1376101 - Remove some unnecessary read barriers

Status: RESOLVED FIXED

(Core :: JavaScript: GC, enhancement)

Description

[Jon Coppeard (:jonco)]

Attachment: remove-read-barrier

We only need a read barrier for a weak pointer on operations where the pointer can escape.  We don't need one for operations such as calculating the hash or matching an entry in a hash table.

This came up in the context of sweeping hash tables off main thread.

This patch removes a bunch of places this happens and lets us an assertion that read barriers only happen on the main thread.

Comment 1

[Steve Fink [:sfink] [:s:]]

Comment on attachment 8881040 [details] [diff] [review]
remove-read-barrier

Review of attachment 8881040 [details] [diff] [review]:
-----------------------------------------------------------------

Oh, nice. I knew we had a bunch of these; I hadn't realized we were this close to being able to assert.

::: js/src/gc/Barrier.h
@@ +587,5 @@
>          this->post(JS::GCPolicy<T>::initial(), v);
>      }
>  
> +    // Copy is creates a new weak edge but the wrapped pointer does not escape,
> +    // so we no read barrier is necessary.

2 typos (grammaticos?) in the comment.

Comment 2

[Jon Coppeard (:jonco)]

(In reply to Steve Fink [:sfink] [:s:] from comment #1)
> 2 typos (grammaticos?) in the comment.

Ugh, sorry, will fix.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/d53c5407081e
Remove some unnecessary read barriers r=sfink

Comment 4

[Jon Coppeard (:jonco)]

Attachment: bug1376101-remove-more-read-barriers

And another one.

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/4ece199e28df
Remove some more unnecessary read barriers r=sfink

Comment 6

[Carsten Book [:Tomcat]]

https://hg.mozilla.org/mozilla-central/rev/d53c5407081e
https://hg.mozilla.org/mozilla-central/rev/4ece199e28df