1128644 - Use common offsets for unboxed layouts which are prefixes of each other

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Brian Hackett [Laid off!]]

Attachment: patch

If one object has a constructor like:

function Foo() {
  this.a = ...;
  this.b = ...;
  this.c = ...;
}

And another object has a constructor like:

function Bar() {
  this.b = ...;
  this.a = ...;
}

If both Foo and Bar are unboxed, we have control over the offsets given to each of their properties.  It would be nice if they used the same offsets for their properties that overlap, as if access sites are used with both Foo and Bar objects this would allow us to avoid having to branch on the type of the object.

The attached patch adds a simple variant of this optimization for unboxed objects.  When we make a new unboxed layout, we look at all the other unboxed layouts in the compartment (hopefully there won't be that many) to look for one which is a subset of the one we are creating.  If we find such a layout we use its offsets for the properties common to the two layouts.

This wins about 5% for me on octane-deltablue, when using unboxed objects.

Comment 1

[Brian Hackett [Laid off!]]

Oh, after this patch deltablue is <5% faster with unboxed objects on x86, but about 20% faster on x64.

Comment 2

[Luke Wagner [:luke]]

(In reply to Brian Hackett (:bhackett) from comment #0)
> When we make a new unboxed layout, we look at all the other
> unboxed layouts in the compartment (hopefully there won't be that many) to
> look for one which is a subset of the one we are creating.

If we did see this become a problem (say in a compiled C# codebase that has thousands of classes) then presumably we could do something better than linear search (e.g., by maintaining the layouts in a trie by field/type).

Comment 3

[Brian Hackett [Laid off!]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/358cc96cf83c

Comment 4

[Wes Kocher (:KWierso) (Not reading bugmail; email directly if needed)]

https://hg.mozilla.org/mozilla-central/rev/358cc96cf83c