Note
Go to the end to download the full example code.
Export a model with a control flow (If)¶
Control flow cannot be exported with a change.
The code of the model can be changed or patched
to introduce function torch.cond()
.
A model with a test¶
import torch
We define a model with a control flow (-> graph break)
class ForwardWithControlFlowTest(torch.nn.Module):
def forward(self, x):
if x.sum():
return x * 2
return -x
class ModelWithControlFlow(torch.nn.Module):
def __init__(self):
super().__init__()
self.mlp = torch.nn.Sequential(
torch.nn.Linear(3, 2),
torch.nn.Linear(2, 1),
ForwardWithControlFlowTest(),
)
def forward(self, x):
out = self.mlp(x)
return out
model = ModelWithControlFlow()
Let’s check it runs.
x = torch.randn(1, 3)
model(x)
tensor([[0.1805]], grad_fn=<MulBackward0>)
As expected, it does not export.
try:
torch.export.export(model, (x,))
raise AssertionError("This export should failed unless pytorch now supports this model.")
except Exception as e:
print(e)
def forward(self, arg0_1: "f32[2, 3]", arg1_1: "f32[2]", arg2_1: "f32[1, 2]", arg3_1: "f32[1]", arg4_1: "f32[1, 3]"):
# File: /home/xadupre/vv/this312/lib/python3.12/site-packages/torch/nn/modules/linear.py:125 in forward, code: return F.linear(input, self.weight, self.bias)
linear: "f32[1, 2]" = torch.ops.aten.linear.default(arg4_1, arg0_1, arg1_1); arg4_1 = arg0_1 = arg1_1 = None
linear_1: "f32[1, 1]" = torch.ops.aten.linear.default(linear, arg2_1, arg3_1); linear = arg2_1 = arg3_1 = None
# File: /home/xadupre/github/onnx-diagnostic/_doc/examples/plot_export_cond.py:24 in forward, code: if x.sum():
sum_1: "f32[]" = torch.ops.aten.sum.default(linear_1); linear_1 = None
ne: "b8[]" = torch.ops.aten.ne.Scalar(sum_1, 0); sum_1 = None
item: "Sym(Eq(u0, 1))" = torch.ops.aten.item.default(ne); ne = item = None
/home/xadupre/vv/this312/lib/python3.12/site-packages/torch/backends/mkldnn/__init__.py:78: UserWarning: TF32 acceleration on top of oneDNN is available for Intel GPUs. The current Torch version does not have Intel GPU Support. (Triggered internally at /pytorch/aten/src/ATen/Context.cpp:148.)
torch._C._set_onednn_allow_tf32(_allow_tf32)
Could not guard on data-dependent expression Eq(u0, 1) (unhinted: Eq(u0, 1)). (Size-like symbols: none)
Caused by: (_export/non_strict_utils.py:683 in __torch_function__)
For more information, run with TORCH_LOGS="dynamic"
For extended logs when we create symbols, also add TORCHDYNAMO_EXTENDED_DEBUG_CREATE_SYMBOL="u0"
If you suspect the guard was triggered from C++, add TORCHDYNAMO_EXTENDED_DEBUG_CPP=1
For more debugging help, see https://docs.google.com/document/d/1HSuTTVvYH1pTew89Rtpeu84Ht3nQEFTYhAX3Ypa_xJs/edit?usp=sharing
For C++ stack trace, run with TORCHDYNAMO_EXTENDED_DEBUG_CPP=1
The following call raised this error:
File "/home/xadupre/github/onnx-diagnostic/_doc/examples/plot_export_cond.py", line 24, in forward
if x.sum():
Suggested Patch¶
Let’s avoid the graph break by replacing the forward.
def new_forward(x):
def identity2(x):
return x * 2
def neg(x):
return -x
return torch.cond(x.sum() > 0, identity2, neg, (x,))
print("the list of submodules")
for name, mod in model.named_modules():
print(name, type(mod))
if isinstance(mod, ForwardWithControlFlowTest):
mod.forward = new_forward
the list of submodules
<class '__main__.ModelWithControlFlow'>
mlp <class 'torch.nn.modules.container.Sequential'>
mlp.0 <class 'torch.nn.modules.linear.Linear'>
mlp.1 <class 'torch.nn.modules.linear.Linear'>
mlp.2 <class '__main__.ForwardWithControlFlowTest'>
Let’s see what the fx graph looks like.
ep = torch.export.export(model, (x,))
print(ep.graph)
/home/xadupre/vv/this312/lib/python3.12/site-packages/torch/backends/mkldnn/__init__.py:78: UserWarning: TF32 acceleration on top of oneDNN is available for Intel GPUs. The current Torch version does not have Intel GPU Support. (Triggered internally at /pytorch/aten/src/ATen/Context.cpp:148.)
torch._C._set_onednn_allow_tf32(_allow_tf32)
graph():
%p_mlp_0_weight : [num_users=1] = placeholder[target=p_mlp_0_weight]
%p_mlp_0_bias : [num_users=1] = placeholder[target=p_mlp_0_bias]
%p_mlp_1_weight : [num_users=1] = placeholder[target=p_mlp_1_weight]
%p_mlp_1_bias : [num_users=1] = placeholder[target=p_mlp_1_bias]
%x : [num_users=1] = placeholder[target=x]
%linear : [num_users=1] = call_function[target=torch.ops.aten.linear.default](args = (%x, %p_mlp_0_weight, %p_mlp_0_bias), kwargs = {})
%linear_1 : [num_users=2] = call_function[target=torch.ops.aten.linear.default](args = (%linear, %p_mlp_1_weight, %p_mlp_1_bias), kwargs = {})
%sum_1 : [num_users=1] = call_function[target=torch.ops.aten.sum.default](args = (%linear_1,), kwargs = {})
%gt : [num_users=1] = call_function[target=torch.ops.aten.gt.Scalar](args = (%sum_1, 0), kwargs = {})
%true_graph_0 : [num_users=1] = get_attr[target=true_graph_0]
%false_graph_0 : [num_users=1] = get_attr[target=false_graph_0]
%cond : [num_users=1] = call_function[target=torch.ops.higher_order.cond](args = (%gt, %true_graph_0, %false_graph_0, [%linear_1]), kwargs = {})
%getitem : [num_users=1] = call_function[target=operator.getitem](args = (%cond, 0), kwargs = {})
return (getitem,)
Total running time of the script: (0 minutes 5.919 seconds)
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