self, PyObject 
     PyObject *return_value = NULL;
     PyObject *chars = NULL;
 
-    if (!_PyArg_UnpackStack(args, nargs, "rstrip",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "rstrip",
         0, 1,
-        &chars)) {
+        &chars))) {
         goto exit;
     }
 
@@ -600,8 +600,8 @@ unicode_replace(PyObject *self, PyObject
     PyObject *new;
     Py_ssize_t count = -1;
 
-    if (!_PyArg_ParseStack(args, nargs, "UU|n:replace",
-        &old, &new, &count)) {
+    if (_Py_unlikely(!_PyArg_ParseStack(args, nargs, "UU|n:replace",
+        &old, &new, &count))) {
         goto exit;
     }
 
@@ -635,8 +635,8 @@ unicode_rjust(PyObject *self, PyObject *
     Py_ssize_t width;
     Py_UCS4 fillchar = ' ';
 
-    if (!_PyArg_ParseStack(args, nargs, "n|O&:rjust",
-        &width, convert_uc, &fillchar)) {
+    if (_Py_unlikely(!_PyArg_ParseStack(args, nargs, "n|O&:rjust",
+        &width, convert_uc, &fillchar))) {
         goto exit;
     }
 
@@ -840,8 +840,8 @@ unicode_maketrans(void *null, PyObject *
     PyObject *y = NULL;
     PyObject *z = NULL;
 
-    if (!_PyArg_ParseStack(args, nargs, "O|UU:maketrans",
-        &x, &y, &z)) {
+    if (_Py_unlikely(!_PyArg_ParseStack(args, nargs, "O|UU:maketrans",
+        &x, &y, &z))) {
         goto exit;
     }
 
@@ -962,4 +962,4 @@ unicode_sizeof(PyObject *self, PyObject 
 {
     return unicode_sizeof_impl(self);
 }
-/*[clinic end generated code: output=88b06f61edd282f9 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=294a5e2489e94722 input=a9049054013a1b77]*/
diff -r 82d1c8d15e18 Objects/descrobject.c
--- a/Objects/descrobject.c	Mon Feb 06 10:41:46 2017 +0200
+++ b/Objects/descrobject.c	Mon Feb 06 10:27:12 2017 +0100
@@ -257,7 +257,7 @@ PyObject *
     PyObject *self, *result;
 
     /* Make sure that the first argument is acceptable as 'self' */
-    if (nargs < 1) {
+    if (_Py_unlikely(nargs < 1)) {
         PyErr_Format(PyExc_TypeError,
                      "descriptor '%V' of '%.100s' "
                      "object needs an argument",
@@ -266,8 +266,9 @@ PyObject *
         return NULL;
     }
     self = args[0];
-    if (!_PyObject_RealIsSubclass((PyObject *)Py_TYPE(self),
-                                  (PyObject *)PyDescr_TYPE(descr))) {
+    if (!_Py_unlikely(_PyObject_RealIsSubclass(
+                            (PyObject *)Py_TYPE(self),
+                            (PyObject *)PyDescr_TYPE(descr)))) {
         PyErr_Format(PyExc_TypeError,
                      "descriptor '%V' "
                      "requires a '%.100s' object "
diff -r 82d1c8d15e18 Objects/methodobject.c
--- a/Objects/methodobject.c	Mon Feb 06 10:41:46 2017 +0200
+++ b/Objects/methodobject.c	Mon Feb 06 10:27:12 2017 +0100
@@ -237,14 +237,14 @@ PyObject *
     switch (flags)
     {
     case METH_NOARGS:
-        if (nargs != 0) {
+        if (_Py_unlikely(nargs != 0)) {
             PyErr_Format(PyExc_TypeError,
                 "%.200s() takes no arguments (%zd given)",
                 method->ml_name, nargs);
             return NULL;
         }
 
-        if (nkwargs) {
+        if (_Py_unlikely(nkwargs)) {
             goto no_keyword_error;
         }
 
@@ -252,14 +252,14 @@ PyObject *
         break;
 
     case METH_O:
-        if (nargs != 1) {
+        if (_Py_unlikely(nargs != 1)) {
             PyErr_Format(PyExc_TypeError,
                 "%.200s() takes exactly one argument (%zd given)",
                 method->ml_name, nargs);
             return NULL;
         }
 
-        if (nkwargs) {
+        if (_Py_unlikely(nkwargs)) {
             goto no_keyword_error;
         }
 
@@ -278,21 +278,21 @@ PyObject *
            and a temporary dict for keyword arguments */
         PyObject *argtuple;
 
-        if (!(flags & METH_KEYWORDS) && nkwargs) {
+        if (!(flags & METH_KEYWORDS) && _Py_unlikely(nkwargs)) {
             goto no_keyword_error;
         }
 
         argtuple = _PyStack_AsTuple(args, nargs);
-        if (argtuple == NULL) {
+        if (_Py_unlikely(argtuple == NULL)) {
             return NULL;
         }
 
         if (flags & METH_KEYWORDS) {
             PyObject *kwdict;
 
-            if (nkwargs > 0) {
+            if (_Py_unlikely(nkwargs > 0)) {
                 kwdict = _PyStack_AsDict(args + nargs, kwnames);
-                if (kwdict == NULL) {
+                if (_Py_unlikely(kwdict == NULL)) {
                     Py_DECREF(argtuple);
                     return NULL;
                 }
diff -r 82d1c8d15e18 Python/bltinmodule.c
--- a/Python/bltinmodule.c	Mon Feb 06 10:41:46 2017 +0200
+++ b/Python/bltinmodule.c	Mon Feb 06 10:27:12 2017 +0100
@@ -1483,7 +1483,7 @@ builtin_len(PyObject *module, PyObject *
     Py_ssize_t res;
 
     res = PyObject_Size(obj);
-    if (res < 0 && PyErr_Occurred())
+    if (_Py_unlikely(res < 0) && PyErr_Occurred())
         return NULL;
     return PyLong_FromSsize_t(res);
 }
diff -r 82d1c8d15e18 Python/ceval.c
--- a/Python/ceval.c	Mon Feb 06 10:41:46 2017 +0200
+++ b/Python/ceval.c	Mon Feb 06 10:27:12 2017 +0100
@@ -4946,7 +4946,7 @@ fast_function(PyObject *func, PyObject *
     PyObject *argdefs = PyFunction_GET_DEFAULTS(func);
     PyObject *kwdefs, *closure, *name, *qualname;
     PyObject **d;
-    Py_ssize_t nkwargs = (kwnames == NULL) ? 0 : PyTuple_GET_SIZE(kwnames);
+    Py_ssize_t nkwargs = _Py_likely(kwnames == NULL) ? 0 : PyTuple_GET_SIZE(kwnames);
     Py_ssize_t nd;
 
     assert(PyFunction_Check(func));
@@ -5035,8 +5035,8 @@ PyObject *
         }
     }
 
-    nk = (kwargs != NULL) ? PyDict_GET_SIZE(kwargs) : 0;
-    if (nk != 0) {
+    nk = _Py_unlikely(kwargs != NULL) ? PyDict_GET_SIZE(kwargs) : 0;
+    if (_Py_unlikely(nk != 0)) {
         Py_ssize_t pos, i;
 
         /* Issue #29318: Caller and callee functions must not share the
diff -r 82d1c8d15e18 Python/clinic/bltinmodule.c.h
--- a/Python/clinic/bltinmodule.c.h	Mon Feb 06 10:41:46 2017 +0200
+++ b/Python/clinic/bltinmodule.c.h	Mon Feb 06 10:27:12 2017 +0100
@@ -92,8 +92,8 @@ builtin_format(PyObject *module, PyObjec
     PyObject *value;
     PyObject *format_spec = NULL;
 
-    if (!_PyArg_ParseStack(args, nargs, "O|U:format",
-        &value, &format_spec)) {
+    if (_Py_unlikely(!_PyArg_ParseStack(args, nargs, "O|U:format",
+        &value, &format_spec))) {
         goto exit;
     }
 
@@ -201,9 +201,9 @@ builtin_divmod(PyObject *module, PyObjec
     PyObject *x;
     PyObject *y;
 
-    if (!_PyArg_UnpackStack(args, nargs, "divmod",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "divmod",
         2, 2,
-        &x, &y)) {
+        &x, &y))) {
         goto exit;
     }
 
@@ -243,9 +243,9 @@ builtin_eval(PyObject *module, PyObject 
     PyObject *globals = Py_None;
     PyObject *locals = Py_None;
 
-    if (!_PyArg_UnpackStack(args, nargs, "eval",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "eval",
         1, 3,
-        &source, &globals, &locals)) {
+        &source, &globals, &locals))) {
         goto exit;
     }
 
@@ -285,9 +285,9 @@ builtin_exec(PyObject *module, PyObject 
     PyObject *globals = Py_None;
     PyObject *locals = Py_None;
 
-    if (!_PyArg_UnpackStack(args, nargs, "exec",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "exec",
         1, 3,
-        &source, &globals, &locals)) {
+        &source, &globals, &locals))) {
         goto exit;
     }
 
@@ -342,9 +342,9 @@ builtin_hasattr(PyObject *module, PyObje
     PyObject *obj;
     PyObject *name;
 
-    if (!_PyArg_UnpackStack(args, nargs, "hasattr",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "hasattr",
         2, 2,
-        &obj, &name)) {
+        &obj, &name))) {
         goto exit;
     }
 
@@ -392,9 +392,9 @@ builtin_setattr(PyObject *module, PyObje
     PyObject *name;
     PyObject *value;
 
-    if (!_PyArg_UnpackStack(args, nargs, "setattr",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "setattr",
         3, 3,
-        &obj, &name, &value)) {
+        &obj, &name, &value))) {
         goto exit;
     }
 
@@ -428,9 +428,9 @@ builtin_delattr(PyObject *module, PyObje
     PyObject *obj;
     PyObject *name;
 
-    if (!_PyArg_UnpackStack(args, nargs, "delattr",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "delattr",
         2, 2,
-        &obj, &name)) {
+        &obj, &name))) {
         goto exit;
     }
 
@@ -542,9 +542,9 @@ builtin_pow(PyObject *module, PyObject *
     PyObject *y;
     PyObject *z = Py_None;
 
-    if (!_PyArg_UnpackStack(args, nargs, "pow",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "pow",
         2, 3,
-        &x, &y, &z)) {
+        &x, &y, &z))) {
         goto exit;
     }
 
@@ -581,9 +581,9 @@ builtin_input(PyObject *module, PyObject
     PyObject *return_value = NULL;
     PyObject *prompt = NULL;
 
-    if (!_PyArg_UnpackStack(args, nargs, "input",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "input",
         0, 1,
-        &prompt)) {
+        &prompt))) {
         goto exit;
     }
 
@@ -630,9 +630,9 @@ builtin_sum(PyObject *module, PyObject *
     PyObject *iterable;
     PyObject *start = NULL;
 
-    if (!_PyArg_UnpackStack(args, nargs, "sum",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "sum",
         1, 2,
-        &iterable, &start)) {
+        &iterable, &start))) {
         goto exit;
     }
 
@@ -669,9 +669,9 @@ builtin_isinstance(PyObject *module, PyO
     PyObject *obj;
     PyObject *class_or_tuple;
 
-    if (!_PyArg_UnpackStack(args, nargs, "isinstance",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "isinstance",
         2, 2,
-        &obj, &class_or_tuple)) {
+        &obj, &class_or_tuple))) {
         goto exit;
     }
 
@@ -708,9 +708,9 @@ builtin_issubclass(PyObject *module, PyO
     PyObject *cls;
     PyObject *class_or_tuple;
 
-    if (!_PyArg_UnpackStack(args, nargs, "issubclass",
+    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "issubclass",
         2, 2,
-        &cls, &class_or_tuple)) {
+        &cls, &class_or_tuple))) {
         goto exit;
     }
 
@@ -722,4 +722,4 @@ builtin_issubclass(PyObject *module, PyO
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=3234725ef4d8bbf1 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=7cad436471be2881 input=a9049054013a1b77]*/
diff -r 82d1c8d15e18 Python/getargs.c
--- a/Python/getargs.c	Mon Feb 06 10:41:46 2017 +0200
+++ b/Python/getargs.c	Mon Feb 06 10:27:12 2017 +0100
@@ -282,9 +282,10 @@ vgetargs1_impl(PyObject *compat_args, Py
             if (level == 0)
                 max++;
             level++;
-            if (level >= 30)
+            if (_Py_unlikely(level >= 30)) {
                 Py_FatalError("too many tuple nesting levels "
                               "in argument format string");
+            }
             break;
         case ')':
             if (level == 0)
@@ -317,8 +318,9 @@ vgetargs1_impl(PyObject *compat_args, Py
         }
     }
 
-    if (level != 0)
+    if (_Py_unlikely(level != 0)) {
         Py_FatalError(/* '(' */ "missing ')' in getargs format");
+    }
 
     if (min < 0)
         min = max;
@@ -327,7 +329,7 @@ vgetargs1_impl(PyObject *compat_args, Py
 
     if (max > STATIC_FREELIST_ENTRIES) {
         freelist.entries = PyMem_NEW(freelistentry_t, max);
-        if (freelist.entries == NULL) {
+        if (_Py_unlikely(freelist.entries == NULL)) {
             PyErr_NoMemory();
             return 0;
         }
@@ -366,7 +368,7 @@ vgetargs1_impl(PyObject *compat_args, Py
         }
     }
 
-    if (nargs < min || max < nargs) {
+    if (_Py_unlikely(nargs < min || max < nargs)) {
         if (message == NULL)
             PyErr_Format(PyExc_TypeError,
                          "%.150s%s takes %s %d argument%s (%ld given)",
@@ -388,15 +390,15 @@ vgetargs1_impl(PyObject *compat_args, Py
         msg = convertitem(stack[i], &format, p_va,
                           flags, levels, msgbuf,
                           sizeof(msgbuf), &freelist);
-        if (msg) {
+        if (_Py_unlikely(msg)) {
             seterror(i+1, msg, levels, fname, message);
             return cleanreturn(0, &freelist);
         }
     }
 
-    if (*format != '\0' && !Py_ISALPHA(Py_CHARMASK(*format)) &&
-        *format != '(' &&
-        *format != '|' && *format != ':' && *format != ';') {
+    if (_Py_unlikely(*format != '\0' && !Py_ISALPHA(Py_CHARMASK(*format)) &&
+                     *format != '(' &&
+                     *format != '|' && *format != ':' && *format != ';')) {
         PyErr_Format(PyExc_SystemError,
                      "bad format string: %.200s", formatsave);
         return cleanreturn(0, &freelist);
@@ -522,7 +524,7 @@ converttuple(PyObject *arg, const char *
             n++;
     }
 
-    if (!PySequence_Check(arg) || PyBytes_Check(arg)) {
+    if (_Py_unlikely(!PySequence_Check(arg) || PyBytes_Check(arg))) {
         levels[0] = 0;
         PyOS_snprintf(msgbuf, bufsize,
                       toplevel ? "expected %d arguments, not %.50s" :
@@ -533,7 +535,7 @@ converttuple(PyObject *arg, const char *
     }
 
     len = PySequence_Size(arg);
-    if (len != n) {
+    if (_Py_unlikely(len != n)) {
         levels[0] = 0;
         if (toplevel) {
             PyOS_snprintf(msgbuf, bufsize,
@@ -554,7 +556,7 @@ converttuple(PyObject *arg, const char *
         const char *msg;
         PyObject *item;
         item = PySequence_GetItem(arg, i);
-        if (item == NULL) {
+        if (_Py_unlikely(item == NULL)) {
             PyErr_Clear();
             levels[0] = i+1;
             levels[1] = 0;
@@ -565,7 +567,7 @@ converttuple(PyObject *arg, const char *
                           msgbuf, bufsize, freelist);
         /* PySequence_GetItem calls tp->sq_item, which INCREFs */
         Py_XDECREF(item);
-        if (msg != NULL) {
+        if (_Py_unlikely(msg != NULL)) {
             levels[0] = i+1;
             return msg;
         }
@@ -589,14 +591,16 @@ convertitem(PyObject *arg, const char **
         format++;
         msg = converttuple(arg, &format, p_va, flags, levels, msgbuf,
                            bufsize, 0, freelist);
-        if (msg == NULL)
+        if (_Py_likely(msg == NULL)) {
             format++;
+        }
     }
     else {
         msg = convertsimple(arg, &format, p_va, flags,
                             msgbuf, bufsize, freelist);
-        if (msg != NULL)
+        if (_Py_unlikely(msg != NULL)) {
             levels[0] = 0;
+        }
     }
     if (msg == NULL)
         *p_format = format;
diff -r 82d1c8d15e18 Tools/clinic/clinic.py
--- a/Tools/clinic/clinic.py	Mon Feb 06 10:41:46 2017 +0200
+++ b/Tools/clinic/clinic.py	Mon Feb 06 10:27:12 2017 +0100
@@ -830,9 +830,9 @@ class CLanguage(Language):
                 parser_prototype = parser_prototype_fastcall
 
                 parser_definition = parser_body(parser_prototype, normalize_snippet("""
-                    if (!_PyArg_UnpackStack(args, nargs, "{name}",
+                    if (_Py_unlikely(!_PyArg_UnpackStack(args, nargs, "{name}",
                         {unpack_min}, {unpack_max},
-                        {parse_arguments})) {{
+                        {parse_arguments}))) {{
                         goto exit;
                     }}
 
@@ -861,8 +861,8 @@ class CLanguage(Language):
                 parser_prototype = parser_prototype_fastcall
 
                 parser_definition = parser_body(parser_prototype, normalize_snippet("""
-                    if (!_PyArg_ParseStack(args, nargs, "{format_units}:{name}",
-                        {parse_arguments})) {{
+                    if (_Py_unlikely(!_PyArg_ParseStack(args, nargs, "{format_units}:{name}",
+                        {parse_arguments}))) {{
                         goto exit;
                     }}