bpo-40222: "Zero cost" exception handling (GH-25729)

"Zero cost" exception handling.

* Uses a lookup table to determine how to handle exceptions.
* Removes SETUP_FINALLY and POP_TOP block instructions, eliminating (most of) the runtime overhead of try statements.
* Reduces the size of the frame object by about 60%.

Author: markshannon

Doc/library/dis.rst

@@ -616,13 +616,6 @@ the original TOS1.
    .. versionadded:: 3.5
 
 
-.. opcode:: SETUP_ASYNC_WITH
-
-   Creates a new frame object.
-
-   .. versionadded:: 3.5
-
-
 
 **Miscellaneous opcodes**
 
@@ -692,28 +685,29 @@ iterations of the loop.
    opcode implements ``from module import *``.
 
 
-.. opcode:: POP_BLOCK
-
-   Removes one block from the block stack.  Per frame, there is a stack of
-   blocks, denoting :keyword:`try` statements, and such.
-
-
 .. opcode:: POP_EXCEPT
 
-   Removes one block from the block stack. The popped block must be an exception
-   handler block, as implicitly created when entering an except handler.  In
-   addition to popping extraneous values from the frame stack, the last three
-   popped values are used to restore the exception state.
+   Pops three values from the stack, which are used to restore the exception state.
 
 
 .. opcode:: RERAISE
 
     Re-raises the exception currently on top of the stack. If oparg is non-zero,
-    restores ``f_lasti`` of the current frame to its value when the exception was raised.
+    pops an additional value from the stack which is used to set ``f_lasti``
+    of the current frame.
 
     .. versionadded:: 3.9
 
 
+.. opcode:: PUSH_EXC_INFO
+
+    Pops the three values from the stack. Pushes the current exception to the top of the stack.
+    Pushes the three values originally popped back to the stack.
+    Used in exception handlers.
+
+    .. versionadded:: 3.11
+
+
 .. opcode:: WITH_EXCEPT_START
 
     Calls the function in position 7 on the stack with the top three
@@ -724,6 +718,17 @@ iterations of the loop.
     .. versionadded:: 3.9
 
 
+.. opcode:: POP_EXCEPT_AND_RERAISE
+
+    Pops the exception currently on top of the stack. Pops the integer value on top
+    of the stack and sets the ``f_lasti`` attribute of the frame with that value.
+    Then pops the next exception from the stack uses it to restore the current exception.
+    Finally it re-raises the originally popped exception.
+    Used in excpetion handler cleanup.
+
+    .. versionadded:: 3.11
+
+
 .. opcode:: LOAD_ASSERTION_ERROR
 
    Pushes :exc:`AssertionError` onto the stack.  Used by the :keyword:`assert`
@@ -738,18 +743,15 @@ iterations of the loop.
    by :opcode:`CALL_FUNCTION` to construct a class.
 
 
-.. opcode:: SETUP_WITH (delta)
+.. opcode:: BEFORE_WITH (delta)
 
    This opcode performs several operations before a with block starts.  First,
    it loads :meth:`~object.__exit__` from the context manager and pushes it onto
    the stack for later use by :opcode:`WITH_EXCEPT_START`.  Then,
-   :meth:`~object.__enter__` is called, and a finally block pointing to *delta*
-   is pushed.  Finally, the result of calling the ``__enter__()`` method is pushed onto
-   the stack.  The next opcode will either ignore it (:opcode:`POP_TOP`), or
-   store it in (a) variable(s) (:opcode:`STORE_FAST`, :opcode:`STORE_NAME`, or
-   :opcode:`UNPACK_SEQUENCE`).
+   :meth:`~object.__enter__` is called. Finally, the result of calling the
+   ``__enter__()`` method is pushed onto the stack.
 
-   .. versionadded:: 3.2
+   .. versionadded:: 3.11
 
 
 .. opcode:: COPY_DICT_WITHOUT_KEYS
@@ -1039,12 +1041,6 @@ All of the following opcodes use their arguments.
    Loads the global named ``co_names[namei]`` onto the stack.
 
 
-.. opcode:: SETUP_FINALLY (delta)
-
-   Pushes a try block from a try-finally or try-except clause onto the block
-   stack.  *delta* points to the finally block or the first except block.
-
-
 .. opcode:: LOAD_FAST (var_num)
 
    Pushes a reference to the local ``co_varnames[var_num]`` onto the stack.

Doc/whatsnew/3.11.rst

@@ -91,10 +91,12 @@ Optimizations
 =============
 
 
+
+
 Build and C API Changes
 =======================
 
-
+* :c:func:`PyFrame_BlockSetup` and :c:func:`PyFrame_BlockPop` have been removed.
 
 Deprecated
 ==========

Include/cpython/code.h

@@ -40,6 +40,7 @@ struct PyCodeObject {
     PyObject *co_name;          /* unicode (name, for reference) */
     PyObject *co_linetable;     /* string (encoding addr<->lineno mapping) See
                                    Objects/lnotab_notes.txt for details. */
+    PyObject *co_exceptiontable; /* Byte string encoding exception handling table */
     void *co_zombieframe;       /* for optimization only (see frameobject.c) */
     PyObject *co_weakreflist;   /* to support weakrefs to code objects */
     /* Scratch space for extra data relating to the code object.
@@ -117,12 +118,12 @@ PyAPI_DATA(PyTypeObject) PyCode_Type;
 PyAPI_FUNC(PyCodeObject *) PyCode_New(
         int, int, int, int, int, PyObject *, PyObject *,
         PyObject *, PyObject *, PyObject *, PyObject *,
-        PyObject *, PyObject *, int, PyObject *);
+        PyObject *, PyObject *, int, PyObject *, PyObject *);
 
 PyAPI_FUNC(PyCodeObject *) PyCode_NewWithPosOnlyArgs(
         int, int, int, int, int, int, PyObject *, PyObject *,
         PyObject *, PyObject *, PyObject *, PyObject *,
-        PyObject *, PyObject *, int, PyObject *);
+        PyObject *, PyObject *, int, PyObject *, PyObject *);
         /* same as struct above */
 
 /* Creates a new empty code object with the specified source location. */

Include/cpython/frameobject.h

@@ -34,18 +34,14 @@ struct _frame {
     PyObject *f_locals;         /* local symbol table (any mapping) */
     PyObject **f_valuestack;    /* points after the last local */
     PyObject *f_trace;          /* Trace function */
-    int f_stackdepth;           /* Depth of value stack */
-    char f_trace_lines;         /* Emit per-line trace events? */
-    char f_trace_opcodes;       /* Emit per-opcode trace events? */
-
     /* Borrowed reference to a generator, or NULL */
     PyObject *f_gen;
-
+    int f_stackdepth;           /* Depth of value stack */
     int f_lasti;                /* Last instruction if called */
     int f_lineno;               /* Current line number. Only valid if non-zero */
-    int f_iblock;               /* index in f_blockstack */
     PyFrameState f_state;       /* What state the frame is in */
-    PyTryBlock f_blockstack[CO_MAXBLOCKS]; /* for try and loop blocks */
+    char f_trace_lines;         /* Emit per-line trace events? */
+    char f_trace_opcodes;       /* Emit per-opcode trace events? */
     PyObject *f_localsplus[1];  /* locals+stack, dynamically sized */
 };
 
@@ -77,11 +73,6 @@ _PyFrame_New_NoTrack(PyThreadState *, PyFrameConstructor *, PyObject *);
 
 /* The rest of the interface is specific for frame objects */
 
-/* Block management functions */
-
-PyAPI_FUNC(void) PyFrame_BlockSetup(PyFrameObject *, int, int, int);
-PyAPI_FUNC(PyTryBlock *) PyFrame_BlockPop(PyFrameObject *);
-
 /* Conversions between "fast locals" and locals in dictionary */
 
 PyAPI_FUNC(void) PyFrame_LocalsToFast(PyFrameObject *, int);

Include/opcode.h

@@ -80,9 +80,9 @@ class struct_frozen(Structure):
                 continue
             items.append((entry.name.decode("ascii"), entry.size))
 
-        expected = [("__hello__", 137),
-                    ("__phello__", -137),
-                    ("__phello__.spam", 137),
+        expected = [("__hello__", 142),
+                    ("__phello__", -142),
+                    ("__phello__.spam", 142),
                     ]
         self.assertEqual(items, expected, "PyImport_FrozenModules example "
             "in Doc/library/ctypes.rst may be out of date")