gh-99254: remove all unused consts from code objects

Lib/importlib/_bootstrap_external.py

@@ -425,6 +425,7 @@ def _write_atomic(path, data, mode=0o666):
 #     Python 3.12a1 3509 (Conditional jumps only jump forward)
 #     Python 3.12a1 3510 (FOR_ITER leaves iterator on the stack)
 #     Python 3.12a1 3511 (Add STOPITERATION_ERROR instruction)
+#     Python 3.12a1 3512 (Remove all unused consts from code objects)
 
 #     Python 3.13 will start with 3550
 
@@ -437,7 +438,7 @@ def _write_atomic(path, data, mode=0o666):
 # Whenever MAGIC_NUMBER is changed, the ranges in the magic_values array
 # in PC/launcher.c must also be updated.
 
-MAGIC_NUMBER = (3511).to_bytes(2, 'little') + b'\r\n'
+MAGIC_NUMBER = (3512).to_bytes(2, 'little') + b'\r\n'
 
 _RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little')  # For import.c
 

Lib/test/test_compile.py

@@ -670,7 +670,7 @@ def test_merge_code_attrs(self):
         self.assertIs(f1.__code__.co_linetable, f2.__code__.co_linetable)
 
     @support.cpython_only
-    def test_strip_unused_consts(self):
+    def test_remove_unused_consts(self):
         def f():
             "docstring"
             if True:
@@ -679,7 +679,41 @@ def f():
                 return "unused"
 
         self.assertEqual(f.__code__.co_consts,
-                         ("docstring", True, "used"))
+                         ("docstring", "used"))
+
+    @support.cpython_only
+    def test_remove_unused_consts_no_docstring(self):
+        # the first item (None for no docstring in this case) is
+        # always retained.
+        def f():
+            if True:
+                return "used"
+            else:
+                return "unused"
+
+        self.assertEqual(f.__code__.co_consts,
+                         (None, "used"))
+
+    @support.cpython_only
+    def test_remove_unused_consts_extended_args(self):
+        N = 1000
+        code = ["def f():\n"]
+        code.append("\ts = ''\n")
+        code.append("\tfor i in range(1):\n")
+        for i in range(N):
+            code.append(f"\t\tif True: s += 't{i}'\n")
+            code.append(f"\t\tif False: s += 'f{i}'\n")
+        code.append("\treturn s\n")
+
+        code = "".join(code)
+        g = {}
+        eval(compile(code, "file.py", "exec"), g)
+        exec(code, g)
+        f = g['f']
+        expected = tuple([None, '', 1] + [f't{i}' for i in range(N)])
+        self.assertEqual(f.__code__.co_consts, expected)
+        expected = "".join(expected[3:])
+        self.assertEqual(expected, f())
 
     # Stripping unused constants is not a strict requirement for the
     # Python semantics, it's a more an implementation detail.

Lib/test/test_dis.py

@@ -168,13 +168,13 @@ def bug1333982(x=[]):
 %3d        RESUME                   0
 
 %3d        LOAD_ASSERTION_ERROR
-           LOAD_CONST               2 (<code object <listcomp> at 0x..., file "%s", line %d>)
+           LOAD_CONST               1 (<code object <listcomp> at 0x..., file "%s", line %d>)
            MAKE_FUNCTION            0
            LOAD_FAST                0 (x)
            GET_ITER
            CALL                     0
 
-%3d        LOAD_CONST               3 (1)
+%3d        LOAD_CONST               2 (1)
 
 %3d        BINARY_OP                0 (+)
            CALL                     0
@@ -1446,9 +1446,9 @@ def jumpy():
 # End fodder for opinfo generation tests
 expected_outer_line = 1
 _line_offset = outer.__code__.co_firstlineno - 1
-code_object_f = outer.__code__.co_consts[3]
+code_object_f = outer.__code__.co_consts[1]
 expected_f_line = code_object_f.co_firstlineno - _line_offset
-code_object_inner = code_object_f.co_consts[3]
+code_object_inner = code_object_f.co_consts[1]
 expected_inner_line = code_object_inner.co_firstlineno - _line_offset
 expected_jumpy_line = 1
 
@@ -1485,21 +1485,21 @@ def _prepare_test_cases():
   Instruction(opname='MAKE_CELL', opcode=135, arg=0, argval='a', argrepr='a', offset=0, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='MAKE_CELL', opcode=135, arg=1, argval='b', argrepr='b', offset=2, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='RESUME', opcode=151, arg=0, argval=0, argrepr='', offset=4, starts_line=1, is_jump_target=False, positions=None),
-  Instruction(opname='LOAD_CONST', opcode=100, arg=7, argval=(3, 4), argrepr='(3, 4)', offset=6, starts_line=2, is_jump_target=False, positions=None),
+  Instruction(opname='LOAD_CONST', opcode=100, arg=5, argval=(3, 4), argrepr='(3, 4)', offset=6, starts_line=2, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_CLOSURE', opcode=136, arg=0, argval='a', argrepr='a', offset=8, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_CLOSURE', opcode=136, arg=1, argval='b', argrepr='b', offset=10, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='BUILD_TUPLE', opcode=102, arg=2, argval=2, argrepr='', offset=12, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='LOAD_CONST', opcode=100, arg=3, argval=code_object_f, argrepr=repr(code_object_f), offset=14, starts_line=None, is_jump_target=False, positions=None),
+  Instruction(opname='LOAD_CONST', opcode=100, arg=1, argval=code_object_f, argrepr=repr(code_object_f), offset=14, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='MAKE_FUNCTION', opcode=132, arg=9, argval=9, argrepr='defaults, closure', offset=16, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='STORE_FAST', opcode=125, arg=2, argval='f', argrepr='f', offset=18, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_GLOBAL', opcode=116, arg=1, argval='print', argrepr='NULL + print', offset=20, starts_line=7, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_DEREF', opcode=137, arg=0, argval='a', argrepr='a', offset=32, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_DEREF', opcode=137, arg=1, argval='b', argrepr='b', offset=34, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='LOAD_CONST', opcode=100, arg=4, argval='', argrepr="''", offset=36, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='LOAD_CONST', opcode=100, arg=5, argval=1, argrepr='1', offset=38, starts_line=None, is_jump_target=False, positions=None),
+  Instruction(opname='LOAD_CONST', opcode=100, arg=2, argval='', argrepr="''", offset=36, starts_line=None, is_jump_target=False, positions=None),
+  Instruction(opname='LOAD_CONST', opcode=100, arg=3, argval=1, argrepr='1', offset=38, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='BUILD_LIST', opcode=103, arg=0, argval=0, argrepr='', offset=40, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='BUILD_MAP', opcode=105, arg=0, argval=0, argrepr='', offset=42, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='LOAD_CONST', opcode=100, arg=6, argval='Hello world!', argrepr="'Hello world!'", offset=44, starts_line=None, is_jump_target=False, positions=None),
+  Instruction(opname='LOAD_CONST', opcode=100, arg=4, argval='Hello world!', argrepr="'Hello world!'", offset=44, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='CALL', opcode=171, arg=7, argval=7, argrepr='', offset=46, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='POP_TOP', opcode=1, arg=None, argval=None, argrepr='', offset=56, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_FAST', opcode=124, arg=2, argval='f', argrepr='f', offset=58, starts_line=8, is_jump_target=False, positions=None),
@@ -1511,13 +1511,13 @@ def _prepare_test_cases():
   Instruction(opname='MAKE_CELL', opcode=135, arg=0, argval='c', argrepr='c', offset=2, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='MAKE_CELL', opcode=135, arg=1, argval='d', argrepr='d', offset=4, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='RESUME', opcode=151, arg=0, argval=0, argrepr='', offset=6, starts_line=2, is_jump_target=False, positions=None),
-  Instruction(opname='LOAD_CONST', opcode=100, arg=4, argval=(5, 6), argrepr='(5, 6)', offset=8, starts_line=3, is_jump_target=False, positions=None),
+  Instruction(opname='LOAD_CONST', opcode=100, arg=2, argval=(5, 6), argrepr='(5, 6)', offset=8, starts_line=3, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_CLOSURE', opcode=136, arg=3, argval='a', argrepr='a', offset=10, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_CLOSURE', opcode=136, arg=4, argval='b', argrepr='b', offset=12, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_CLOSURE', opcode=136, arg=0, argval='c', argrepr='c', offset=14, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_CLOSURE', opcode=136, arg=1, argval='d', argrepr='d', offset=16, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='BUILD_TUPLE', opcode=102, arg=4, argval=4, argrepr='', offset=18, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='LOAD_CONST', opcode=100, arg=3, argval=code_object_inner, argrepr=repr(code_object_inner), offset=20, starts_line=None, is_jump_target=False, positions=None),
+  Instruction(opname='LOAD_CONST', opcode=100, arg=1, argval=code_object_inner, argrepr=repr(code_object_inner), offset=20, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='MAKE_FUNCTION', opcode=132, arg=9, argval=9, argrepr='defaults, closure', offset=22, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='STORE_FAST', opcode=125, arg=2, argval='inner', argrepr='inner', offset=24, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_GLOBAL', opcode=116, arg=1, argval='print', argrepr='NULL + print', offset=26, starts_line=5, is_jump_target=False, positions=None),

Misc/NEWS.d/next/Core and Builtins/2022-11-08-17-47-10.gh-issue-99254.RSvyFt.rst

@@ -0,0 +1 @@
+The compiler now removes all unused constants from code objects (except the first one, which may be a docstring).

Python/compile.c

@@ -8472,7 +8472,7 @@ static int
 optimize_cfg(cfg_builder *g, PyObject *consts, PyObject *const_cache);
 
 static int
-trim_unused_consts(basicblock *entryblock, PyObject *consts);
+remove_unused_consts(basicblock *entryblock, PyObject *consts);
 
 /* Duplicates exit BBs, so that line numbers can be propagated to them */
 static int
@@ -8813,6 +8813,9 @@ assemble(struct compiler *c, int addNone)
     if (add_checks_for_loads_of_uninitialized_variables(g->g_entryblock, c) < 0) {
         goto error;
     }
+    if (remove_unused_consts(g->g_entryblock, consts)) {
+        goto error;
+    }
 
     /** line numbers (TODO: move this before optimization stage) */
     if (duplicate_exits_without_lineno(g) < 0) {
@@ -8844,10 +8847,6 @@ assemble(struct compiler *c, int addNone)
     /* Can't modify the bytecode after computing jump offsets. */
     assemble_jump_offsets(g->g_entryblock);
 
-    if (trim_unused_consts(g->g_entryblock, consts)) {
-        goto error;
-    }
-
     /* Create assembler */
     if (!assemble_init(&a, c->u->u_firstlineno))
         goto error;
@@ -9706,32 +9705,106 @@ optimize_cfg(cfg_builder *g, PyObject *consts, PyObject *const_cache)
     return 0;
 }
 
-// Remove trailing unused constants.
+
 static int
-trim_unused_consts(basicblock *entryblock, PyObject *consts)
+remove_unused_consts(basicblock *entryblock, PyObject *consts)
 {
     assert(PyList_CheckExact(consts));
+    Py_ssize_t nconsts = PyList_GET_SIZE(consts);
+    if (nconsts == 0) {
+        return 0;  /* nothing to do */
+    }
 
+    Py_ssize_t *index_map = NULL;
+    Py_ssize_t *reverse_index_map = NULL;
+    int err = 1;
+
+    index_map = PyMem_Malloc(nconsts * sizeof(Py_ssize_t));
+    if (index_map == NULL) {
+        goto end;
+    }
+    for (Py_ssize_t i = 1; i < nconsts; i++) {
+        index_map[i] = -1;
+    }
     // The first constant may be docstring; keep it always.
-    int max_const_index = 0;
+    index_map[0] = 0;
+
+    /* mark used consts */
     for (basicblock *b = entryblock; b != NULL; b = b->b_next) {
         for (int i = 0; i < b->b_iused; i++) {
-            if ((b->b_instr[i].i_opcode == LOAD_CONST ||
-                b->b_instr[i].i_opcode == KW_NAMES) &&
-                    b->b_instr[i].i_oparg > max_const_index) {
-                max_const_index = b->b_instr[i].i_oparg;
+            if (b->b_instr[i].i_opcode == LOAD_CONST ||
+                b->b_instr[i].i_opcode == KW_NAMES) {
+
+                int index = b->b_instr[i].i_oparg;
+                index_map[index] = index;
             }
         }
     }
-    if (max_const_index+1 < PyList_GET_SIZE(consts)) {
-        //fprintf(stderr, "removing trailing consts: max=%d, size=%d\n",
-        //        max_const_index, (int)PyList_GET_SIZE(consts));
-        if (PyList_SetSlice(consts, max_const_index+1,
-                            PyList_GET_SIZE(consts), NULL) < 0) {
-            return 1;
+    /* now index_map[i] == i if consts[i] is used, -1 otherwise */
+
+    /* condense consts */
+    Py_ssize_t n_used_consts = 0;
+    for (int i = 0; i < nconsts; i++) {
+        if (index_map[i] != -1) {
+            assert(index_map[i] == i);
+            index_map[n_used_consts++] = index_map[i];
         }
     }
-    return 0;
+    if (n_used_consts == nconsts) {
+        /* nothing to do */
+        err = 0;
+        goto end;
+    }
+
+    /* move all used consts to the beginning of the consts list */
+    assert(n_used_consts < nconsts);
+    for (Py_ssize_t i = 0; i < n_used_consts; i++) {
+        Py_ssize_t old_index = index_map[i];
+        assert(i <= old_index && old_index < nconsts);
+        if (i != old_index) {
+            PyObject *value = PyList_GET_ITEM(consts, index_map[i]);
+            assert(value != NULL);
+            PyList_SetItem(consts, i, Py_NewRef(value));
+        }
+    }
+
+    /* truncate the consts list at its new size */
+    if (PyList_SetSlice(consts, n_used_consts, nconsts, NULL) < 0) {
+        goto end;
+    }
+
+    /* adjust const indices in the bytecode */
+    reverse_index_map = PyMem_Malloc(nconsts * sizeof(Py_ssize_t));
+    if (reverse_index_map == NULL) {
+        goto end;
+    }
+    for (Py_ssize_t i = 0; i < nconsts; i++) {
+        reverse_index_map[i] = -1;
+    }
+    for (Py_ssize_t i = 0; i < n_used_consts; i++) {
+        assert(index_map[i] != -1);
+        assert(reverse_index_map[index_map[i]] == -1);
+        reverse_index_map[index_map[i]] = i;
+    }
+
+    for (basicblock *b = entryblock; b != NULL; b = b->b_next) {
+        for (int i = 0; i < b->b_iused; i++) {
+            if (b->b_instr[i].i_opcode == LOAD_CONST ||
+                b->b_instr[i].i_opcode == KW_NAMES) {
+
+                int index = b->b_instr[i].i_oparg;
+                assert(reverse_index_map[index] >= 0);
+                assert(reverse_index_map[index] < n_used_consts);
+                b->b_instr[i].i_oparg = (int)reverse_index_map[index];
+            }
+        }
+    }
+
+    err = 0;
+end:
+    PyMem_Free(index_map);
+    PyMem_Free(reverse_index_map);
+    return err;
 }
 
 static inline int