743406 - GC: only set tracing location if we call Mark

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Terrence Cole [:terrence]]

Attachment: v0: fixup an oversite

If we set the trace location, but then don't call mark (for example because !v->isMarkable()), then the location won't be unset and will be wrong for the next mark function we call.

Comment 1

[Terrence Cole [:terrence]]

Attachment: v1: forgot to qref

Sorry about that!

Comment 2

[Terrence Cole [:terrence]]

Comment on attachment 613049 [details] [diff] [review]
v1: forgot to qref

Actually, it seems that this doesn't make much sense for the nested call from MapObject::mark to MarkValueUnbarriered -- in this case we clobber the real address with the stack address.

I'm going to give this some thought over the weekend and try to solve this correctly.

Comment 3

[Terrence Cole [:terrence]]

Attachment: v2: Proven working with the verifier.

Had to broaden the condition allowing us to set realLocation.  It now allows only the outermost call and the reset call after it is used.

Unfortunately, it is still pretty subtle to use correctly: e.g. if you are going to mark a Value, you need to check if it isMarkable before calling JS_SET_TRACING_LOCATION because we only clear after marking currently.  We could add an unconditional reset at the end of MarkValueInternal and MarkIdInternal, but this data flow is rapidly turning into spaghetti.

Comment 4

[Terrence Cole [:terrence]]

This was a lot simpler than I was making it.  Added the trivial fix to:
https://hg.mozilla.org/integration/mozilla-inbound/rev/0cd6ed757687
since it was needed there.