gh-125063: Emit slices as constants in the bytecode compiler (#125064)

* Make slices marshallable

* Emit slices as constants

* Update Python/marshal.c

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>

* Refactor codegen_slice into two functions so it
always has the same net effect

* Fix for free-threaded builds

* Simplify marshal loading of slices

* Only return SUCCESS/ERROR from codegen_slice

---------

Co-authored-by: Mark Shannon <mark@hotpy.org>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>

Include/internal/pycore_magic_number.h

@@ -259,6 +259,7 @@ Known values:
     Python 3.14a1 3605 (Move ENTER_EXECUTOR to opcode 255)
     Python 3.14a1 3606 (Specialize CALL_KW)
     Python 3.14a1 3607 (Add pseudo instructions JUMP_IF_TRUE/FALSE)
+    Python 3.14a1 3608 (Add support for slices)
 
     Python 3.15 will start with 3650
 
@@ -271,7 +272,7 @@ PC/launcher.c must also be updated.
 
 */
 
-#define PYC_MAGIC_NUMBER 3607
+#define PYC_MAGIC_NUMBER 3608
 /* This is equivalent to converting PYC_MAGIC_NUMBER to 2 bytes
    (little-endian) and then appending b'\r\n'. */
 #define PYC_MAGIC_NUMBER_TOKEN \

Lib/test/test_compile.py

@@ -1386,6 +1386,14 @@ class TestSpecifics(unittest.TestCase):
                     actual += 1
             self.assertEqual(actual, expected)
 
+        def check_consts(func, typ, expected):
+            slice_consts = 0
+            consts = func.__code__.co_consts
+            for instr in dis.Bytecode(func):
+                if instr.opname == "LOAD_CONST" and isinstance(consts[instr.oparg], typ):
+                    slice_consts += 1
+            self.assertEqual(slice_consts, expected)
+
         def load():
             return x[a:b] + x [a:] + x[:b] + x[:]
 
@@ -1401,15 +1409,30 @@ class TestSpecifics(unittest.TestCase):
         def aug():
             x[a:b] += y
 
-        check_op_count(load, "BINARY_SLICE", 4)
+        def aug_const():
+            x[1:2] += y
+
+        def compound_const_slice():
+            x[1:2:3, 4:5:6] = y
+
+        check_op_count(load, "BINARY_SLICE", 3)
         check_op_count(load, "BUILD_SLICE", 0)
-        check_op_count(store, "STORE_SLICE", 4)
+        check_consts(load, slice, 1)
+        check_op_count(store, "STORE_SLICE", 3)
         check_op_count(store, "BUILD_SLICE", 0)
+        check_consts(store, slice, 1)
         check_op_count(long_slice, "BUILD_SLICE", 1)
         check_op_count(long_slice, "BINARY_SLICE", 0)
         check_op_count(aug, "BINARY_SLICE", 1)
         check_op_count(aug, "STORE_SLICE", 1)
         check_op_count(aug, "BUILD_SLICE", 0)
+        check_op_count(aug_const, "BINARY_SLICE", 0)
+        check_op_count(aug_const, "STORE_SLICE", 0)
+        check_consts(aug_const, slice, 1)
+        check_op_count(compound_const_slice, "BINARY_SLICE", 0)
+        check_op_count(compound_const_slice, "BUILD_SLICE", 0)
+        check_consts(compound_const_slice, slice, 0)
+        check_consts(compound_const_slice, tuple, 1)
 
     def test_compare_positions(self):
         for opname_prefix, op in [

Objects/codeobject.c

@@ -2336,6 +2336,7 @@ _PyCode_ConstantKey(PyObject *op)
     if (op == Py_None || op == Py_Ellipsis
        || PyLong_CheckExact(op)
        || PyUnicode_CheckExact(op)
+       || PySlice_Check(op)
           /* code_richcompare() uses _PyCode_ConstantKey() internally */
        || PyCode_Check(op))
     {

Objects/sliceobject.c

@@ -343,7 +343,7 @@ Create a slice object.  This is used for extended slicing (e.g. a[0:10:2]).");
 static void
 slice_dealloc(PySliceObject *r)
 {
-    _PyObject_GC_UNTRACK(r);
+    PyObject_GC_UnTrack(r);
     Py_DECREF(r->step);
     Py_DECREF(r->start);
     Py_DECREF(r->stop);

Python/codegen.c

@@ -194,6 +194,7 @@ static int codegen_visit_expr(compiler *, expr_ty);
 static int codegen_augassign(compiler *, stmt_ty);
 static int codegen_annassign(compiler *, stmt_ty);
 static int codegen_subscript(compiler *, expr_ty);
+static int codegen_slice_two_parts(compiler *, expr_ty);
 static int codegen_slice(compiler *, expr_ty);
 
 static bool are_all_items_const(asdl_expr_seq *, Py_ssize_t, Py_ssize_t);
@@ -5005,12 +5006,8 @@ codegen_visit_expr(compiler *c, expr_ty e)
         }
         break;
     case Slice_kind:
-    {
+        RETURN_IF_ERROR(codegen_slice(c, e));
-        int n = codegen_slice(c, e);
-        RETURN_IF_ERROR(n);
-        ADDOP_I(c, loc, BUILD_SLICE, n);
         break;
-    }
     case Name_kind:
         return codegen_nameop(c, loc, e->v.Name.id, e->v.Name.ctx);
     /* child nodes of List and Tuple will have expr_context set */
@@ -5023,9 +5020,22 @@ codegen_visit_expr(compiler *c, expr_ty e)
 }
 
 static bool
-is_two_element_slice(expr_ty s)
+is_constant_slice(expr_ty s)
 {
     return s->kind == Slice_kind &&
+        (s->v.Slice.lower == NULL ||
+         s->v.Slice.lower->kind == Constant_kind) &&
+        (s->v.Slice.upper == NULL ||
+         s->v.Slice.upper->kind == Constant_kind) &&
+        (s->v.Slice.step == NULL ||
+         s->v.Slice.step->kind == Constant_kind);
+}
+
+static bool
+should_apply_two_element_slice_optimization(expr_ty s)
+{
+    return !is_constant_slice(s) &&
+           s->kind == Slice_kind &&
            s->v.Slice.step == NULL;
 }
 
@@ -5046,8 +5056,8 @@ codegen_augassign(compiler *c, stmt_ty s)
         break;
     case Subscript_kind:
         VISIT(c, expr, e->v.Subscript.value);
-        if (is_two_element_slice(e->v.Subscript.slice)) {
+        if (should_apply_two_element_slice_optimization(e->v.Subscript.slice)) {
-            RETURN_IF_ERROR(codegen_slice(c, e->v.Subscript.slice));
+            RETURN_IF_ERROR(codegen_slice_two_parts(c, e->v.Subscript.slice));
             ADDOP_I(c, loc, COPY, 3);
             ADDOP_I(c, loc, COPY, 3);
             ADDOP_I(c, loc, COPY, 3);
@@ -5084,7 +5094,7 @@ codegen_augassign(compiler *c, stmt_ty s)
         ADDOP_NAME(c, loc, STORE_ATTR, e->v.Attribute.attr, names);
         break;
     case Subscript_kind:
-        if (is_two_element_slice(e->v.Subscript.slice)) {
+        if (should_apply_two_element_slice_optimization(e->v.Subscript.slice)) {
             ADDOP_I(c, loc, SWAP, 4);
             ADDOP_I(c, loc, SWAP, 3);
             ADDOP_I(c, loc, SWAP, 2);
@@ -5231,8 +5241,10 @@ codegen_subscript(compiler *c, expr_ty e)
     }
 
     VISIT(c, expr, e->v.Subscript.value);
-    if (is_two_element_slice(e->v.Subscript.slice) && ctx != Del) {
+    if (should_apply_two_element_slice_optimization(e->v.Subscript.slice) &&
-        RETURN_IF_ERROR(codegen_slice(c, e->v.Subscript.slice));
+        ctx != Del
+    ) {
+        RETURN_IF_ERROR(codegen_slice_two_parts(c, e->v.Subscript.slice));
         if (ctx == Load) {
             ADDOP(c, loc, BINARY_SLICE);
         }
@@ -5254,15 +5266,9 @@ codegen_subscript(compiler *c, expr_ty e)
     return SUCCESS;
 }
 
-/* Returns the number of the values emitted,
- * thus are needed to build the slice, or -1 if there is an error. */
 static int
-codegen_slice(compiler *c, expr_ty s)
+codegen_slice_two_parts(compiler *c, expr_ty s)
 {
-    int n = 2;
-    assert(s->kind == Slice_kind);
-
-    /* only handles the cases where BUILD_SLICE is emitted */
     if (s->v.Slice.lower) {
         VISIT(c, expr, s->v.Slice.lower);
     }
@@ -5277,11 +5283,45 @@ codegen_slice(compiler *c, expr_ty s)
         ADDOP_LOAD_CONST(c, LOC(s), Py_None);
     }
 
+    return 0;
+}
+
+static int
+codegen_slice(compiler *c, expr_ty s)
+{
+    int n = 2;
+    assert(s->kind == Slice_kind);
+
+    if (is_constant_slice(s)) {
+        PyObject *start = NULL;
+        if (s->v.Slice.lower) {
+            start = s->v.Slice.lower->v.Constant.value;
+        }
+        PyObject *stop = NULL;
+        if (s->v.Slice.upper) {
+            stop = s->v.Slice.upper->v.Constant.value;
+        }
+        PyObject *step = NULL;
+        if (s->v.Slice.step) {
+            step = s->v.Slice.step->v.Constant.value;
+        }
+        PyObject *slice = PySlice_New(start, stop, step);
+        if (slice == NULL) {
+            return ERROR;
+        }
+        ADDOP_LOAD_CONST_NEW(c, LOC(s), slice);
+        return SUCCESS;
+    }
+
+    RETURN_IF_ERROR(codegen_slice_two_parts(c, s));
+
     if (s->v.Slice.step) {
         n++;
         VISIT(c, expr, s->v.Slice.step);
     }
-    return n;
+
+    ADDOP_I(c, LOC(s), BUILD_SLICE, n);
+    return SUCCESS;
 }
 
 

Python/marshal.c

@@ -76,6 +76,7 @@ module marshal
 #define TYPE_UNKNOWN            '?'
 #define TYPE_SET                '<'
 #define TYPE_FROZENSET          '>'
+#define TYPE_SLICE              ':'
 #define FLAG_REF                '\x80' /* with a type, add obj to index */
 
 #define TYPE_ASCII              'a'
@@ -613,6 +614,13 @@ w_complex_object(PyObject *v, char flag, WFILE *p)
         w_pstring(view.buf, view.len, p);
         PyBuffer_Release(&view);
     }
+    else if (PySlice_Check(v)) {
+        PySliceObject *slice = (PySliceObject *)v;
+        W_TYPE(TYPE_SLICE, p);
+        w_object(slice->start, p);
+        w_object(slice->stop, p);
+        w_object(slice->step, p);
+    }
     else {
         W_TYPE(TYPE_UNKNOWN, p);
         p->error = WFERR_UNMARSHALLABLE;
@@ -1534,6 +1542,32 @@ r_object(RFILE *p)
         retval = Py_NewRef(v);
         break;
 
+    case TYPE_SLICE:
+    {
+        Py_ssize_t idx = r_ref_reserve(flag, p);
+        PyObject *stop = NULL;
+        PyObject *step = NULL;
+        PyObject *start = r_object(p);
+        if (start == NULL) {
+            goto cleanup;
+        }
+        stop = r_object(p);
+        if (stop == NULL) {
+            goto cleanup;
+        }
+        step = r_object(p);
+        if (step == NULL) {
+            goto cleanup;
+        }
+        retval = PySlice_New(start, stop, step);
+        r_ref_insert(retval, idx, flag, p);
+    cleanup:
+        Py_XDECREF(start);
+        Py_XDECREF(stop);
+        Py_XDECREF(step);
+        break;
+    }
+
     default:
         /* Bogus data got written, which isn't ideal.
            This will let you keep working and recover. */