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flatten.py
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flatten.py
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import ast
from typing import List, Tuple, Union, Set
from .node_visitor import MyNodeVisitor, NodeNotSupportedError
from .util import assign, load, clone_node, parse_ast_expr, parse_ast_stmt
# Typing shorthands.
ActionsT = List[ast.stmt]
VisitExprReturnT = Tuple[ast.expr, ActionsT] # (result expr node, actions)
VisitSliceReturnT = Tuple[ast.slice, ActionsT]
def _not_expr(expr: ast.expr) -> ast.expr:
"""Generates AST node for `not expr`."""
return ast.UnaryOp(op=ast.Not(), operand=expr)
class Flatten(MyNodeVisitor):
"""Produces a flattened-out version of an AST. Also applies desugaring and other transformations.
The resulting AST is essentially in three-address code, except that some literals (e.g., list literals) and
structured control flow (e.g., `if` and `while` blocks) are kept.
See the `flatten_module` function below for usage.
"""
def __init__(self, ignored: Set[ast.AST]) -> None:
self._ignored = ignored
self._next_symbol_id = 0
# Stack containing loop condition actions for each loop we're in, i.e., for a loop `while __x_0:`, the
# statements that compute `__x_0`. For each loop, its condition actions are added before the start of the
# loop, at the end of the loop body, and before each `continue` statement within the loop body. A for-loop
# has no condition actions because the iterator is only created once.
self.loop_cond_actions: List[List[ast.stmt]] = []
super(Flatten, self).__init__()
def next_symbol_id(self) -> str:
"""Returns a `Name` node with the next unused symbol name, to be stored into."""
symbol_id = f"__x_{self._next_symbol_id}"
self._next_symbol_id += 1
return symbol_id
# Expressions
def visit_expr(self, expr: ast.expr) -> VisitExprReturnT:
"""The evaluation of the returned result expression node must have no side effect."""
assert expr not in self._ignored, "Expressions cannot be ignored"
return self.visit(expr)
def visit_Attribute(self, attr: ast.Attribute) -> VisitExprReturnT:
value, value_actions = self.visit_expr(attr.value)
attr_flattened = ast.Attribute(value=value, attr=attr.attr, ctx=attr.ctx)
ctx = attr.ctx
if isinstance(ctx, ast.Load):
# Store the attribute's value into a symbol.
result_id = self.next_symbol_id()
assign_node = assign(result_id, attr_flattened)
return load(result_id), value_actions + [assign_node]
elif isinstance(ctx, (ast.Store, ast.Del)):
# Don't evaluate the attribute.
return attr_flattened, value_actions
else:
raise NodeNotSupportedError(attr, "Attribute context not supported")
def visit_BinOp(self, binop: ast.BinOp) -> VisitExprReturnT:
# `BinOp` doesn't include boolean operators, which have their own AST type `BoolOp`.
left, left_actions = self.visit_expr(binop.left)
right, right_actions = self.visit_expr(binop.right)
binop_flattened = ast.BinOp(left=left, op=binop.op, right=right)
result_id = self.next_symbol_id()
result_node = assign(result_id, binop_flattened)
return load(result_id), left_actions + right_actions + [result_node]
def visit_UnaryOp(self, unaryop: ast.UnaryOp) -> VisitExprReturnT:
operand, operand_actions = self.visit_expr(unaryop.operand)
unaryop_flattened = ast.UnaryOp(op=unaryop.op, operand=operand)
result_id = self.next_symbol_id()
result_node = assign(result_id, unaryop_flattened)
return load(result_id), operand_actions + [result_node]
def visit_BoolOp(self, boolop: ast.BoolOp) -> VisitExprReturnT:
"""
Due to short-circuiting, desugars boolop into nested if-statements before being flattened.
For example, this expression::
x = v1 and v2 and v3
gets desugared into::
x = v1
if x: # if_test (`x` for `and`, `not x` for `or`)
x = v2
if x:
x = v3
This statement block then gets flattened. The strategy is similar for the `or` operation.
"""
result_id = self.next_symbol_id()
result_node = load(result_id)
if_test: ast.expr
if isinstance(boolop.op, ast.And):
if_test = result_node
elif isinstance(boolop.op, ast.Or):
if_test = ast.UnaryOp(op=ast.Not(), operand=result_node)
else:
assert False, f"BoolOp operation not recognized: {boolop}"
body: List[ast.stmt] = [assign(result_id, boolop.values[-1])]
for value in reversed(boolop.values[:-1]): # Iteratively wrap body in if-statements.
body = [ast.If(test=if_test, body=body, orelse=[])]
body.insert(0, assign(result_id, value))
return load(result_id), self.visit_stmt_list(body)
def visit_Bytes(self, b: ast.Bytes) -> VisitExprReturnT:
return b, []
def visit_Compare(self, cmp: ast.Compare) -> VisitExprReturnT:
result_actions = []
left, left_actions = self.visit_expr(cmp.left)
result_actions.extend(left_actions)
comparators = []
for comparator in cmp.comparators:
comparator, comparator_actions = self.visit_expr(comparator)
result_actions.extend(comparator_actions)
comparators.append(comparator)
cmp_flattened = ast.Compare(left=left, ops=cmp.ops, comparators=comparators)
result_id = self.next_symbol_id()
assign_node = assign(result_id, cmp_flattened)
return load(result_id), result_actions + [assign_node]
def visit_Call(self, call: ast.Call) -> VisitExprReturnT:
result_actions = []
func, func_actions = self.visit_expr(call.func)
result_actions.extend(func_actions)
args = []
for arg in call.args:
arg_flattened, arg_actions = self.visit_expr(arg)
result_actions.extend(arg_actions)
args.append(arg_flattened)
keywords = []
for kw in call.keywords:
kw_value_flattened, kw_value_actions = self.visit_expr(kw.value)
result_actions.extend(kw_value_actions)
keywords.append(ast.keyword(arg=kw.arg, value=kw_value_flattened))
result_id = self.next_symbol_id()
result_actions.append(assign(result_id, ast.Call(func, args, keywords)))
return load(result_id), result_actions
def visit_Dict(self, dic: ast.Dict) -> VisitExprReturnT:
actions = []
keys = []
values = []
for key, value in zip(dic.keys, dic.values):
# Evaluation order determined through experimentation: value1, key1, value2, key2, ...
value_flattened, value_actions = self.visit_expr(value)
actions.extend(value_actions)
values.append(value_flattened)
key_flattened = None
if key is not None:
key_flattened, key_actions = self.visit_expr(key)
actions.extend(key_actions)
keys.append(key_flattened)
dic_flattened = ast.Dict(keys=keys, values=values)
return dic_flattened, actions
def visit_ListComp(self, list_comp: ast.ListComp) -> VisitExprReturnT:
"""
Desugars a list comprehension into nested for-loops and if-statements before flattening.
For example, this expression::
l = [y for x in it if cond for y in x]
gets desugared into::
l = []
for x in it:
if cond:
for y in x:
l.append(y)
This statement block then gets flattened.
"""
result_id = self.next_symbol_id()
# First, desugar the comprehension into nested for-loops and if-statements.
# Iteratively wrap `body` in for-loops and if-statements.
body: ast.stmt = ast.Expr(clone_node(
parse_ast_expr(f"{result_id}.append()"),
args=[list_comp.elt]
))
for comp in reversed(list_comp.generators):
if comp.is_async: # type: ignore # Mypy doesn't recognize the `is_async` attribute of `comprehension`.
raise NodeNotSupportedError(comp, "Asynchronous comprehension not supported")
for if_test in reversed(comp.ifs):
body = ast.If(test=if_test, body=[body], orelse=[])
body = ast.For(target=comp.target, iter=comp.iter, body=[body], orelse=[])
# Now that we've gotten rid of the comprehension, flatten the resulting action.
define_list = parse_ast_stmt(f"{result_id} = []")
return load(result_id), [define_list] + self.visit_stmt(body)
def visit_Name(self, name: ast.Name) -> VisitExprReturnT:
return name, []
def visit_Str(self, string: ast.Str) -> VisitExprReturnT:
return string, []
def visit_NameConstant(self, name_const: ast.NameConstant) -> VisitExprReturnT:
return name_const, []
def visit_Num(self, num: ast.Num) -> VisitExprReturnT:
return num, []
def _visit_sequence_literal(self, lit: Union[ast.Tuple, ast.List]) -> VisitExprReturnT:
result_actions = []
elts = []
for elt in lit.elts:
elt_flattened, elt_actions = self.visit_expr(elt)
result_actions.extend(elt_actions)
elts.append(elt_flattened)
flattened = clone_node(lit, elts=elts)
return flattened, result_actions
def visit_Tuple(self, tup: ast.Tuple) -> VisitExprReturnT:
return self._visit_sequence_literal(tup)
def visit_List(self, lst: ast.List) -> VisitExprReturnT:
return self._visit_sequence_literal(lst)
def visit_Starred(self, starred: ast.Starred) -> VisitExprReturnT:
value, value_actions = self.visit_expr(starred.value)
return ast.Starred(value=value, ctx=starred.ctx), value_actions
def visit_Subscript(self, subscript: ast.Subscript) -> VisitExprReturnT:
value, value_actions = self.visit_expr(subscript.value)
sl, slice_actions = self.visit_slice(subscript.slice)
ctx = subscript.ctx
subscript_flattened = ast.Subscript(value=value, slice=sl, ctx=ctx)
actions = value_actions + slice_actions
if isinstance(ctx, ast.Load):
result_id = self.next_symbol_id()
assign_node = assign(result_id, subscript_flattened)
return load(result_id), actions + [assign_node]
elif isinstance(ctx, (ast.Store, ast.Del)):
return subscript_flattened, actions
raise NodeNotSupportedError(subscript, "Subscript context not supported")
def visit_expr_list(self, exprs: List[ast.expr]) -> Tuple[List[ast.expr], ActionsT]:
exprs_flattened = []
actions = []
for expr in exprs:
flattened_expr, expr_actions = self.visit_expr(expr)
exprs_flattened.append(flattened_expr)
actions.extend(expr_actions)
return exprs_flattened, actions
# Statements
def visit_stmt(self, stmt: ast.stmt) -> ActionsT:
if stmt in self._ignored:
return [stmt] # Don't flatten.
return self.visit(stmt)
def visit_Assert(self, asr: ast.Assert) -> ActionsT:
test, test_actions = self.visit_expr(asr.test)
msg_actions: ActionsT
if asr.msg is None:
msg, msg_actions = None, []
else:
msg, msg_actions = self.visit_expr(asr.msg)
result_node = ast.Assert(test=test, msg=msg)
return test_actions + msg_actions + [result_node]
def visit_Assign(self, ass: ast.Assign) -> ActionsT:
value, value_actions = self.visit_expr(ass.value)
targets, targets_actions = self.visit_expr_list(ass.targets)
result_node = ast.Assign(targets=targets, value=value)
return value_actions + targets_actions + [result_node]
def visit_AugAssign(self, aug_assign: ast.AugAssign) -> ActionsT:
value, value_actions = self.visit_expr(aug_assign.value)
target, target_actions = self.visit_expr(aug_assign.target)
result_node = ast.AugAssign(target=target, op=aug_assign.op, value=value)
return value_actions + target_actions + [result_node]
def visit_Break(self, br: ast.Break) -> ActionsT:
return [br]
def visit_ClassDef(self, class_def: ast.ClassDef) -> ActionsT:
bases, bases_actions = self.visit_expr_list(class_def.bases)
keywords = []
keywords_actions = []
for kw in class_def.keywords:
kw_value_flattened, kw_value_actions = self.visit_expr(kw.value)
keywords_actions.extend(kw_value_actions)
keywords.append(ast.keyword(arg=kw.arg, value=kw_value_flattened))
body = self.visit_stmt_list(class_def.body)
if class_def.decorator_list:
raise NodeNotSupportedError(class_def, "ClassDef decorators not supported")
result_node = ast.ClassDef(name=class_def.name, bases=bases, keywords=keywords, body=body,
decorator_list=class_def.decorator_list)
return bases_actions + keywords_actions + [result_node]
def visit_Continue(self, cont_stmt: ast.Continue) -> ActionsT:
# Before going into the next iteration, emit statements to re-evaluate the loop condition.
return self.loop_cond_actions[-1] + [cont_stmt]
def visit_Expr(self, expr: ast.Expr) -> ActionsT:
_, actions = self.visit_expr(expr.value)
return actions
def visit_If(self, if_stmt: ast.If) -> ActionsT:
test, test_actions = self.visit_expr(if_stmt.test)
body = self.visit_stmt_list(if_stmt.body)
orelse = self.visit_stmt_list(if_stmt.orelse)
result_node = ast.If(test=test, body=body, orelse=orelse)
return test_actions + [result_node]
def visit_Import(self, imp: ast.Import) -> ActionsT:
return [imp]
def visit_ImportFrom(self, imp_from: ast.ImportFrom) -> ActionsT:
return [imp_from]
def visit_Return(self, ret: ast.Return) -> ActionsT:
if ret.value is None: # A bare `return`.
return [ret]
value, value_actions = self.visit_expr(ret.value)
result_node = ast.Return(value=value)
return value_actions + [result_node]
def visit_While(self, while_stmt: ast.While) -> ActionsT:
test, test_actions = self.visit_expr(while_stmt.test)
self.loop_cond_actions.append(test_actions)
body = self.visit_stmt_list(while_stmt.body)
self.loop_cond_actions.pop()
body += test_actions # Re-compute loop condition at the end of loop body.
if while_stmt.orelse:
raise NodeNotSupportedError(while_stmt, "While statement orelse not supported.")
result_node = ast.While(test=test, body=body, orelse=[])
return test_actions + [result_node]
def visit_For(self, for_stmt: ast.For) -> ActionsT:
# Create the iterator explicitly.
wrapped_iter = clone_node(parse_ast_expr("iter()"), args=[for_stmt.iter])
for_stmt = clone_node(for_stmt, iter=wrapped_iter)
# Actually flatten the for-loop.
target, target_actions = self.visit_expr(for_stmt.target)
for_iter, iter_actions = self.visit_expr(for_stmt.iter)
self.loop_cond_actions.append([]) # For-loop has no condition actions.
body = self.visit_stmt_list(for_stmt.body)
self.loop_cond_actions.pop()
if for_stmt.orelse:
raise NodeNotSupportedError(for_stmt, "For statement orelse not supported.")
result_node = ast.For(target=target, iter=for_iter, body=body, orelse=[])
return target_actions + iter_actions + [result_node]
def visit_stmt_list(self, stmts: List[ast.stmt]) -> ActionsT:
"""Flattens a block of statements."""
result_actions = []
for stmt in stmts:
curr_actions = self.visit_stmt(stmt)
result_actions.extend(curr_actions)
return result_actions
def visit_FunctionDef(self, func_def: ast.FunctionDef) -> ActionsT:
body = self.visit_stmt_list(func_def.body)
for d in func_def.decorator_list:
# TODO(zhangwen): this is a hack to specifically allow the `on_coordinator` decorator.
if isinstance(d, ast.Name) and d.id == "on_coordinator": # `@on_coordinator`
continue
if isinstance(d, ast.Attribute) and isinstance(d.value, ast.Name) and d.value.id == "rt" \
and d.attr == "on_coordinator": # `@rt.on_coordinator`
continue
raise NodeNotSupportedError(d, "Function decorator not supported")
return [ast.FunctionDef(name=func_def.name, args=func_def.args, body=body,
decorator_list=func_def.decorator_list)]
def visit_Pass(self, pass_stmt: ast.Pass) -> ActionsT:
return [pass_stmt]
# Slices
def visit_slice(self, sl: ast.slice) -> VisitSliceReturnT:
assert sl not in self._ignored, "Slices cannot be ignored"
return self.visit(sl)
def visit_Index(self, index: ast.Index) -> VisitSliceReturnT:
value_flattened, value_actions = self.visit_expr(index.value)
return ast.Index(value=value_flattened), value_actions
def visit_Slice(self, sl: ast.Slice) -> VisitSliceReturnT:
actions = []
if sl.lower is None:
lower_flattened = None
else:
lower_flattened, lower_actions = self.visit_expr(sl.lower)
actions.extend(lower_actions)
if sl.upper is None:
upper_flattened = None
else:
upper_flattened, upper_actions = self.visit_expr(sl.upper)
actions.extend(upper_actions)
if sl.step is None:
step_flattened = None
else:
step_flattened, step_actions = self.visit_expr(sl.step)
actions.extend(step_actions)
return ast.Slice(lower=lower_flattened, upper=upper_flattened, step=step_flattened), actions
def visit_ExtSlice(self, ext_sl: ast.ExtSlice) -> VisitSliceReturnT:
dims = []
actions = []
for dim in ext_sl.dims:
dim_flattened, dim_actions = self.visit_slice(dim)
dims.append(dim_flattened)
actions.extend(dim_actions)
return ast.ExtSlice(dims=dims), actions
# Module
def visit_Module(self, mod: ast.Module) -> ast.Module:
body = self.visit_stmt_list(mod.body)
return ast.Module(body=body)
def flatten_module(mod: ast.Module, ignored: Set[ast.AST]) -> ast.Module:
return Flatten(ignored=ignored).visit_Module(mod)