[CLangBackend] Meet unsupported operator: Constant

[robinvanemden]

What to add:

When converting a model containing a Constant type operator to C using the CLangBackend, onnc exits prematurely with the following message:

Meet unsupported operator: Constant(name="Constant_72")

Why it is necessary:

Constant is a common operator.

How to achieve it:

The "constant" operator is currently commented out in CLangBackend's RegisterLowers():

// FIXME: #include <onnc/Transforms/TensorSel/Standards/ConstantLower.h>
// TODO: #include <onnc/Transforms/TensorSel/Standards/ConstantFillLower.h>

So presumably, fixing ConstantLower would solve this issue?

[a127a127]

Wow, it's weird. Due to historical reasons, ONNC handles Constant in ONNX IR level.
The "Constant" operator should be removed in ONNX IR level by this pass: https://github.com/ONNC/onnc/blob/master/lib/Transforms/ExtractConstToInitializer.cpp
And that pass should be added in addStandardTensorSel():
https://github.com/ONNC/onnc/blob/master/lib/Target/TargetStandardPasses.cpp#L43
https://github.com/ONNC/onnc/blob/master/lib/Target/CLang/CLangBackend.cpp#L171

Fixing ConstantLower would solve this "Meet unsupported operator" error, but I'm not sure following passes could handle Constant operator correctly.

[robinvanemden]

Thanks! That ought to help me pinpoint the issue - I will see if I am able to fix it myself using your information.