744609 - GC: expose BufferableRef API

Status: RESOLVED INVALID

(Core :: JavaScript Engine, defect)

Description

[Terrence Cole [:terrence]]

There are 4 write buffers with known types: pointer-to-id, pointer-to-value, pointer-to-object, and reference-to-slot.  The GC will know the internal details of these and make them fast, because they are the common cases.  There is also a 5th, catch-all "general" write buffer that stores the long-tail in a slower, GC-opaque format.  Items storable in the generic write buffer need to derive from the pure-virtual class BufferableRef.

Even though we won't have a real writebuffer implementation in-tree for awhile yet, it would be very nice to have the places where we need manual post barriers in-tree because:
  - They won't get broken or lost.
  - In many cases I'm going to be removing automatic post barriers when I add manual post barriers: this way the reviews can cover both changes at once.
  - It will be possible for SpiderMonkey features developed in the next few months to be "generational-ready" (I'll skip the logo requirements) without coordinating with me specifically.

Comment 1

[Terrence Cole [:terrence]]

Attachment: v0

Comment 2

[Bill McCloskey [inactive unless it's an emergency] (:billm)]

Comment on attachment 614479 [details] [diff] [review]
v0

Review of attachment 614479 [details] [diff] [review]:
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Could you add something to the comment so people know that it's for generational GC?

::: js/src/gc/StoreBuffer.h
@@ +16,5 @@
> +namespace js {
> +namespace gc {
> +
> +/*
> + * Represents an abstract reference.  Entries in the store buffer that cannot be

One space after the period.

@@ +17,5 @@
> +namespace gc {
> +
> +/*
> + * Represents an abstract reference.  Entries in the store buffer that cannot be
> + * represented with the simple pointer-to-a-pointer scheme, must derive from

No comma here.

Comment 3

[Terrence Cole [:terrence]]

Thanks for taking a look at this! I've updated my atrocious grammar. I see a way now where we won't need to expose BufferableRef outside of #ifdef JSGC_GENERATIONAL, so I'm going to fold this into the generational mega-patch.