Is it reasonable to add API for adding params for a BasicBlock?

[hsqStephenZhang]

Background

1. When implementing mem2reg, I have to patch koopa and add this method to DataFlowGraph
2. there are some existing discussion on the current design, see https://pku-minic.github.io/online-doc/#/lv9p-reincarnation/ssa-form

Proposal

#[must_use = "must fix arguments in predecessors' terminators to match the params"]
  pub fn append_bb_param(&mut self, bb: BasicBlock, ty: crate::ir::Type) -> Value {
    assert!(!ty.is_unit(), "parameter type must not be `unit`!");
    let index = self.bb(bb).params().len();
    let value = self.new_value_data(crate::ir::values::BlockArgRef::new_data(index, ty));
    self.bb_mut(bb).params_mut().push(value);
    value
  }

I think it's a reasonable design, otherwise user might have to clone a brand new BB just to add one BasicBlock param/Phi, or they should patch their own koopa.

We could apply this API to FunctionArgRef if it's wanted

[hsqStephenZhang]

Also, should removing a BasicBlock param be supported as a helper function? current DCE addressed this issue through a roundabout way

koopa/koopa/examples/opt/dce.rs

Line 105 in 6a06fb2

data.dfg_mut().bb_mut(*b).params_mut().retain(|_| {

, but it will not update the BlockArgRef's index value after the update of the basic block params.