Direct-style continuations on unoptimized core

[b-studios]

When not opimizing core, continuations are not inlined, complicating the direct-style analysis.

For the example in #842, this currently yields:

repro.js:365
  let v_r_0 = undefined;
      ^

SyntaxError: Identifier 'v_r_0' has already been declared

since the generated code looks like this:

function println_0(ans_0, ks_0, k_0) {
  let v_r_0 = undefined;
  let v_r_0 = undefined;
  switch (ans_0.__tag) {
    case 0:  return () => b_k_0(ks_0);
    case 1:  break;
  }
  const v_r_1 = $effekt.println("n");
  v_r_0 = v_r_1;
  return () => k_0(v_r_0, ks_0);
  const v_r_2 = $effekt.println("y");
  v_r_0 = v_r_2;
  return () => k_0(v_r_0, ks_0);
}

Disabling the direct style optimization by commenting out this

effekt/effekt/shared/src/main/scala/effekt/generator/js/TransformerCps.scala

Lines 223 to 228 in 5599ca1

	case cps.Stmt.LetCont(id, Cont.ContLam(params, ks, body), body2) if canBeDirect(id, body2) =>
	Binding { k =>
	params.map { p => js.Let(nameDef(p), js.Undefined) } :::
	toJS(body2)(using markDirectStyle(id, params, ks)).stmts ++
	toJS(maintainDirectStyle(ks, body)).run(k)
	}

resolves the issue, but is slow.

[b-studios]

This is the CPS IR for println

let k3755 = { (v_r_27793745) | ks3756 =>
  jump k3754(v_r_27793745) @ ks3756
}
let b_k_27763741 = { () | ks3757 =>
  let v_r_27753743 = println1("y")
  jump k3755(v_r_27753743) @ ks3757
}
let k3753 = { (v_r_27793745) | ks3758 =>
  jump k3754(v_r_27793745) @ ks3758
}
let b_k_27783742 = { () | ks3759 =>
  let v_r_27773744 = println1("n")
  jump k3753(v_r_27773744) @ ks3759
}
ans2747 match {
  case Yes2753 () => {
    jump b_k_27763741() @ ks3760
  }
  case No2754 () => {
    jump b_k_27783742() @ ks3760
  }
}

which it believes is direct style in k3755.

[b-studios]

It also suffices to make the rhs of this

effekt/effekt/shared/src/main/scala/effekt/generator/js/TransformerCps.scala

Line 514 in 5599ca1

case Stmt.Jump(k2, vargs, ks2) => vargs.forall(notIn)

to always return false, which effectively requires all branches to return to the same continuation. This again might be too conservative since under certain conditions we can convert a program to direct style (for the fast path), but still keep non-local jumps to other continuations.