bpo-42217: compiler: merge same co_code and co_linetable objects

Lib/test/test_compile.py

@@ -632,6 +632,17 @@ def check_same_constant(const):
         self.check_constant(f1, frozenset({0}))
         self.assertTrue(f1(0))
 
+    # Merging equal co_linetable and co_code is not a strict requirement
+    # for the Python semantics, it's a more an implementation detail.
+    @support.cpython_only
+    def test_merge_code_attrs(self):
+        # See https://bugs.python.org/issue42217
+        f1 = lambda x: x.y.z
+        f2 = lambda a: a.b.c
+
+        self.assertIs(f1.__code__.co_linetable, f2.__code__.co_linetable)
+        self.assertIs(f1.__code__.co_code, f2.__code__.co_code)
+
     # This is a regression test for a CPython specific peephole optimizer
     # implementation bug present in a few releases.  It's assertion verifies
     # that peephole optimization was actually done though that isn't an

Misc/NEWS.d/next/Core and Builtins/2020-10-31-16-54-00.bpo-42217.GdcHe5.rst

@@ -0,0 +1 @@
+Make the compiler merges same co_code and co_linetable objects in a module like already did for co_consts.

Python/compile.c

@@ -5806,14 +5806,12 @@ compute_code_flags(struct compiler *c)
     return flags;
 }
 
-// Merge *tuple* with constant cache.
+// Merge *obj* with constant cache.
 // Unlike merge_consts_recursive(), this function doesn't work recursively.
 static int
-merge_const_tuple(struct compiler *c, PyObject **tuple)
+merge_const_one(struct compiler *c, PyObject **obj)
 {
-    assert(PyTuple_CheckExact(*tuple));
-
-    PyObject *key = _PyCode_ConstantKey(*tuple);
+    PyObject *key = _PyCode_ConstantKey(*obj);
     if (key == NULL) {
         return 0;
     }
@@ -5824,14 +5822,18 @@ merge_const_tuple(struct compiler *c, PyObject **tuple)
     if (t == NULL) {
         return 0;
     }
-    if (t == key) {  // tuple is new constant.
+    if (t == key) {  // obj is new constant.
         return 1;
     }
 
-    PyObject *u = PyTuple_GET_ITEM(t, 1);
-    Py_INCREF(u);
-    Py_DECREF(*tuple);
-    *tuple = u;
+    if (PyTuple_CheckExact(t)) {
+        // t is still borrowed reference
+        t = PyTuple_GET_ITEM(t, 1);
+    }
+
+    Py_INCREF(t);
+    Py_DECREF(*obj);
+    *obj = t;
     return 1;
 }
 
@@ -5861,10 +5863,10 @@ makecode(struct compiler *c, struct assembler *a, PyObject *consts)
     if (!freevars)
         goto error;
 
-    if (!merge_const_tuple(c, &names) ||
-            !merge_const_tuple(c, &varnames) ||
-            !merge_const_tuple(c, &cellvars) ||
-            !merge_const_tuple(c, &freevars))
+    if (!merge_const_one(c, &names) ||
+            !merge_const_one(c, &varnames) ||
+            !merge_const_one(c, &cellvars) ||
+            !merge_const_one(c, &freevars))
     {
         goto error;
     }
@@ -5881,7 +5883,7 @@ makecode(struct compiler *c, struct assembler *a, PyObject *consts)
     if (consts == NULL) {
         goto error;
     }
-    if (!merge_const_tuple(c, &consts)) {
+    if (!merge_const_one(c, &consts)) {
         Py_DECREF(consts);
         goto error;
     }
@@ -6028,10 +6030,18 @@ assemble(struct compiler *c, int addNone)
         goto error;
     }
 
-    if (_PyBytes_Resize(&a.a_lnotab, a.a_lnotab_off) < 0)
+    if (_PyBytes_Resize(&a.a_lnotab, a.a_lnotab_off) < 0) {
         goto error;
-    if (_PyBytes_Resize(&a.a_bytecode, a.a_offset * sizeof(_Py_CODEUNIT)) < 0)
+    }
+    if (!merge_const_one(c, &a.a_lnotab)) {
         goto error;
+    }
+    if (_PyBytes_Resize(&a.a_bytecode, a.a_offset * sizeof(_Py_CODEUNIT)) < 0) {
+        goto error;
+    }
+    if (!merge_const_one(c, &a.a_bytecode)) {
+        goto error;
+    }
 
     co = makecode(c, &a, consts);
  error: