diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-02-03-21-12-39.bpo-39087.YnbUpL.rst b/Misc/NEWS.d/next/Core and Builtins/2020-02-03-21-12-39.bpo-39087.YnbUpL.rst
new file mode 100644
index 00000000000..847f78f5b18
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-02-03-21-12-39.bpo-39087.YnbUpL.rst	
@@ -0,0 +1,2 @@
+Optimize :c:func:`PyUnicode_AsUTF8` and :c:func:`PyUnicode_AsUTF8AndSize`
+slightly when they need to create internal UTF-8 cache.
diff --git a/Objects/stringlib/codecs.h b/Objects/stringlib/codecs.h
index 269a5581f70..eb42e071751 100644
--- a/Objects/stringlib/codecs.h
+++ b/Objects/stringlib/codecs.h
@@ -256,8 +256,9 @@ InvalidContinuation3:
 /* UTF-8 encoder specialized for a Unicode kind to avoid the slow
    PyUnicode_READ() macro. Delete some parts of the code depending on the kind:
    UCS-1 strings don't need to handle surrogates for example. */
-Py_LOCAL_INLINE(PyObject *)
-STRINGLIB(utf8_encoder)(PyObject *unicode,
+Py_LOCAL_INLINE(char *)
+STRINGLIB(utf8_encoder)(_PyBytesWriter *writer,
+                        PyObject *unicode,
                         STRINGLIB_CHAR *data,
                         Py_ssize_t size,
                         _Py_error_handler error_handler,
@@ -277,17 +278,16 @@ STRINGLIB(utf8_encoder)(PyObject *unicode,
 #else /*  STRINGLIB_SIZEOF_CHAR == 4 */
     const Py_ssize_t max_char_size = 4;
 #endif
-    _PyBytesWriter writer;
 
     assert(size >= 0);
-    _PyBytesWriter_Init(&writer);
-
     if (size > PY_SSIZE_T_MAX / max_char_size) {
         /* integer overflow */
-        return PyErr_NoMemory();
+        PyErr_NoMemory();
+        return NULL;
     }
 
-    p = _PyBytesWriter_Alloc(&writer, size * max_char_size);
+    _PyBytesWriter_Init(writer);
+    p = _PyBytesWriter_Alloc(writer, size * max_char_size);
     if (p == NULL)
         return NULL;
 
@@ -323,7 +323,7 @@ STRINGLIB(utf8_encoder)(PyObject *unicode,
                 endpos++;
 
             /* Only overallocate the buffer if it's not the last write */
-            writer.overallocate = (endpos < size);
+            writer->overallocate = (endpos < size);
 
             switch (error_handler)
             {
@@ -347,8 +347,8 @@ STRINGLIB(utf8_encoder)(PyObject *unicode,
 
             case _Py_ERROR_BACKSLASHREPLACE:
                 /* subtract preallocated bytes */
-                writer.min_size -= max_char_size * (endpos - startpos);
-                p = backslashreplace(&writer, p,
+                writer->min_size -= max_char_size * (endpos - startpos);
+                p = backslashreplace(writer, p,
                                      unicode, startpos, endpos);
                 if (p == NULL)
                     goto error;
@@ -357,8 +357,8 @@ STRINGLIB(utf8_encoder)(PyObject *unicode,
 
             case _Py_ERROR_XMLCHARREFREPLACE:
                 /* subtract preallocated bytes */
-                writer.min_size -= max_char_size * (endpos - startpos);
-                p = xmlcharrefreplace(&writer, p,
+                writer->min_size -= max_char_size * (endpos - startpos);
+                p = xmlcharrefreplace(writer, p,
                                       unicode, startpos, endpos);
                 if (p == NULL)
                     goto error;
@@ -387,10 +387,10 @@ STRINGLIB(utf8_encoder)(PyObject *unicode,
                     goto error;
 
                 /* subtract preallocated bytes */
-                writer.min_size -= max_char_size * (newpos - startpos);
+                writer->min_size -= max_char_size * (newpos - startpos);
 
                 if (PyBytes_Check(rep)) {
-                    p = _PyBytesWriter_WriteBytes(&writer, p,
+                    p = _PyBytesWriter_WriteBytes(writer, p,
                                                   PyBytes_AS_STRING(rep),
                                                   PyBytes_GET_SIZE(rep));
                 }
@@ -406,7 +406,7 @@ STRINGLIB(utf8_encoder)(PyObject *unicode,
                         goto error;
                     }
 
-                    p = _PyBytesWriter_WriteBytes(&writer, p,
+                    p = _PyBytesWriter_WriteBytes(writer, p,
                                                   PyUnicode_DATA(rep),
                                                   PyUnicode_GET_LENGTH(rep));
                 }
@@ -420,7 +420,7 @@ STRINGLIB(utf8_encoder)(PyObject *unicode,
 
             /* If overallocation was disabled, ensure that it was the last
                write. Otherwise, we missed an optimization */
-            assert(writer.overallocate || i == size);
+            assert(writer->overallocate || i == size);
         }
         else
 #if STRINGLIB_SIZEOF_CHAR > 2
@@ -449,14 +449,13 @@ STRINGLIB(utf8_encoder)(PyObject *unicode,
     Py_XDECREF(error_handler_obj);
     Py_XDECREF(exc);
 #endif
-    return _PyBytesWriter_Finish(&writer, p);
+    return p;
 
 #if STRINGLIB_SIZEOF_CHAR > 1
  error:
     Py_XDECREF(rep);
     Py_XDECREF(error_handler_obj);
     Py_XDECREF(exc);
-    _PyBytesWriter_Dealloc(&writer);
     return NULL;
 #endif
 }
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index ee6d3dfd394..e0a666f70da 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -3991,11 +3991,11 @@ PyUnicode_FSDecoder(PyObject* arg, void* addr)
 }
 
 
+static int unicode_fill_utf8(PyObject *unicode);
+
 const char *
 PyUnicode_AsUTF8AndSize(PyObject *unicode, Py_ssize_t *psize)
 {
-    PyObject *bytes;
-
     if (!PyUnicode_Check(unicode)) {
         PyErr_BadArgument();
         return NULL;
@@ -4004,21 +4004,9 @@ PyUnicode_AsUTF8AndSize(PyObject *unicode, Py_ssize_t *psize)
         return NULL;
 
     if (PyUnicode_UTF8(unicode) == NULL) {
-        assert(!PyUnicode_IS_COMPACT_ASCII(unicode));
-        bytes = _PyUnicode_AsUTF8String(unicode, NULL);
-        if (bytes == NULL)
-            return NULL;
-        _PyUnicode_UTF8(unicode) = PyObject_MALLOC(PyBytes_GET_SIZE(bytes) + 1);
-        if (_PyUnicode_UTF8(unicode) == NULL) {
-            PyErr_NoMemory();
-            Py_DECREF(bytes);
+        if (unicode_fill_utf8(unicode) == -1) {
             return NULL;
         }
-        _PyUnicode_UTF8_LENGTH(unicode) = PyBytes_GET_SIZE(bytes);
-        memcpy(_PyUnicode_UTF8(unicode),
-                  PyBytes_AS_STRING(bytes),
-                  _PyUnicode_UTF8_LENGTH(unicode) + 1);
-        Py_DECREF(bytes);
     }
 
     if (psize)
@@ -5381,10 +5369,6 @@ static PyObject *
 unicode_encode_utf8(PyObject *unicode, _Py_error_handler error_handler,
                     const char *errors)
 {
-    enum PyUnicode_Kind kind;
-    void *data;
-    Py_ssize_t size;
-
     if (!PyUnicode_Check(unicode)) {
         PyErr_BadArgument();
         return NULL;
@@ -5397,9 +5381,12 @@ unicode_encode_utf8(PyObject *unicode, _Py_error_handler error_handler,
         return PyBytes_FromStringAndSize(PyUnicode_UTF8(unicode),
                                          PyUnicode_UTF8_LENGTH(unicode));
 
-    kind = PyUnicode_KIND(unicode);
-    data = PyUnicode_DATA(unicode);
-    size = PyUnicode_GET_LENGTH(unicode);
+    enum PyUnicode_Kind kind = PyUnicode_KIND(unicode);
+    void *data = PyUnicode_DATA(unicode);
+    Py_ssize_t size = PyUnicode_GET_LENGTH(unicode);
+
+    _PyBytesWriter writer;
+    char *end;
 
     switch (kind) {
     default:
@@ -5407,12 +5394,73 @@ unicode_encode_utf8(PyObject *unicode, _Py_error_handler error_handler,
     case PyUnicode_1BYTE_KIND:
         /* the string cannot be ASCII, or PyUnicode_UTF8() would be set */
         assert(!PyUnicode_IS_ASCII(unicode));
-        return ucs1lib_utf8_encoder(unicode, data, size, error_handler, errors);
+        end = ucs1lib_utf8_encoder(&writer, unicode, data, size, error_handler, errors);
+        break;
     case PyUnicode_2BYTE_KIND:
-        return ucs2lib_utf8_encoder(unicode, data, size, error_handler, errors);
+        end = ucs2lib_utf8_encoder(&writer, unicode, data, size, error_handler, errors);
+        break;
     case PyUnicode_4BYTE_KIND:
-        return ucs4lib_utf8_encoder(unicode, data, size, error_handler, errors);
+        end = ucs4lib_utf8_encoder(&writer, unicode, data, size, error_handler, errors);
+        break;
     }
+
+    if (end == NULL) {
+        _PyBytesWriter_Dealloc(&writer);
+        return NULL;
+    }
+    return _PyBytesWriter_Finish(&writer, end);
+}
+
+static int
+unicode_fill_utf8(PyObject *unicode)
+{
+    /* the string cannot be ASCII, or PyUnicode_UTF8() would be set */
+    assert(!PyUnicode_IS_ASCII(unicode));
+
+    enum PyUnicode_Kind kind = PyUnicode_KIND(unicode);
+    void *data = PyUnicode_DATA(unicode);
+    Py_ssize_t size = PyUnicode_GET_LENGTH(unicode);
+
+    _PyBytesWriter writer;
+    char *end;
+
+    switch (kind) {
+    default:
+        Py_UNREACHABLE();
+    case PyUnicode_1BYTE_KIND:
+        end = ucs1lib_utf8_encoder(&writer, unicode, data, size,
+                                   _Py_ERROR_STRICT, NULL);
+        break;
+    case PyUnicode_2BYTE_KIND:
+        end = ucs2lib_utf8_encoder(&writer, unicode, data, size,
+                                   _Py_ERROR_STRICT, NULL);
+        break;
+    case PyUnicode_4BYTE_KIND:
+        end = ucs4lib_utf8_encoder(&writer, unicode, data, size,
+                                   _Py_ERROR_STRICT, NULL);
+        break;
+    }
+    if (end == NULL) {
+        _PyBytesWriter_Dealloc(&writer);
+        return -1;
+    }
+
+    char *start = writer.use_small_buffer ? writer.small_buffer :
+                    PyBytes_AS_STRING(writer.buffer);
+    Py_ssize_t len = end - start;
+
+    char *cache = PyObject_MALLOC(len + 1);
+    if (cache == NULL) {
+        _PyBytesWriter_Dealloc(&writer);
+        PyErr_NoMemory();
+        return -1;
+    }
+    _PyUnicode_UTF8(unicode) = cache;
+    _PyUnicode_UTF8_LENGTH(unicode) = len;
+    memcpy(cache, start, len);
+    cache[len] = '\0';
+    _PyBytesWriter_Dealloc(&writer);
+    return 0;
 }
 
 PyObject *