[Relay] VM prunes global functions with no direct calls.

[abergeron]

It seems there is a problem in the VM compiler for indirect calls to global functions that aren't used elsewhere.

I have a simple code block that reproduces it:

import tvm
from tvm import relay

ctx = tvm.ndarray.context('cpu', 0)

mod = relay.Module({})

fn1 = relay.Function([], relay.const(1))
fn2 = relay.Function([], relay.const(2))

g1 = relay.GlobalVar('g1')
g2 = relay.GlobalVar('g2')
mod[g1] = fn1
mod[g2] = fn2

p = relay.var('p', 'bool')
mod['main'] = relay.Function([p], relay.Call(relay.If(p, g1, g2), []))

print(str(mod))

vm = relay.create_executor(mod=mod, ctx=ctx, target='llvm', kind="vm")

relay_out = vm.evaluate()(True)

print("Relay result:\n", relay_out)

This errors at the relay.create_executor line saying there is no definition for g1.

• If you change the calls in the main functions to use the fn1 and fn2 instead of the global variables, it works.
• If you move the Call inside the If it works.

So it seems that because g1 and g2 don't get directly called they get removed at some point.

[zhiics]

@abergeron Thanks for reporting this. There is pass in VM that removes the "unused" global functions. It probably removed these functions.

[zhiics]

@abergeron #4700 fixes this.