Use a walking visitor to traverse JSObject structure.

Use a walking visitor to traverse JSObject structure. The purpose is to prepare for more complex context-dependent walks of the structure, needed for allocation site and pretenuring work. Different visitors can be created that annotate the object in various ways.

BUG=
R=yangguo@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=17001

[mvstanton, 2013-09-27 11:53:59]

Hi Yang, here is the visitor I was talking about.
Thanks,
--Michael

[Yang, 2013-09-30 09:37:25]

LGTM with comment.

Also, I think a few test cases for DeepCopy are a good investment.

https://codereview.chromium.org/25025002/diff/3001/src/objects.cc
File src/objects.cc (right):

https://codereview.chromium.org/25025002/diff/3001/src/objects.cc#newcode5627
src/objects.cc:5627: return StructureWalk(value, this);
Why do you need to pass 'this' explicitly? The copy visitor class inherits (the
non-static) StructureWalk, wouldn't it pass itself implicitly as receiver
anyways?

[Yang, 2013-09-30 09:38:38]

On 2013/09/30 09:37:25, Yang wrote:
> LGTM with comment.
> 
> Also, I think a few test cases for DeepCopy are a good investment.
> 
> https://codereview.chromium.org/25025002/diff/3001/src/objects.cc
> File src/objects.cc (right):
> 
> https://codereview.chromium.org/25025002/diff/3001/src/objects.cc#newcode5627
> src/objects.cc:5627: return StructureWalk(value, this);
> Why do you need to pass 'this' explicitly? The copy visitor class inherits
(the
> non-static) StructureWalk, wouldn't it pass itself implicitly as receiver
> anyways?

Oh and also, this CL increases the stack size per recursion. Do we rely on
DeepCopy to be able to deal with a certain nesting level without throwing stack
overflow?

[mvstanton, 2013-09-30 11:35:50]

Thanks for the look!
1) Addressed the visitor parameter to StructureWalk (it is unneeded)
2) The tests mjsunit/big-[object|array]-literal cover things nicely, including
the stack overflow check. I beefed it up to expect the exact right kind of
exception in the CL.
3) Although the stack depth increases with the virtual dispatch, it didn't have
a noticeable effect on test runs. In release builds, the stack overflow occurs
at a object literal size of 421, on a master build and a build with this CL.
Thanks again,
--Michael

https://codereview.chromium.org/25025002/diff/3001/src/objects.cc
File src/objects.cc (right):

https://codereview.chromium.org/25025002/diff/3001/src/objects.cc#newcode5627
src/objects.cc:5627: return StructureWalk(value, this);
On 2013/09/30 09:37:25, Yang wrote:
> Why do you need to pass 'this' explicitly? The copy visitor class inherits
(the
> non-static) StructureWalk, wouldn't it pass itself implicitly as receiver
> anyways?

thx for that, first StructureWalk was a static function, not a memeber of the
class, this is a leftover artifact. Fixed!

[mvstanton, 2013-09-30 11:37:05]

Committed patchset #4 manually as r17001 (presubmit successful).