Issue #14744: Use the new _PyUnicodeWriter internal API to speed up str%args and str.format(args)

 * Formatting string, int, float and complex use the _PyUnicodeWriter API. It
   avoids a temporary buffer in most cases.
 * Add _PyUnicodeWriter_WriteStr() to restore the PyAccu optimization: just
   keep a reference to the string if the output is only composed of one string
 * Disable overallocation when formatting the last argument of str%args and
   str.format(args)
 * Overallocation allocates at least 100 characters: add min_length attribute
   to the _PyUnicodeWriter structure
 * Add new private functions: _PyUnicode_FastCopyCharacters(),
   _PyUnicode_FastFill() and _PyUnicode_FromASCII()

The speed up is around 20% in average.

1 files changed, 20 insertions, 7 deletions

diff --git a/Objects/floatobject.c b/Objects/floatobject.c
index 07d31b2..3c742c3 100644
--- a/Objects/floatobject.c
+++ b/Objects/floatobject.c
@@ -267,13 +267,15 @@ static PyObject *
 float_repr(PyFloatObject *v)
 {
     PyObject *result;
-    char *buf = PyOS_double_to_string(PyFloat_AS_DOUBLE(v),
-                                      'r', 0,
-                                      Py_DTSF_ADD_DOT_0,
-                                      NULL);
+    char *buf;
+
+    buf = PyOS_double_to_string(PyFloat_AS_DOUBLE(v),
+                                'r', 0,
+                                Py_DTSF_ADD_DOT_0,
+                                NULL);
     if (!buf)
         return PyErr_NoMemory();
-    result = PyUnicode_FromString(buf);
+    result = _PyUnicode_FromASCII(buf, strlen(buf));
     PyMem_Free(buf);
     return result;
 }
@@ -1703,11 +1705,22 @@ static PyObject *
 float__format__(PyObject *self, PyObject *args)
 {
     PyObject *format_spec;
+    _PyUnicodeWriter writer;
+    int ret;
 
     if (!PyArg_ParseTuple(args, "U:__format__", &format_spec))
         return NULL;
-    return _PyFloat_FormatAdvanced(self, format_spec, 0,
-                                   PyUnicode_GET_LENGTH(format_spec));
+
+    _PyUnicodeWriter_Init(&writer, 0);
+    ret = _PyFloat_FormatAdvancedWriter(
+        &writer,
+        self,
+        format_spec, 0, PyUnicode_GET_LENGTH(format_spec));
+    if (ret == -1) {
+        _PyUnicodeWriter_Dealloc(&writer);
+        return NULL;
+    }
+    return _PyUnicodeWriter_Finish(&writer);
 }
 
 PyDoc_STRVAR(float__format__doc,