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Constant fold enum member initializers. #917
Description
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.
Activity
RyanCavanaugh commentedon Oct 18, 2014
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 commentedon Oct 25, 2014
I think I'll take this on.
vladima commentedon Oct 25, 2014
Oops, I've already started working on this one.
From: Daniel Rosenwassermailto:notifications@github.com
Sent: ý10/ý25/ý2014 11:22 AM
To: Microsoft/TypeScriptmailto:TypeScript@noreply.github.com
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 commentedon Oct 25, 2014
Oops 😄 - I currently propagate arithmetic ops on constants but not previously declared enums. You're probably farther than me, right?
vladima commentedon Nov 4, 2014
covered by #970