765297 - free up bits in String::lengthAndFlags for new string subtypes

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Luke Wagner [:luke]]

Attachment: patch

There are a few interesting optimizations that require adding new string subtypes (e.g., bug 654190) but can't because the 4 flag bits in lengthAndFlags are a bit crowded.  Currently, ropes and dependent strings get their own bit (which allows a single cmp to test), but only testing the rope bit (or their absence: isLinear) is hot.  This patch frees up the dependent string bit which should give us enough room for the next few optimizations.

Comment 1

[Nicholas Nethercote [inactive]]

Comment on attachment 633577 [details] [diff] [review]
patch

Review of attachment 633577 [details] [diff] [review]:
-----------------------------------------------------------------

::: js/src/vm/String.cpp
@@ +336,2 @@
>  
> +    /* TODO: explain why undepended. */

?

::: js/src/vm/String.h
@@ +92,2 @@
>   *  |
> + * JSLinearString (abstract)     chars / may not be null-terminated

Can you change this to "might"?  I always find "may" ambiguous -- does it mean "might" or "must"?

Comment 2

[Luke Wagner [:luke]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/613262f091e6

Comment 3

[Ed Morley [:emorley]]

https://hg.mozilla.org/mozilla-central/rev/613262f091e6