1434263 - Give JSString an IsLinear flag instead of IsFlat

Status: RESOLVED FIXED

(Core :: JavaScript Engine, enhancement)

Description

[Jan de Mooij [:jandem]]

Attachment: Patch

For bug 1434230 we want strings to have an explicit IsLinear flag - the current (flags & TYPE_FLAGS_MASK) != 0 predicate for isLinear() is hard to work with for that bug.

I think it makes sense to get rid of the IsFlat bit. JSString::isFlat() can be defined as isLinear() && !isDependent() && !isExternal() - that's slightly slower but flat strings will hopefully be removed at some point (bug 1330776) and isLinear() is the much hotter query.

Another option is to have both IsFlat/IsLinear bits; I can do that too if you think it's better.

Comment 1

[Luke Wagner [:luke]]

Comment on attachment 8946639 [details] [diff] [review]
Patch

Review of attachment 8946639 [details] [diff] [review]:
-----------------------------------------------------------------

It's nice that compile errors mostly (completely?) pointed out every place that needed updating.

::: js/src/vm/String.h
@@ +219,5 @@
>       *
>       *   String        Instance     Subtype
>       *   type          encoding     predicate
>       *   ------------------------------------
> +     *   Rope          000000       xxxxx0

Naturally, now we must find something to do with these other 7 bits ;)

Comment 2

[Jan de Mooij [:jandem]]

With the current approach in bug 1434230 I don't strictly need this patch anymore, but it still makes it easier to reason about the new code, so I'll just land it.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/a806fd1e5427
Refactor JSString to have an IsLinear flag instead of IsFlat. r=luke

Comment 4

[Natalia Csoregi [:nataliaCs]]

https://hg.mozilla.org/mozilla-central/rev/a806fd1e5427