@@ -777,6 +777,14 @@ All of the following opcodes use their arguments.
    .. versionadded:: 3.6
 
 
+.. opcode:: BUILD_STRING (count)
+
+   Concatenates *count* strings from the stack and pushes the resulting string
+   onto the stack.
+
+   .. versionadded:: 3.6
+
+
 .. opcode:: LOAD_ATTR (namei)
 
    Replaces TOS with ``getattr(TOS, co_names[namei])``.
 
@@ -123,6 +123,7 @@ extern "C" {
 #define SETUP_ASYNC_WITH        154
 #define FORMAT_VALUE            155
 #define BUILD_CONST_KEY_MAP     156
+#define BUILD_STRING            157
 
 /* EXCEPT_HANDLER is a special, implicit block type which is created when
    entering an except handler. It is not an opcode but we define it here
 
@@ -1955,6 +1955,14 @@ PyAPI_FUNC(PyObject*) PyUnicode_Join(
     PyObject *seq               /* Sequence object */
     );
 
+#ifndef Py_LIMITED_API
+PyAPI_FUNC(PyObject *) _PyUnicode_JoinArray(
+    PyObject *separator,
+    PyObject **items,
+    Py_ssize_t seqlen
+    );
+#endif /* Py_LIMITED_API */
+
 /* Return 1 if substr matches str[start:end] at the given tail end, 0
    otherwise. */
 
 
@@ -232,7 +232,8 @@ def _write_atomic(path, data, mode=0o666):
 #     Python 3.6a1  3370 (16 bit wordcode)
 #     Python 3.6a1  3371 (add BUILD_CONST_KEY_MAP opcode #27140)
 #     Python 3.6a1  3372 (MAKE_FUNCTION simplification, remove MAKE_CLOSURE
-                          #27095)
+#                         #27095)
+#     Python 3.6b1  3373 (add BUILD_STRING opcode #27078)
 #
 # MAGIC must change whenever the bytecode emitted by the compiler may no
 # longer be understood by older implementations of the eval loop (usually
@@ -241,7 +242,7 @@ def _write_atomic(path, data, mode=0o666):
 # Whenever MAGIC_NUMBER is changed, the ranges in the magic_values array
 # in PC/launcher.c must also be updated.
 
-MAGIC_NUMBER = (3372).to_bytes(2, 'little') + b'\r\n'
+MAGIC_NUMBER = (3373).to_bytes(2, 'little') + b'\r\n'
 _RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little')  # For import.c
 
 _PYCACHE = '__pycache__'
 
@@ -213,5 +213,6 @@ def jabs_op(name, op):
 
 def_op('FORMAT_VALUE', 155)
 def_op('BUILD_CONST_KEY_MAP', 156)
+def_op('BUILD_STRING', 157)
 
 del def_op, name_op, jrel_op, jabs_op
@@ -10,6 +10,8 @@ What's New in Python 3.6.0 beta 1
 Core and Builtins
 -----------------
 
+- Issue #27078: Added BUILD_STRING opcode.  Optimized f-strings evaluation.
+
 - Issue #17884: Python now requires systems with inttypes.h and stdint.h
 
 - Issue #27961: Require platforms to support ``long long``. Python hasn't
 
@@ -675,13 +675,31 @@ PyObject_Format(PyObject *obj, PyObject *format_spec)
     PyObject *result = NULL;
     _Py_IDENTIFIER(__format__);
 
+    if (format_spec != NULL && !PyUnicode_Check(format_spec)) {
+        PyErr_Format(PyExc_SystemError,
+                     "Format specifier must be a string, not %.200s",
+                     Py_TYPE(format_spec)->tp_name);
+        return NULL;
+    }
+
+    /* Fast path for common types. */
+    if (format_spec == NULL || PyUnicode_GET_LENGTH(format_spec) == 0) {
+        if (PyUnicode_CheckExact(obj)) {
+            Py_INCREF(obj);
+            return obj;
+        }
+        if (PyLong_CheckExact(obj)) {
+            return PyObject_Str(obj);
+        }
+    }
+
     /* If no format_spec is provided, use an empty string */
     if (format_spec == NULL) {
         empty = PyUnicode_New(0, 0);
         format_spec = empty;
     }
 
-    /* Find the (unbound!) __format__ method (a borrowed reference) */
+    /* Find the (unbound!) __format__ method */
     meth = _PyObject_LookupSpecial(obj, &PyId___format__);
     if (meth == NULL) {
         if (!PyErr_Occurred())
 
@@ -9892,20 +9892,10 @@ case_operation(PyObject *self,
 PyObject *
 PyUnicode_Join(PyObject *separator, PyObject *seq)
 {
-    PyObject *sep = NULL;
-    Py_ssize_t seplen;
-    PyObject *res = NULL; /* the result */
-    PyObject *fseq;          /* PySequence_Fast(seq) */
-    Py_ssize_t seqlen;       /* len(fseq) -- number of items in sequence */
+    PyObject *res;
+    PyObject *fseq;
+    Py_ssize_t seqlen;
     PyObject **items;
-    PyObject *item;
-    Py_ssize_t sz, i, res_offset;
-    Py_UCS4 maxchar;
-    Py_UCS4 item_maxchar;
-    int use_memcpy;
-    unsigned char *res_data = NULL, *sep_data = NULL;
-    PyObject *last_obj;
-    unsigned int kind = 0;
 
     fseq = PySequence_Fast(seq, "can only join an iterable");
     if (fseq == NULL) {
@@ -9916,21 +9906,39 @@ PyUnicode_Join(PyObject *separator, PyObject *seq)
      * so we are sure that fseq won't be mutated.
      */
 
+    items = PySequence_Fast_ITEMS(fseq);
     seqlen = PySequence_Fast_GET_SIZE(fseq);
+    res = _PyUnicode_JoinArray(separator, items, seqlen);
+    Py_DECREF(fseq);
+    return res;
+}
+
+PyObject *
+_PyUnicode_JoinArray(PyObject *separator, PyObject **items, Py_ssize_t seqlen)
+{
+    PyObject *res = NULL; /* the result */
+    PyObject *sep = NULL;
+    Py_ssize_t seplen;
+    PyObject *item;
+    Py_ssize_t sz, i, res_offset;
+    Py_UCS4 maxchar;
+    Py_UCS4 item_maxchar;
+    int use_memcpy;
+    unsigned char *res_data = NULL, *sep_data = NULL;
+    PyObject *last_obj;
+    unsigned int kind = 0;
+
     /* If empty sequence, return u"". */
     if (seqlen == 0) {
-        Py_DECREF(fseq);
         _Py_RETURN_UNICODE_EMPTY();
     }
 
     /* If singleton sequence with an exact Unicode, return that. */
     last_obj = NULL;
-    items = PySequence_Fast_ITEMS(fseq);
     if (seqlen == 1) {
         if (PyUnicode_CheckExact(items[0])) {
             res = items[0];
             Py_INCREF(res);
-            Py_DECREF(fseq);
             return res;
         }
         seplen = 0;
@@ -10065,13 +10073,11 @@ PyUnicode_Join(PyObject *separator, PyObject *seq)
         assert(res_offset == PyUnicode_GET_LENGTH(res));
     }
 
-    Py_DECREF(fseq);
     Py_XDECREF(sep);
     assert(_PyUnicode_CheckConsistency(res, 1));
     return res;
 
   onError:
-    Py_DECREF(fseq);
     Py_XDECREF(sep);
     Py_XDECREF(res);
     return NULL;
 
@@ -1089,7 +1089,7 @@ static PYC_MAGIC magic_values[] = {
     { 3190, 3230, L"3.3" },
     { 3250, 3310, L"3.4" },
     { 3320, 3351, L"3.5" },
-    { 3360, 3371, L"3.6" },
+    { 3360, 3373, L"3.6" },
     { 0 }
 };
 
 
@@ -2538,6 +2538,24 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
             DISPATCH();
         }
 
+        TARGET(BUILD_STRING) {
+            PyObject *str;
+            PyObject *empty = PyUnicode_New(0, 0);
+            if (empty == NULL) {
+                goto error;
+            }
+            str = _PyUnicode_JoinArray(empty, stack_pointer - oparg, oparg);
+            Py_DECREF(empty);
+            if (str == NULL)
+                goto error;
+            while (--oparg >= 0) {
+                PyObject *item = POP();
+                Py_DECREF(item);
+            }
+            PUSH(str);
+            DISPATCH();
+        }
+
         TARGET(BUILD_TUPLE) {
             PyObject *tup = PyTuple_New(oparg);
             if (tup == NULL)