gh-119118: Fix performance regression in tokenize module

Misc/NEWS.d/next/Library/2024-05-28-12-15-03.gh-issue-119118.FMKz1F.rst

@@ -0,0 +1 @@
+Fix performance regression in the :mod:`tokenize` module by caching the `line` token attribute and calculating the column offset more efficiently.

Python/Python-tokenize.c

@@ -32,6 +32,12 @@ typedef struct
 {
     PyObject_HEAD struct tok_state *tok;
     int done;
+
+    /* Needed to cache line for performance */
+    const char *last_line_start;
+    Py_ssize_t last_line_size;
+    PyObject *last_line;
+    Py_ssize_t byte_col_offset_diff;
 } tokenizeriterobject;
 
 /*[clinic input]
@@ -68,6 +74,12 @@ tokenizeriter_new_impl(PyTypeObject *type, PyObject *readline,
         self->tok->tok_extra_tokens = 1;
     }
     self->done = 0;
+
+    self->last_line_start = NULL;
+    self->last_line_size = -2;
+    self->last_line = NULL;
+    self->byte_col_offset_diff = 0;
+
     return (PyObject *)self;
 }
 
@@ -210,7 +222,20 @@ tokenizeriter_next(tokenizeriterobject *it)
         if (size >= 1 && it->tok->implicit_newline) {
             size -= 1;
         }
-        line = PyUnicode_DecodeUTF8(line_start, size, "replace");
+
+        if (line_start != it->last_line_start || size != it->last_line_size) {
+            // Line has changed since last token, so we fetch the new line and cache it
+            // in the iter object.
+            Py_XDECREF(it->last_line);
+            line = PyUnicode_DecodeUTF8(line_start, size, "replace");
+            it->last_line = line;
+            it->last_line_start = line_start;
+            it->last_line_size = size;
+            it->byte_col_offset_diff = 0;
+        } else {
+            // Line hasn't changed so we reuse the cached one.
+            line = it->last_line;
+        }
     }
     if (line == NULL) {
         Py_DECREF(str);
@@ -222,10 +247,25 @@ tokenizeriter_next(tokenizeriterobject *it)
     Py_ssize_t col_offset = -1;
     Py_ssize_t end_col_offset = -1;
     if (token.start != NULL && token.start >= line_start) {
-        col_offset = _PyPegen_byte_offset_to_character_offset(line, token.start - line_start);
+        Py_ssize_t byte_offset = token.start - line_start;
+        col_offset = byte_offset - it->byte_col_offset_diff;
     }
     if (token.end != NULL && token.end >= it->tok->line_start) {
-        end_col_offset = _PyPegen_byte_offset_to_character_offset_raw(it->tok->line_start, token.end - it->tok->line_start);
+        Py_ssize_t end_byte_offset = token.end - it->tok->line_start;
+        if (lineno == end_lineno) {
+            // If the whole token is at the same line, we can just use the token.start
+            // buffer for figuring out the new column offset, since using line is not
+            // performant for very long lines.
+            Py_ssize_t token_byte_offset = token.end - token.start;
+            Py_ssize_t token_col_offset = _PyPegen_byte_offset_to_character_offset_raw(
+                token.start, token_byte_offset
+            );
+            end_col_offset = col_offset + token_col_offset;
+            it->byte_col_offset_diff += token_byte_offset - token_col_offset;
+        } else {
+            end_col_offset = _PyPegen_byte_offset_to_character_offset_raw(it->tok->line_start, end_byte_offset);
+            it->byte_col_offset_diff += end_byte_offset - end_col_offset;
+        }
     }
 
     if (it->tok->tok_extra_tokens) {
@@ -262,7 +302,7 @@ tokenizeriter_next(tokenizeriterobject *it)
         }
     }
 
-    result = Py_BuildValue("(iN(nn)(nn)N)", type, str, lineno, col_offset, end_lineno, end_col_offset, line);
+    result = Py_BuildValue("(iN(nn)(nn)O)", type, str, lineno, col_offset, end_lineno, end_col_offset, line);
 exit:
     _PyToken_Free(&token);
     if (type == ENDMARKER) {