Module SSA

Provides classes for working with static single assignment form (SSA).

We compute SSA form based on the intra-procedural CFG, without any call graph information. This means that we have to make worst-case assumptions about the possible effects of function calls and yield:

• For a variable x declared in a function f, if x has assignments in a function other than f, then any function call and yield expression is assumed to write x.
• If x is not written outside f, then function calls can never affect x, while yield expressions in functions other than f still may affect it.

This is modeled as follows.

Within each function g that accesses a variable x declared in an enclosing function f, we introduce a pseudo-assignment to x called a capture of x at the beginning of g that (conceptually) captures the current value of x.

Additionally, we introduce re-captures for x in the following places:

• At any function call and yield, if x is assigned outside f.
• At any yield outside f, if x is not assigned outside f.

Re-captures are introduced only where needed, that is, where there is a live use of x after the re-capture.

To see why re-captures need to be placed at yield expressions, consider the following function:

function k() {
  var x = 0;

  function* iter() {
    console.log(x);
    yield;
    console.log(x);
  }

  var gen = iter();
  gen.next();
  ++x;
  gen.next();
}

Here, iter has a capture for x at its beginning, and a re-capture at the yield to reflect the fact that x is incremented between the two console.log calls.

In the above example, x is only assigned inside its declaring function k, so function calls and yield expressions inside k cannot affect it.

Consider another example:

function* k() {
  var x = 0;
  console.log(x);
  yield () => ++x;
  console.log(x);
}
var gen = k();
gen.next().value();
gen.next();

Here, x is assigned outside its declaring function k, so the yield expression in k induces a re-capture of x to reflect the fact that x is incremented between the two console.log calls.