Constant fold enum member initializers.

[mbebenita]

Consider the following enum definition:

enum Flags {
    A = 1,
    B = 2,
    C = A | B // Should be constant folded to 3.
}

In most cases, JS engines can optimize simple expressions like these. But they can't do much about accesses to Flags.C.

switch (x) {
  case Flags.A: ... break; // Compiled as case 1: ...
  case Flags.B: ... break; // Compiled as case 2: ...
  case Flags.C: ... break; // Compiled as Flags.C: ...  
}

This prevents JS engines from using a table jump and causes them to fall back on a sequential if/else if implementation for the switch which is all sorts of terrible.

[RyanCavanaugh]

We've talked about this before as something we'd like to do -- the old compiler would do this under an experimental flag and it was nice for a small perf boost. This also would be a necessary first step for a "zero emit" enum that some we had interest in.

As a rough outline I'd expect this to support math over |, &, ~, +, -, integer literals, and previously-declared enum members.

[DanielRosenwasser]

I think I'll take this on.

[vladima]

Oops, I've already started working on this one.

---

From: Daniel Rosenwassermailto:[email protected]
Sent: ý10/ý25/ý2014 11:22 AM
To: Microsoft/TypeScriptmailto:[email protected]
Subject: Re: [TypeScript] Constant fold enum member initializers. (#917)

I think I'll take this on.

—
Reply to this email directly or view it on GitHubhttps://github.com//issues/917#issuecomment-60491992.

[DanielRosenwasser]

Oops 😄 - I currently propagate arithmetic ops on constants but not previously declared enums. You're probably farther than me, right?

[vladima]

covered by #970