1576885 - Reduce size of Maybe<SharedImmutableString>

Status: RESOLVED FIXED

(Core :: JavaScript Engine, task, P2)

Description

[Ted Campbell [:tcampbell]]

In Bug 1575077 we start making more use of the SharedImmutableStringsCache and should reduce the size of references to shrink ScriptSource by 10 words per top-level script.

It should be possible to move the cache pointer from SharedImmutableString into the StringBox itself, and then allowing this to be a nullable type to replace the use of Maybe.

Comment 1

[Ted Campbell [:tcampbell]]

The first step would be to understand the relation between

• SharedImmutableString
• SharedImmutableStringCache
• SharedImmutableStringCache::Inner
• SharedImmutableStringCache::StringBox

Since multiple SharedImmutableString may point to the same StringBox, moving the cache_ field from SharedImmutableString to StringBox should save us memory in general.

Comment 2

[Ted Campbell [:tcampbell]]

Part 2 of this bug would be to to have SharedImmutableString support null strings (eg, box_ being nullptr). If we did that, we could replace uses of Maybe<SharedImmutableString> and save more memory.

Comment 3

[Ashita]

Attachment: Bug 1576885: Reduce size of Maybe<SharedImmutableString> r=tcampbell

Comment 4

[Ted Campbell [:tcampbell]]

For Part 2 there are probably a few steps:

• Add a new Constructor to SharedImmutableString that takes no arguments and sets box_ to nullptr. This will represent a missing string. Today we use Maybe<SharedImmutableString> set to Nothing to represent that. See here for documents on Maybe.
• Use that new constructor in a dummy place and fix crashes. For example, add SharedImmutableString _dummy; after this. When you try to run the jsshell, you will see assertion failures such as here. We need to teach the code how to deal with box_ being null and add an "if (box)" around certain code.
• Add an "operator bool" to SharedImmutableString similar to here. This will let us write more natural code later.
• Replace uses of Maybe<SharedImmutableString> with just SharedImmutableString (and the same for SharedImmutableTwoByteString) here. You will also need to change our getOrCreate function we have been working on to stop returning a Maybe type.

(Once again, I recommend doing one step at a time and trying to get it to compile)

Comment 5

[Dorel Luca [:dluca]]

https://hg.mozilla.org/mozilla-central/rev/2102776da1b2