@jenkins-droid test this please.

Question about the Solution:
1. The stickification is delayed to lowering constant to llvm. In this lowering, can the space for krnl.Global be freed one-by-one while new llvm constant is created? I am comparing the situation when onnx.Constant is converted to stickified krnl.Global in onnx to zhigh in original code.
2. Following the previous point, I'd like to know whether the benefit of this PR comes from storing constant in file instead of in-memory, or from delaying the stickification operation.

I updated the figure in this PR since previous figure might be confusing.
The solution does not use krnl.Global. Instead, zlow.StickifiedConstant is used and lowered to llvm. When krnl.Global is used, memory usage can not be reduced. KrnlGlobal.cpp is also modified in this PR, but this modification is for using the ConstantOpInterface.
When zlow.StickifiedConstant is lowered to llvm, the data in value attribute is stickified, stored into a file, then deleted after writing. Original non-stickified data is alive, but stickified data is not alive.
The benefit comes from delaying the stickification, but it only benefits when storing into file.

The previous figure is fine. I knew that this PR replaced some krnl.GlobalOps (result of stickifiedConstant) with zlowstickifiedConstant. The previous figure showed the reason why the ConstantOpInterface is introduced.

@chentong319 Thanks for the comments. I'm now looking into RNN group and inheritance in Op definition.

Our RNN group did not use inheritance. We just use a common utility function for core part of different RNN operations.

I'm considering to use common functions.

I'm investigating how we can commonize extractConstantsToFile() for KrnlGlobalOp and ZLowStickifiedConstantOp. Currently it is called here. In the function, ConstantOpInterface is used for KrnlGlobalOp and ZLowStickifiedConstOp. I'm considering to commonize it using template, then call it here such as extractConstantsToFile<KrnlGlobalOp> or extractConstantsToFile<ZLowStickifiedConstantOp>, but I think we can't set operation name here. Do you know good way?

If you use the Op matching rule for rewrite, the op is cast automatically.
In your this implementation, you are traverse the ops by yourself. The op has to be cast.
Then you can call the member functions and pass the results as parameters to the common utility function.
If you feel it is too complicated, it is okay to me to use ConstantOpInterface (perhaps with another less general name).

@chentong319 Thanks for the investigations and comments!

I tried to implement some other way such as common functions and Trait, but I couldn't write it clean. So, I would like to keep using OpInterface.
How about using KrnlGlobalOpInterface? I used the opInterface to use the same functions with KrnlGlobalOp in zlowStickifiedConstantOp. I can replace if you have better candidate.

Could you review again?

@chentong319 Do you mind reviewing it? You have been very involved with this PR. Thanks