bpo-37348: optimize decoding ASCII string (GH-14283)

`_PyUnicode_Writer` is a relatively complex structure.  Initializing it is significant overhead when decoding short ASCII string.

Misc/NEWS.d/next/Core and Builtins/2019-06-23-00-26-30.bpo-37348.pp8P-x.rst

+Optimized decoding short ASCII string with UTF-8 and ascii codecs.
+``b"foo".decode()`` is about 15% faster.  Patch by Inada Naoki.

Objects/unicodeobject.c

@@ -265,6 +265,8 @@ unicode_fill(enum PyUnicode_Kind kind, void *data, Py_UCS4 value,
 /* Forward declaration */
 static inline int
 _PyUnicodeWriter_WriteCharInline(_PyUnicodeWriter *writer, Py_UCS4 ch);
+static inline void
+_PyUnicodeWriter_InitWithBuffer(_PyUnicodeWriter *writer, PyObject *buffer);
 static PyObject *
 unicode_encode_utf8(PyObject *unicode, _Py_error_handler error_handler,
                     const char *errors);
@@ -4877,16 +4879,6 @@ unicode_decode_utf8(const char *s, Py_ssize_t size,
                     _Py_error_handler error_handler, const char *errors,
                     Py_ssize_t *consumed)
 {
-    _PyUnicodeWriter writer;
-    const char *starts = s;
-    const char *end = s + size;
-
-    Py_ssize_t startinpos;
-    Py_ssize_t endinpos;
-    const char *errmsg = "";
-    PyObject *error_handler_obj = NULL;
-    PyObject *exc = NULL;
-
     if (size == 0) {
         if (consumed)
             *consumed = 0;
@@ -4900,13 +4892,29 @@ unicode_decode_utf8(const char *s, Py_ssize_t size,
         return get_latin1_char((unsigned char)s[0]);
     }
 
-    _PyUnicodeWriter_Init(&writer);
-    writer.min_length = size;
-    if (_PyUnicodeWriter_Prepare(&writer, writer.min_length, 127) == -1)
-        goto onError;
+    const char *starts = s;
+    const char *end = s + size;
+
+    // fast path: try ASCII string.
+    PyObject *u = PyUnicode_New(size, 127);
+    if (u == NULL) {
+        return NULL;
+    }
+    s += ascii_decode(s, end, PyUnicode_DATA(u));
+    if (s == end) {
+        return u;
+    }
+
+    // Use _PyUnicodeWriter after fast path is failed.
+    _PyUnicodeWriter writer;
+    _PyUnicodeWriter_InitWithBuffer(&writer, u);
+    writer.pos = s - starts;
+
+    Py_ssize_t startinpos, endinpos;
+    const char *errmsg = "";
+    PyObject *error_handler_obj = NULL;
+    PyObject *exc = NULL;
 
-    writer.pos = ascii_decode(s, end, writer.data);
-    s += writer.pos;
     while (s < end) {
         Py_UCS4 ch;
         int kind = writer.kind;
@@ -6451,7 +6459,7 @@ PyUnicode_DecodeRawUnicodeEscape(const char *s,
        length after conversion to the true value. (But decoding error
        handler might have to resize the string) */
     _PyUnicodeWriter_Init(&writer);
-     writer.min_length = size;
+    writer.min_length = size;
     if (_PyUnicodeWriter_Prepare(&writer, size, 127) < 0) {
         goto onError;
     }
@@ -6975,13 +6983,7 @@ PyUnicode_DecodeASCII(const char *s,
                       const char *errors)
 {
     const char *starts = s;
-    _PyUnicodeWriter writer;
-    int kind;
-    void *data;
-    Py_ssize_t startinpos;
-    Py_ssize_t endinpos;
-    Py_ssize_t outpos;
-    const char *e;
+    const char *e = s + size;
     PyObject *error_handler_obj = NULL;
     PyObject *exc = NULL;
     _Py_error_handler error_handler = _Py_ERROR_UNKNOWN;
@@ -6993,20 +6995,25 @@ PyUnicode_DecodeASCII(const char *s,
     if (size == 1 && (unsigned char)s[0] < 128)
         return get_latin1_char((unsigned char)s[0]);
 
-    _PyUnicodeWriter_Init(&writer);
-    writer.min_length = size;
-    if (_PyUnicodeWriter_Prepare(&writer, writer.min_length, 127) < 0)
+    // Shortcut for simple case
+    PyObject *u = PyUnicode_New(size, 127);
+    if (u == NULL) {
         return NULL;
+    }
+    Py_ssize_t outpos = ascii_decode(s, e, PyUnicode_DATA(u));
+    if (outpos == size) {
+        return u;
+    }
 
-    e = s + size;
-    data = writer.data;
-    outpos = ascii_decode(s, e, (Py_UCS1 *)data);
+    _PyUnicodeWriter writer;
+    _PyUnicodeWriter_InitWithBuffer(&writer, u);
     writer.pos = outpos;
-    if (writer.pos == size)
-        return _PyUnicodeWriter_Finish(&writer);
 
-    s += writer.pos;
-    kind = writer.kind;
+    s += outpos;
+    int kind = writer.kind;
+    void *data = writer.data;
+    Py_ssize_t startinpos, endinpos;
+
     while (s < e) {
         unsigned char c = (unsigned char)*s;
         if (c < 128) {
@@ -13506,6 +13513,16 @@ _PyUnicodeWriter_Init(_PyUnicodeWriter *writer)
     assert(writer->kind <= PyUnicode_1BYTE_KIND);
 }
 
+// Initialize _PyUnicodeWriter with initial buffer
+static inline void
+_PyUnicodeWriter_InitWithBuffer(_PyUnicodeWriter *writer, PyObject *buffer)
+{
+    memset(writer, 0, sizeof(*writer));
+    writer->buffer = buffer;
+    _PyUnicodeWriter_Update(writer);
+    writer->min_length = writer->size;
+}
+
 int
 _PyUnicodeWriter_PrepareInternal(_PyUnicodeWriter *writer,
                                  Py_ssize_t length, Py_UCS4 maxchar)