# mypy: allow-untyped-decorators # mypy: allow-untyped-defs import contextlib import logging import warnings from typing import Any, Callable, Optional, Union import torch import torch._subclasses.functional_tensor import torch.utils._pytree as pytree from torch._C import DispatchKey from torch._C._functorch import ( _add_batch_dim, get_unwrapped, is_batchedtensor, maybe_get_bdim, ) from torch._dispatch.python import suspend_functionalization from torch._functorch.utils import exposed_in from torch._higher_order_ops.utils import ( _has_potential_branch_input_alias, _has_potential_branch_input_mutation, _maybe_run_with_interpreter, _set_compilation_env, reenter_make_fx, save_tensors_and_symints_for_backward, saved_tensors_and_symints, unique_graph_id, UnsupportedAliasMutationException, validate_subgraph_args_types, ) from torch._ops import HigherOrderOperator from torch._subclasses.fake_tensor import FakeTensor, FakeTensorMode from torch._subclasses.functional_tensor import disable_functional_mode from torch.fx.experimental.proxy_tensor import ( _temp_remove_metadata_torch_function_mode, _temp_remove_pre_dispatch_torch_function_mode, disable_proxy_modes_tracing, ProxyTorchDispatchMode, track_tensor_tree, ) from torch.utils._python_dispatch import _get_current_dispatch_mode from .utils import _from_fun, _maybe_fake_prop_ignore_unbacked, create_fw_bw_graph log = logging.getLogger(__name__) """ We're going to define a `cond_op` operation. In order to do this, we need implementations for each of the dispatch keys. """ class CondOp(HigherOrderOperator): def __init__(self): super().__init__("cond") def __call__(self, pred, true_fn, false_fn, operands): validate_subgraph_args_types(operands) return super().__call__(pred, true_fn, false_fn, operands) cond_op = CondOp() @exposed_in("torch") def cond( pred: Union[bool, int, float, torch.Tensor], true_fn: Callable, false_fn: Callable, operands: Union[tuple, list] = (), ) -> Any: r""" Conditionally applies `true_fn` or `false_fn`. .. warning:: `torch.cond` is a prototype feature in PyTorch. It has limited support for input and output types and doesn't support training currently. Please look forward to a more stable implementation in a future version of PyTorch. Read more about feature classification at: https://pytorch.org/blog/pytorch-feature-classification-changes/#prototype `cond` is structured control flow operator. That is, it is like a Python if-statement, but has restrictions on `true_fn`, `false_fn`, and `operands` that enable it to be capturable using torch.compile and torch.export. Assuming the constraints on `cond`'s arguments are met, `cond` is equivalent to the following:: def cond(pred, true_branch, false_branch, operands): if pred: return true_branch(*operands) else: return false_branch(*operands) Args: pred (Union[bool, torch.Tensor]): A boolean expression or a tensor with one element, indicating which branch function to apply. true_fn (Callable): A callable function (a -> b) that is within the scope that is being traced. false_fn (Callable): A callable function (a -> b) that is within the scope that is being traced. The true branch and false branch must have consistent input and outputs, meaning the inputs have to be the same, and the outputs have to be the same type and shape. operands (Tuple of possibly nested dict/list/tuple of torch.Tensor): A tuple of inputs to the true/false functions. It can be empty if true_fn/false_fn doesn't require input. Defaults to (). Example:: def true_fn(x: torch.Tensor): return x.cos() def false_fn(x: torch.Tensor): return x.sin() return cond(x.shape[0] > 4, true_fn, false_fn, (x,)) Restrictions: - The conditional statement (aka `pred`) must meet one of the following constraints: - It's a `torch.Tensor` with only one element, and torch.bool dtype - It's a boolean expression, e.g. `x.shape[0] > 10` or `x.dim() > 1 and x.shape[1] > 10` - The branch function (aka `true_fn`/`false_fn`) must meet all of the following constraints: - The function signature must match with operands. - The function must return a tensor with the same metadata, e.g. shape, dtype, etc. - The function cannot have in-place mutations on inputs or global variables. (Note: in-place tensor operations such as `add_` for intermediate results are allowed in a branch) """ if torch.compiler.is_dynamo_compiling(): return cond_op(pred, true_fn, false_fn, operands) from torch._dynamo.backends.debugging import ( make_eager_backend_with_torch_function_mode, ) if isinstance(pred, (bool, int, float)): # This is the non-strict export case. Strict export and torch.compile are # handled above in dynamo. if torch.compiler.is_compiling(): warnings.warn( "Pred is a Python constant. When used with torch.cond, it specializes on one of the branches." " If you want torch.cond to preserve two branches, please make the predicate a boolean tensor or a SymBool.", UserWarning, ) # This is the eager case. We can just run the true or false branch. if pred: return true_fn(*operands) else: return false_fn(*operands) def _validate_input(pred, true_fn, false_fn, operands): if not isinstance(pred, (bool, torch.Tensor, torch.SymBool)): raise RuntimeError(f"Expected pred to be bool or tensor, but got {pred}.") if isinstance(pred, torch.Tensor) and pred.numel() != 1: raise RuntimeError( f"Expected pred to be bool or single-element tensor, but got {pred}." ) if not callable(true_fn) or not callable(false_fn): raise RuntimeError("Expect both branches to be callable.") if not isinstance(operands, (tuple, list)) or pytree.tree_any( lambda t: not isinstance(t, torch.Tensor), operands ): raise RuntimeError( "Expect operands to be a tuple of possibly nested dict/list/tuple that only " f"consists of tensor leaves, but got {operands}." ) _validate_input(pred, true_fn, false_fn, operands) if not torch._dynamo.is_dynamo_supported(): raise RuntimeError("torch.cond requires dynamo support.") # Dynamo is expecting a callable with "__code__" attribute. # We cannot directly pass cond_op to it. So we wrap it in a dummy function. def _cond_op_wrapper(*args, **kwargs): return cond_op(*args, **kwargs) with _set_compilation_env(), torch._dynamo.utils.disable_cache_limit(), _temp_remove_pre_dispatch_torch_function_mode(): with _temp_remove_metadata_torch_function_mode() as metadata_mode: if metadata_mode: backend = make_eager_backend_with_torch_function_mode(metadata_mode) else: backend = "eager" return torch.compile(_cond_op_wrapper, backend=backend, fullgraph=True)( pred, true_fn, false_fn, operands ) def create_fw_bw_graph_branches(true_fn, false_fn, *operands): # See Note [HOP create fw_bw graph] in create_fw_bw_graph in utils.py with suspend_functionalization(), disable_functional_mode(): with disable_proxy_modes_tracing(): fw_inputs = pytree.tree_map(_from_fun, operands) fw_outputs_true = pytree.tree_map( _from_fun, _maybe_fake_prop_ignore_unbacked(true_fn, fw_inputs) ) if any( not isinstance(out, torch.Tensor) for out in fw_outputs_true if out is not None ): raise RuntimeError( "Expect outputs of true_fn to only contains tensors or None. " f"Got types {[type(out) for out in fw_outputs_true]}." ) fw_outputs_false = pytree.tree_map( _from_fun, _maybe_fake_prop_ignore_unbacked(false_fn, fw_inputs) ) if any( not isinstance(out, torch.Tensor) for out in fw_outputs_false if out is not None ): raise RuntimeError( "Expect outputs of false_fn to only contains tensors or None. " f"Got types {[type(out) for out in fw_outputs_false]}." ) # TODO: There is a major issue that the create_fw_bw in the higher_order_op is invoked twice: # Once in the forward path (as it should) and once in the backward path, where it shouldn't be called # If we can get rid of the second invokation, it would simplify this function fw_true_graph, joint_true_graph = create_fw_bw_graph( true_fn, False, fw_inputs, fw_outputs_true ) fw_false_graph, joint_false_graph = create_fw_bw_graph( false_fn, False, fw_inputs, fw_outputs_false ) return fw_true_graph, fw_false_graph, joint_true_graph, joint_false_graph def trace_cond(proxy_mode, func_overload, pred, true_fn, false_fn, operands): assert isinstance( operands, (list, tuple) ), f"Cond operands must be a list or tuple of tensors and SymInts {operands}" true_graph = reenter_make_fx(true_fn)(*operands) false_graph = reenter_make_fx(false_fn)(*operands) true_outs = [] false_outs = [] for node in true_graph.graph.nodes: if node.op == "output": true_outs.extend(node.args) for node in false_graph.graph.nodes: if node.op == "output": false_outs.extend(node.args) flat_true_outs = pytree.arg_tree_leaves(*true_outs) flat_false_outs = pytree.arg_tree_leaves(*false_outs) if len(flat_true_outs) != len(flat_false_outs): raise torch._dynamo.exc.CondOpArgsMismatchError( f"Expected to return same number of outputs but got:" f"\n true branch returns {len(flat_true_outs)} item(s)" f"\n false branch returns {len(flat_false_outs)} item(s)" ) i, true_name = unique_graph_id(proxy_mode, prefix="true_graph") false_name = f"false_graph_{i}" assert not hasattr(proxy_mode.tracer.root, false_name) proxy_mode.tracer.root.register_module(true_name, true_graph) proxy_mode.tracer.root.register_module(false_name, false_graph) args = (pred, true_graph, false_graph, operands) proxy_args = pytree.tree_map(proxy_mode.tracer.unwrap_proxy, args) out_proxy = proxy_mode.tracer.create_proxy( "call_function", func_overload, proxy_args, {} ) out = func_overload(pred, true_graph, false_graph, operands) return track_tensor_tree(out, out_proxy, constant=None, tracer=proxy_mode.tracer) @cond_op.py_impl(DispatchKey.CompositeExplicitAutograd) def cond_op_dense(pred, true_fn, false_fn, operands): assert all( isinstance(o, (torch.Tensor, int)) for o in operands ), f"Dense implementation operands must be a list of tensors and ints {operands}" mode = _get_current_dispatch_mode() assert mode is None, "Mode should never be enabled for CPU/CUDA key" if pred: return true_fn(*operands) else: return false_fn(*operands) class CondAutogradOp(torch.autograd.Function): @staticmethod def forward( ctx, pred, fw_true_graph, fw_false_graph, joint_true_graph, joint_false_graph, *operands, ): ctx._pred = pred ctx._joint_true_graph = joint_true_graph ctx._joint_false_graph = joint_false_graph save_tensors_and_symints_for_backward(ctx, operands) with torch._C._AutoDispatchBelowAutograd(): return cond_op(pred, fw_true_graph, fw_false_graph, operands) @staticmethod def backward(ctx, *flat_grads): operands = saved_tensors_and_symints(ctx) grads = cond_op( ctx._pred, ctx._joint_true_graph, ctx._joint_false_graph, flat_grads + operands, ) return None, None, None, None, None, *grads @cond_op.py_impl(DispatchKey.Autograd) def cond_autograd(pred, true_fn, false_fn, operands): # A shortcut for the case where all inputs don't require gradient, # we skip tracing the forward and backward graph. if pytree.tree_all_only( torch.Tensor, lambda t: not t.requires_grad, # type: ignore[union-attr] (pred, operands), ): with torch._C._AutoDispatchBelowAutograd(): return cond_op(pred, true_fn, false_fn, operands) ( fw_true_graph, fw_false_graph, joint_true_graph, joint_false_graph, ) = create_fw_bw_graph_branches(true_fn, false_fn, *operands) flat_out = CondAutogradOp.apply( pred, fw_true_graph, fw_false_graph, joint_true_graph, joint_false_graph, *operands, ) return flat_out @cond_op.py_impl(ProxyTorchDispatchMode) def inner(mode, pred, true_fn, false_fn, operands): return trace_cond(mode, cond_op, pred, true_fn, false_fn, operands) @cond_op.py_impl(FakeTensorMode) def cond_fake_tensor_mode(mode, pred, true_fn, false_fn, operands): # Ignore here, because if you've gotten here but you're not manually # tracing the inner graphs, that means that you intend to reuse the graph # directly. Which means the old unbacked symbol bindings are appropriate. # This strategy will not work if unbacked symbols can escape. ignore_fresh_unbacked = contextlib.nullcontext() if mode.shape_env: ignore_fresh_unbacked = mode.shape_env.ignore_fresh_unbacked_symbols() with mode, ignore_fresh_unbacked: flat_true_outs, true_out_spec = pytree.tree_flatten(true_fn(*operands)) flat_false_outs, false_out_spec = pytree.tree_flatten(false_fn(*operands)) if true_out_spec != false_out_spec: raise RuntimeError( "Unmatched output spec from torch.cond branches: " f"true branch tree_spec {true_out_spec} vs false branch tree_spec {false_out_spec}." ) merged_outs = [] for true_out, false_out in zip(flat_true_outs, flat_false_outs): merged_outs.append(_merge_tensors(true_out, false_out, mode)) return pytree.tree_unflatten(merged_outs, true_out_spec) def check_tensor_meta_match( t1: torch.Tensor, t2: torch.Tensor, attr_names: tuple[str, ...], msg_prefix: str ) -> None: def _get_attr_maybe_call(t: torch.Tensor, attr_name: str) -> Any: attr = getattr(t, attr_name) if callable(attr): return attr() return attr for attr_name in attr_names: lattr = _get_attr_maybe_call(t1, attr_name) rattr = _get_attr_maybe_call(t2, attr_name) torch._check( lattr == rattr, lambda: f"{msg_prefix} expected same {attr_name} but got {lattr} and {rattr}.", ) def _merge_tensors( a: Optional[torch.Tensor], b: Optional[torch.Tensor], mode: FakeTensorMode ): from torch.fx.experimental.symbolic_shapes import SymIntEqByExpr if a is None or b is None: assert a is None and b is None, (a, b) return None assert type(a) is FakeTensor and type(b) is FakeTensor, (a, type(a), b, type(b)) # Note: we don't check size, stride because # they'll be merged with unbacked symints if they differ. _meta_to_check = { "dtype", "device", "layout", "dim", "is_quantized", "is_conj", "is_sparse", "storage_offset", } check_tensor_meta_match( a, b, tuple(_meta_to_check), msg_prefix="When merging two branches' output in torch.cond, ", ) # NYI assert not a.is_quantized and not b.is_quantized assert not a.is_sparse and not b.is_sparse assert not a.is_conj() and not b.is_conj() """ Step 1: create unbacked symints for sizes that are different along the same axis. For example: a.size is [s0, 4, s0, 5, 4, 5] b.size is [s1, 4, s2, 8, 4, 7] merged_size will be [u0, 4, u1, u2, 4, u3], where u0 has range [min(s0, s1), max(s0, s1)] u1 has range [min(s0, s2), max(s0, s2)] u2 has range [5, 8] u3 has range [5, 7] """ merged_size: list[Union[int, torch.SymInt]] = [] for s0, s1 in zip(a.size(), b.size()): if SymIntEqByExpr(s0) == SymIntEqByExpr(s1): merged_size.append(s0) else: def min_max(s0, s1): def _bound(s0, lower_bound: bool): if isinstance(s0, int): return s0 r = mode.shape_env.var_to_range.get( # type: ignore[union-attr] s0.node.expr, torch.utils._sympy.value_ranges.ValueRanges.unknown(), ) return r.lower if lower_bound else r.upper return min(_bound(s0, True), _bound(s1, True)), max( _bound(s0, False), _bound(s1, False) ) assert mode.shape_env is not None new_size = mode.shape_env.create_unbacked_symint() mode.shape_env.constrain_symbol_range(new_size.node.expr, *min_max(s0, s1)) merged_size.append(new_size) """ This follows the logic in symbolic_shapes._compute_symbolic_stride Step 2: Since tensor stride is an accumulative muliplication of the sizes, which is a permutated (due to view ops) non-decending sequence. Case 1: No size is 1. In this case, strides have unique values. For example, suppose we have a tenosr with: size [3, 4, 3, 5, 4, 5], stride (1200, 300, 1, 12, 3, 60), merged_size [u0, u1, u2, u3, u4, u5]. We visit the strides in ascending order: 1, 3, 12, 60, 300, 1200. In each step, we check whether the current stride is bounded or not and bound next stride by setting. stride_expr[next_stride] = current_stride_expr * current_size_expr 1st round: current_stride is 1, current_size is 3, so next_stride is 1 * 3 = 3, current_stride_expr is set to 1, current_size_expr is u2, so stride_expr[3] is therefore 1 * u2 = u2 2nd round: current_stride is 3, current_size is 4, so next_stride is 3 * 4 = 12, current_stride_expr is stride_expr[3] i.e. u2, current_size_expr is u4, so stride_expr[12] = u2 * u4 ... Case 2: At least one dimension has size 1, which can produce duplicates in strides. In this case, theorectically, we cannot uniquely determine the expr of strides because the accessing stride_expr with same key in different order causes the final stride expression to be different. Suppose we have: size: (3, 1) stride: (1, 1) merged_size: (u0, u1) The stride expr could either be (u1, 1) or (1, u0) depending on whether we start with u1 or u0. For this reason, we try to break tie by sorting via decending index so we always get (u1, 1). Note that backend might optimize the strides anyway so this is usually not a problem as long as two branches matches. See relevant discussions in https://github.com/pytorch/pytorch/issues/142024. Case 3: Dim has 0 stride. 0 stride doesn't participate in the accumulative multiplication of sizes. So they're always treated as constant even if their corresponding size is turned into unbacked symint. Suppose we have: size: (3, 3) stride: (0, 1) merged_size: (u0, u1) The merged stride would be (0, 1) """ def _bound_stride( a_ex_size: torch.Size, b_ex_size: torch.Size, a_ex_stride: tuple[int, ...], b_ex_stride: tuple[int, ...], merged_size: list[Union[int, torch.SymInt]], ) -> list[Union[int, torch.SymInt]]: from torch._inductor.ir import get_stride_order a_sorted_stride_idx = get_stride_order(a_ex_stride, mode.shape_env) b_sorted_stride_idx = get_stride_order(b_ex_stride, mode.shape_env) a_stride_li: list[Optional[tuple[Union[int, torch.SymInt], int]]] = [ None ] * len(a_ex_stride) b_stride_li: list[Optional[tuple[Union[int, torch.SymInt], int]]] = [ None ] * len(b_ex_stride) for i, idx in enumerate(a_sorted_stride_idx): a_stride_li[idx] = (a_ex_stride[i], -i) for i, idx in enumerate(b_sorted_stride_idx): b_stride_li[idx] = (b_ex_stride[i], -i) for a_pair, b_pair in zip(a_stride_li, b_stride_li): assert a_pair is not None and b_pair is not None _, a_idx = a_pair _, b_idx = b_pair if a_idx != b_idx: raise RuntimeError( f"The sorted order of strides of the two branches' output doesn't match." f"this indicates the contiguousness of the two branches are different. " f"True branch has stride {a_ex_stride} but false branch has stride {b_ex_stride}." f"Consider using contiguous() to make the two branches have the same contiguousness." ) def _maybe_expr(s: Union[int, torch.SymInt]): if isinstance(s, int): return s return s.node.expr a_stride_expr: dict[Any, Union[int, torch.SymInt]] = {} b_stride_expr: dict[Any, Union[int, torch.SymInt]] = {} merged_strides: list[Union[int, torch.SymInt]] = [None] * len(a_ex_stride) # type: ignore[list-item] for a_pair, b_pair in zip(a_stride_li, b_stride_li): assert a_pair is not None and b_pair is not None a_val, neg_i = a_pair b_val, _ = b_pair i = -neg_i if a_val == 0: assert b_val == 0, (a_val, b_val) merged_strides[i] = 0 continue if _maybe_expr(a_val) in a_stride_expr: a_expr = a_stride_expr[_maybe_expr(a_val)] assert ( b_stride_expr[_maybe_expr(b_val)] == a_expr ), f"a_stride_expr:{a_stride_expr}, b_stride_expr:{b_stride_expr}" merged_strides[i] = a_expr else: if a_val == 1: assert b_val == 1 a_stride_expr[_maybe_expr(a_val)] = 1 b_stride_expr[_maybe_expr(b_val)] = 1 merged_strides[i] = 1 else: # If we cannot find the expr of a_val in a_stride_expr, it means # the strides is not a simple accumulative multiplication of sizes. # In this case, we cannot determine the expr of strides from the new # shapes so we error out and hint users to call contiguous(). raise RuntimeError( f"It seems one of cond's output stride is not a simple accumulative multiplication of sizes. " f"This could be because cond returns a slice of a tensor, which is not dense in memory. " f"True branch has size {a_ex_size}, stride {a_ex_stride} and false branch has size {b_ex_size} " f"stride {b_ex_stride}. Hint: can call t.contiguous(). " ) nxt_merged_stride_expr = merged_strides[i] * merged_size[i] a_stride_expr[_maybe_expr(a_val * a_ex_size[i])] = nxt_merged_stride_expr b_stride_expr[_maybe_expr(b_val * b_ex_size[i])] = nxt_merged_stride_expr return merged_strides merged_stride: list[Union[int, torch.SymInt]] = _bound_stride( a.size(), b.size(), a.stride(), b.stride(), merged_size ) with mode: return torch.empty_strided( merged_size, merged_stride, dtype=a.dtype, device=a.device ) @cond_op.py_functionalize_impl def cond_func(ctx, pred, true_fn, false_fn, inputs): unwrapped_inputs = ctx.unwrap_tensors(inputs) unwrapped_pred = ctx.unwrap_tensors(pred) with ctx.redispatch_to_next(): functional_true = ctx.functionalize(_maybe_run_with_interpreter(true_fn)) functional_false = ctx.functionalize(_maybe_run_with_interpreter(false_fn)) pre_dispatch = hasattr(ctx, "mode") and ctx.mode.pre_dispatch for branch in [true_fn, false_fn]: if _has_potential_branch_input_mutation( branch, unwrapped_inputs, pre_dispatch=pre_dispatch ): raise UnsupportedAliasMutationException( "One of torch.cond branch might be modifying the input! " "Consider cloning the input before modifying it. " ) for branch in [true_fn, false_fn]: if _has_potential_branch_input_alias( branch, unwrapped_inputs, pre_dispatch=pre_dispatch ): raise UnsupportedAliasMutationException( "One of torch.cond branch might be aliasing the input! " "If you are returning a view of the input, please make sure " "to clone it. " ) cond_return = cond_op( unwrapped_pred, functional_true, functional_false, unwrapped_inputs ) return ctx.wrap_tensors(cond_return) @cond_op.py_impl(torch._C._functorch.TransformType.Vmap) def cond_batch_rule(interpreter, pred, true_fn, false_fn, inputs): assert isinstance( inputs, (list, tuple) ), "Cond inputs must be a list or tuple of tensors" assert all( isinstance(i, torch.Tensor) for i in inputs ), "Cond inputs must be a list of tensors" pred_is_batched = isinstance(pred, torch.Tensor) and is_batchedtensor(pred) pred_ = get_unwrapped(pred) if pred_is_batched else pred # unbatched tensors are not vmapped tensors, in_dims = zip( *[ (get_unwrapped(t), maybe_get_bdim(t)) if is_batchedtensor(t) else (t, None) for t in inputs ] ) if pred_is_batched: # prepend "pred" and vmap everything tensors = (pred_,) + tensors in_dims = (0,) + in_dims def fn(p, *args): t = true_fn(*args) f = false_fn(*args) return torch.where(p, t[0], f[0]) with interpreter.lower(): result = torch.vmap(fn, in_dims=in_dims)(*tensors) else: # predicate is known at this stage and it is a boolean expression or a # tensor with one element. true_fn = torch.vmap(true_fn, in_dims=in_dims) false_fn = torch.vmap(false_fn, in_dims=in_dims) with interpreter.lower(): result = cond_op(pred, true_fn, false_fn, tensors) if not isinstance(result, tuple): result = (result,) lvl = interpreter.level() return tuple([_add_batch_dim(r, 0, lvl) for r in result])