bpo-41323: compiler: Reuse tuple in constant tuple folding (GH-25419)

Misc/NEWS.d/next/Core and Builtins/2021-04-15-10-19-59.bpo-41323.nsvpSg.rst

@@ -0,0 +1,2 @@
+Constant tuple folding in bytecode optimizer now reuses tuple in constant
+table.

Python/compile.c

@@ -6751,7 +6751,7 @@ static int
 normalize_basic_block(basicblock *bb);
 
 static int
-optimize_cfg(struct assembler *a, PyObject *consts);
+optimize_cfg(struct compiler *c, struct assembler *a, PyObject *consts);
 
 static int
 ensure_exits_have_lineno(struct compiler *c);
@@ -6850,7 +6850,7 @@ assemble(struct compiler *c, int addNone)
     if (consts == NULL) {
         goto error;
     }
-    if (optimize_cfg(&a, consts)) {
+    if (optimize_cfg(c, &a, consts)) {
         goto error;
     }
 
@@ -6898,7 +6898,8 @@ assemble(struct compiler *c, int addNone)
    Called with codestr pointing to the first LOAD_CONST.
 */
 static int
-fold_tuple_on_constants(struct instr *inst,
+fold_tuple_on_constants(struct compiler *c,
+                        struct instr *inst,
                         int n, PyObject *consts)
 {
     /* Pre-conditions */
@@ -6923,15 +6924,27 @@ fold_tuple_on_constants(struct instr *inst,
         Py_INCREF(constant);
         PyTuple_SET_ITEM(newconst, i, constant);
     }
-    Py_ssize_t index = PyList_GET_SIZE(consts);
-    if ((size_t)index >= (size_t)INT_MAX - 1) {
+    if (merge_const_one(c, &newconst) == 0) {
         Py_DECREF(newconst);
-        PyErr_SetString(PyExc_OverflowError, "too many constants");
         return -1;
     }
-    if (PyList_Append(consts, newconst)) {
-        Py_DECREF(newconst);
-        return -1;
+
+    Py_ssize_t index;
+    for (index = 0; index < PyList_GET_SIZE(consts); index++) {
+        if (PyList_GET_ITEM(consts, index) == newconst) {
+            break;
+        }
+    }
+    if (index == PyList_GET_SIZE(consts)) {
+        if ((size_t)index >= (size_t)INT_MAX - 1) {
+            Py_DECREF(newconst);
+            PyErr_SetString(PyExc_OverflowError, "too many constants");
+            return -1;
+        }
+        if (PyList_Append(consts, newconst)) {
+            Py_DECREF(newconst);
+            return -1;
+        }
     }
     Py_DECREF(newconst);
     for (int i = 0; i < n; i++) {
@@ -6968,7 +6981,7 @@ eliminate_jump_to_jump(basicblock *bb, int opcode) {
 
 /* Optimization */
 static int
-optimize_basic_block(basicblock *bb, PyObject *consts)
+optimize_basic_block(struct compiler *c, basicblock *bb, PyObject *consts)
 {
     assert(PyList_CheckExact(consts));
     struct instr nop;
@@ -7056,7 +7069,7 @@ optimize_basic_block(basicblock *bb, PyObject *consts)
                     break;
                 }
                 if (i >= oparg) {
-                    if (fold_tuple_on_constants(inst-oparg, oparg, consts)) {
+                    if (fold_tuple_on_constants(c, inst-oparg, oparg, consts)) {
                         goto error;
                     }
                 }
@@ -7390,10 +7403,10 @@ propogate_line_numbers(struct assembler *a) {
 */
 
 static int
-optimize_cfg(struct assembler *a, PyObject *consts)
+optimize_cfg(struct compiler *c, struct assembler *a, PyObject *consts)
 {
     for (basicblock *b = a->a_entry; b != NULL; b = b->b_next) {
-        if (optimize_basic_block(b, consts)) {
+        if (optimize_basic_block(c, b, consts)) {
             return -1;
         }
         clean_basic_block(b, -1);