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gh-125063: Move slice constant-folding to AST #126830

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gh-125063: Move slice constant-folding to AST #126830

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3313406
Fold slices of constants into slice constants
encukou Nov 13, 2024
d4cb362
Remove the codegen optimization
encukou Nov 13, 2024
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7 changes: 7 additions & 0 deletions Lib/test/test_peepholer.py
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Original file line number Diff line number Diff line change
Expand Up @@ -240,6 +240,7 @@ def test_folding_of_binops_on_constants(self):
('a = 2+3', 5), # binary add
('a = 13-4', 9), # binary subtract
('a = (12,13)[1]', 13), # binary subscr
('a = (12,13)[:1]', (12,)), # binary subscr (slice)
('a = 13 << 2', 52), # binary lshift
('a = 13 >> 2', 3), # binary rshift
('a = 13 & 7', 5), # binary and
Expand Down Expand Up @@ -282,11 +283,17 @@ def test_binary_subscr_on_unicode(self):
self.assertInBytecode(code, 'LOAD_CONST', 'f')
self.assertNotInBytecode(code, 'BINARY_SUBSCR')
self.check_lnotab(code)

code = compile('"\u0061\uffff"[1]', '', 'single')
self.assertInBytecode(code, 'LOAD_CONST', '\uffff')
self.assertNotInBytecode(code,'BINARY_SUBSCR')
self.check_lnotab(code)

code = compile('"\u0061\uffffxx"[1::2]', '', 'single')
self.assertInBytecode(code, 'LOAD_CONST', '\uffffx')
self.assertNotInBytecode(code,'BINARY_SUBSCR')
self.check_lnotab(code)

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Strictly speaking this test doesn't belong in test_peepholer now that the optimization is not part of the peephole optimiser.

Where are the other const folding tests?

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You're right, Irit.

This test should be re-written and placed in the test_ast/test_ast.py::ASTOptimiziationTests

# With PEP 393, non-BMP char get optimized
code = compile('"\U00012345"[0]', '', 'single')
self.assertInBytecode(code, 'LOAD_CONST', '\U00012345')
Expand Down
21 changes: 21 additions & 0 deletions Python/ast_opt.c
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Expand Up @@ -586,6 +586,26 @@ fold_subscr(expr_ty node, PyArena *arena, _PyASTOptimizeState *state)
return make_const(node, newval, arena);
}

static int
fold_slice(expr_ty node, PyArena *arena, _PyASTOptimizeState *state)
{
expr_ty lower = node->v.Slice.lower;
expr_ty upper = node->v.Slice.upper;
expr_ty step = node->v.Slice.step;
if ((lower && lower->kind != Constant_kind) ||
(upper && upper->kind != Constant_kind) ||
(step && step->kind != Constant_kind))
{
return 1;
}

PyObject *newval = PySlice_New(
lower ? lower->v.Constant.value : Py_None,
upper ? upper->v.Constant.value : Py_None,
step ? step->v.Constant.value : Py_None);
return make_const(node, newval, arena);
}

/* Change literal list or set of constants into constant
tuple or frozenset respectively. Change literal list of
non-constants into tuple.
Expand Down Expand Up @@ -831,6 +851,7 @@ astfold_expr(expr_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
CALL_OPT(astfold_expr, expr_ty, node_->v.Slice.lower);
CALL_OPT(astfold_expr, expr_ty, node_->v.Slice.upper);
CALL_OPT(astfold_expr, expr_ty, node_->v.Slice.step);
CALL(fold_slice, expr_ty, node_);
break;
case List_kind:
CALL_SEQ(astfold_expr, expr, node_->v.List.elts);
Expand Down
36 changes: 1 addition & 35 deletions Python/codegen.c
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Expand Up @@ -5039,23 +5039,10 @@ codegen_visit_expr(compiler *c, expr_ty e)
return SUCCESS;
}

static bool
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 &&
return s->kind == Slice_kind &&
s->v.Slice.step == NULL;
}

Expand Down Expand Up @@ -5312,27 +5299,6 @@ 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) {
Expand Down
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