How to add a relay pass

Questions
#1

From the tvm docs, there are two ways to implement a pass: Using Python Decorator or Use C++. I found most relay passes are implemented by C++, such as fold_constant, etc,. Only one pass named ChangeBatch(https://github.com/apache/tvm/blob/main/python/tvm/relay/transform/transform.py#L1062) is impled using Python Decorator.

So if I want to add a custom relay pass, is it better to implemented it by C++?

#2

both implementions is ok. in most condition, writing python pass is enough.

#3

But there are few examples of passes writing by python. Only one pass named ChangeBatch is writen by python. So it may be hard for pass beginers.

BTY, if I want to insert a op to the relay graph ir, I should use module-level pass?

#4

Hi, @zhaoyang-star.

I believe that is because Python interface is more recommended for the quick tryout, rather than the formal integration. If you are planning to push your pass to the main branch, I think C++ implementation is generally recommended.

You can either use a module-level pass or a relay function-level pass. But since a function-level pass applies transformation for each relay function independently, you would need to use a module-level pass if you want to make change across functions.

1 Like
#5

Thanks for your kind reply.

#6

@zhaoyang-star There is another pass implemented in python,
the example is here: /path_to_tvm/gallery/how_to/extend_tvm/use_pass_infra.py

@relay.transform.function_pass(opt_level=1)
class CustomPipeline:
    """Simple test function to replace one argument to another."""

    def __init__(self, multiplier):
        self.multiplier = multiplier

    # This function can define a pass.
    def transform_function(self, func, mod, ctx):
        obj = self

        class ReplaceConstant(tvm.relay.ExprMutator):
            def visit_constant(self, c):
                return relay.multiply(obj.multiplier, c)

        return ReplaceConstant().visit(func)


f = example()
mod = tvm.IRModule.from_expr(f)
custom_pass = CustomPipeline(multiplier=relay.const(3, "float32"))
assert custom_pass.info.name == "CustomPipeline"
mod3 = custom_pass(mod)
print(mod3)