[Apache TVM Discuss] [Questions] [PassInfra] Relay Function Pass

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Hello,

In the [PassInfra Design and Development Doc](https://tvm.apache.org/docs/dev/pass_infra.html#pass-objects) the `function_pass` decorator is briefly explained.
In the python codebase, there is also the `class FunctionPass`, objects of this type should be created with `function_pass`. It also states "works on each tvm.relay.Function in a module"

https://github.com/apache/incubator-tvm/blob/master/python/tvm/relay/transform/transform.py#L85

I have the following code:
```
import tvm
from tvm import relay

dtype = 'float32'
ishape = (1, 32, 14, 14)
wshape = (32, 32, 3, 3)
data1 = relay.var("data0", shape=ishape, dtype=dtype)
weight1 = relay.var("weight0", shape=wshape, dtype=dtype)
conv2d_1 = relay.nn.conv2d(data1, weight1, kernel_size=(3, 3), padding=(1, 1))
relu_1 = relay.nn.relu(conv2d_1)
f = relay.Function([data1, weight1], relu_1)
mod = tvm.IRModule()
mod['main'] = f

with tvm.transform.PassContext(opt_level=3):
    opt_mod, _ = relay.optimize(mod, 'llvm') #I am only doing this to force the module to have many internal functions and function calls

x = relay.var("x", shape=(10, 20))
f1 = relay.Function([x], x)
@relay.transform.function_pass(opt_level=1)
class TestReplaceFunc:
    """Simple test function to replace one argument to another. After the first function"""
    def __init__(self, new_func):
        self.new_func = new_func
        self.i = 0
    def transform_function(self, func, mod, ctx):
        if self.i == 0:
            return func
        else:
            self.i =1
            return self.new_func

fpass = TestReplaceFunc(f1)
print(opt_mod)
end_mod = fpass(opt_mod)
print(end_mod)
```

The output of both `opt_mod` and `end_mod` are identical and if I set a break point at the ` if self.i == 0:`statement, I see it only stops once.
The `opt_mod` has many (internal) functions, but only has one global function ('main'). 
 * Why is it not recursing through the rest of the internal functions?

If I do the same script as above but add another global function (ex:`mod['not_main']=another_relay_function`) before optimizing, I also noticed that `opt_mod` only has the 'main' function.
* What pass is deleting the 'not_main' function?
    * there is no call to 'not_main' from 'main', so maybe dead code elimination? but I find it weird that it always preserves the 'main' function,
* Is there a way to recurse through all global functions?
    * the `IRModule` has the attribute `functions`, but its a <Map> and I dont know how to get the keys or an iterator of them

Thank you for the help :slight_smile:





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