Issue #19722: Added opcode.stack_effect(), which accurately

computes the stack effect of bytecode instructions.

Doc/library/dis.rst

@@ -217,6 +217,13 @@ object isn't useful:
    Detect all offsets in the code object *code* which are jump targets, and
    return a list of these offsets.
 
+
+.. function:: stack_effect(opcode, [oparg])
+
+   Compute the stack effect of *opcode* with argument *oparg*.
+
+   .. versionadded:: 3.4
+
 .. _bytecodes:
 
 Python Bytecode Instructions

Include/compile.h

@@ -54,6 +54,9 @@ PyAPI_FUNC(PyFutureFeatures *) PyFuture_FromASTObject(
 /* _Py_Mangle is defined in compile.c */
 PyAPI_FUNC(PyObject*) _Py_Mangle(PyObject *p, PyObject *name);
 
+#define PY_INVALID_STACK_EFFECT INT_MAX
+PyAPI_FUNC(int) PyCompile_OpcodeStackEffect(int opcode, int oparg);
+
 #ifdef __cplusplus
 }
 #endif

Lib/opcode.py

@@ -8,6 +8,19 @@ __all__ = ["cmp_op", "hasconst", "hasname", "hasjrel", "hasjabs",
            "haslocal", "hascompare", "hasfree", "opname", "opmap",
            "HAVE_ARGUMENT", "EXTENDED_ARG", "hasnargs"]
 
+# It's a chicken-and-egg I'm afraid:
+# We're imported before _opcode's made.
+# With exception unheeded
+# (stack_effect is not needed)
+# Both our chickens and eggs are allayed.
+#     --Larry Hastings, 2013/11/23
+
+try:
+    from _opcode import stack_effect
+    __all__.append('stack_effect')
+except ImportError:
+    pass
+
 cmp_op = ('<', '<=', '==', '!=', '>', '>=', 'in', 'not in', 'is',
         'is not', 'exception match', 'BAD')
 

Lib/test/test__opcode.py

@@ -0,0 +1,22 @@
+import dis
+import _opcode
+from test.support import run_unittest
+import unittest
+
+class OpcodeTests(unittest.TestCase):
+
+    def test_stack_effect(self):
+        self.assertEqual(_opcode.stack_effect(dis.opmap['POP_TOP']), -1)
+        self.assertEqual(_opcode.stack_effect(dis.opmap['DUP_TOP_TWO']), 2)
+        self.assertEqual(_opcode.stack_effect(dis.opmap['BUILD_SLICE'], 0), -1)
+        self.assertEqual(_opcode.stack_effect(dis.opmap['BUILD_SLICE'], 1), -1)
+        self.assertEqual(_opcode.stack_effect(dis.opmap['BUILD_SLICE'], 3), -2)
+        self.assertRaises(ValueError, _opcode.stack_effect, 30000)
+        self.assertRaises(ValueError, _opcode.stack_effect, dis.opmap['BUILD_SLICE'])
+        self.assertRaises(ValueError, _opcode.stack_effect, dis.opmap['POP_TOP'], 0)
+
+def test_main():
+    run_unittest(OpcodeTests)
+
+if __name__ == "__main__":
+    test_main()

Misc/NEWS

@@ -67,6 +67,8 @@ Core and Builtins
 
 Library
 -------
+- Issue #19722: Added opcode.stack_effect(), which
+  computes the stack effect of bytecode instructions.
 
 - Issue #19735: Implement private function ssl._create_stdlib_context() to
   create SSLContext objects in Python's stdlib module. It provides a single

Modules/_opcode.c

@@ -0,0 +1,116 @@
+#include "Python.h"
+#include "opcode.h"
+
+
+/*[clinic]
+
+module _opcode
+
+_opcode.stack_effect -> int
+
+  opcode: int
+
+  [
+  oparg: int
+  ]
+  /
+
+Compute the stack effect of the opcode.
+[clinic]*/
+
+PyDoc_STRVAR(_opcode_stack_effect__doc__,
+"Compute the stack effect of the opcode.\n"
+"\n"
+"_opcode.stack_effect(opcode, [oparg])");
+
+#define _OPCODE_STACK_EFFECT_METHODDEF    \
+    {"stack_effect", (PyCFunction)_opcode_stack_effect, METH_VARARGS, _opcode_stack_effect__doc__},
+
+static int
+_opcode_stack_effect_impl(PyObject *module, int opcode, int group_right_1, int oparg);
+
+static PyObject *
+_opcode_stack_effect(PyObject *module, PyObject *args)
+{
+    PyObject *return_value = NULL;
+    int opcode;
+    int group_right_1 = 0;
+    int oparg = 0;
+    int _return_value;
+
+    switch (PyTuple_Size(args)) {
+        case 1:
+            if (!PyArg_ParseTuple(args, "i:stack_effect", &opcode))
+                return NULL;
+            break;
+        case 2:
+            if (!PyArg_ParseTuple(args, "ii:stack_effect", &opcode, &oparg))
+                return NULL;
+            group_right_1 = 1;
+            break;
+        default:
+            PyErr_SetString(PyExc_TypeError, "_opcode.stack_effect requires 1 to 2 arguments");
+            return NULL;
+    }
+    _return_value = _opcode_stack_effect_impl(module, opcode, group_right_1, oparg);
+    if ((_return_value == -1) && PyErr_Occurred())
+        goto exit;
+    return_value = PyLong_FromLong((long)_return_value);
+
+exit:
+    return return_value;
+}
+
+static int
+_opcode_stack_effect_impl(PyObject *module, int opcode, int group_right_1, int oparg)
+/*[clinic checksum: 2312ded40abc9bcbce718942de21f53e61a2dfd3]*/
+{
+    int effect;
+    if (HAS_ARG(opcode)) {        
+        if (!group_right_1) {
+            PyErr_SetString(PyExc_ValueError,
+                    "stack_effect: opcode requires oparg but oparg was not specified");
+            return -1;
+        }
+    }
+    else if (group_right_1) {
+        PyErr_SetString(PyExc_ValueError,
+                "stack_effect: opcode does not permit oparg but oparg was specified");
+        return -1;
+    }
+    effect = PyCompile_OpcodeStackEffect(opcode, oparg);
+    if (effect == PY_INVALID_STACK_EFFECT) {
+            PyErr_SetString(PyExc_ValueError,
+                    "invalid opcode or oparg");
+            return -1;
+    }
+    return effect;
+}
+
+
+
+
+static PyMethodDef
+opcode_functions[] =  {
+    _OPCODE_STACK_EFFECT_METHODDEF
+    {NULL, NULL, 0, NULL}
+};
+
+
+static struct PyModuleDef opcodemodule = {
+    PyModuleDef_HEAD_INIT,
+    "_opcode",
+    "Opcode support module.",
+    -1,
+    opcode_functions,
+    NULL,
+    NULL,
+    NULL,
+    NULL
+};
+
+PyMODINIT_FUNC
+PyInit__opcode(void)
+{
+    return PyModule_Create(&opcodemodule);
+}

Python/compile.c

@@ -853,8 +853,8 @@ compiler_set_lineno(struct compiler *c, int off)
     b->b_instr[off].i_lineno = c->u->u_lineno;
 }
 
-static int
+int
-opcode_stack_effect(int opcode, int oparg)
+PyCompile_OpcodeStackEffect(int opcode, int oparg)
 {
     switch (opcode) {
         case POP_TOP:
@@ -1044,11 +1044,9 @@ opcode_stack_effect(int opcode, int oparg)
         case DELETE_DEREF:
             return 0;
         default:
-            fprintf(stderr, "opcode = %d\n", opcode);
+            return PY_INVALID_STACK_EFFECT;
-            Py_FatalError("opcode_stack_effect()");
-
     }
-    return 0; /* not reachable */
+    return PY_INVALID_STACK_EFFECT; /* not reachable */
 }
 
 /* Add an opcode with no argument.
@@ -3829,7 +3827,7 @@ dfs(struct compiler *c, basicblock *b, struct assembler *a)
 static int
 stackdepth_walk(struct compiler *c, basicblock *b, int depth, int maxdepth)
 {
-    int i, target_depth;
+    int i, target_depth, effect;
     struct instr *instr;
     if (b->b_seen || b->b_startdepth >= depth)
         return maxdepth;
@@ -3837,7 +3835,13 @@ stackdepth_walk(struct compiler *c, basicblock *b, int depth, int maxdepth)
     b->b_startdepth = depth;
     for (i = 0; i < b->b_iused; i++) {
         instr = &b->b_instr[i];
-        depth += opcode_stack_effect(instr->i_opcode, instr->i_oparg);
+        effect = PyCompile_OpcodeStackEffect(instr->i_opcode, instr->i_oparg);
+        if (effect == PY_INVALID_STACK_EFFECT) {
+            fprintf(stderr, "opcode = %d\n", instr->i_opcode);
+            Py_FatalError("PyCompile_OpcodeStackEffect()");
+        }
+        depth += effect;
+
         if (depth > maxdepth)
             maxdepth = depth;
         assert(depth >= 0); /* invalid code or bug in stackdepth() */

setup.py

@@ -596,6 +596,8 @@ class PyBuildExt(build_ext):
         exts.append( Extension('_lsprof', ['_lsprof.c', 'rotatingtree.c']) )
         # static Unicode character database
         exts.append( Extension('unicodedata', ['unicodedata.c']) )
+        # _opcode module
+        exts.append( Extension('_opcode', ['_opcode.c']) )
 
         # Modules with some UNIX dependencies -- on by default:
         # (If you have a really backward UNIX, select and socket may not be