Attempt no. 3 to fix Heap::IsHeapIterable and HeapIterator

Attempt no. 3 to fix Heap::IsHeapIterable and HeapIterator.

Now we remember new space's top pointer after the last GC to find out if there was a new space allocation since the last GC.

Unfortunately, this not completely safe - the debugger has a callback hook (that can call to JS) at the end of the GC epilogue that can in theory allocate and possibly make the heap non-iterable. We can only hope this does not happen.

BUG=373283
R=hpayer@chromium.org
LOG=N

Committed: https://code.google.com/p/v8/source/detail?r=21431

[Hannes Payer (out of office), 2014-05-22 08:41:52]

LGTM

[Jarin, 2014-05-22 11:13:48]

Committed patchset #3 manually as r21431 (presubmit successful).

[yurys, 2014-05-22 11:24:49]

https://codereview.chromium.org/291193005/diff/40001/src/heap-profiler.cc
File src/heap-profiler.cc (left):

https://codereview.chromium.org/291193005/diff/40001/src/heap-profiler.cc#old...
src/heap-profiler.cc:176: heap()->CollectAllGarbage(Heap::kMakeHeapIterableMask,
Does this mean that we don't need to call GC anymore in order to make heap
iterable? If this is the case we could speed up heap profiler in a few places.

[Jarin, 2014-05-22 11:29:37]

On 2014/05/22 11:24:49, yurys wrote:
> https://codereview.chromium.org/291193005/diff/40001/src/heap-profiler.cc
> File src/heap-profiler.cc (left):
> 
>
https://codereview.chromium.org/291193005/diff/40001/src/heap-profiler.cc#old...
> src/heap-profiler.cc:176:
heap()->CollectAllGarbage(Heap::kMakeHeapIterableMask,
> Does this mean that we don't need to call GC anymore in order to make heap
> iterable? If this is the case we could speed up heap profiler in a few places.

Unfortunately no, the iterator now calls the GC in its constructor.

[yurys, 2014-05-22 11:31:38]

On 2014/05/22 11:29:37, jarin wrote:
> On 2014/05/22 11:24:49, yurys wrote:
> > https://codereview.chromium.org/291193005/diff/40001/src/heap-profiler.cc
> > File src/heap-profiler.cc (left):
> > 
> >
>
https://codereview.chromium.org/291193005/diff/40001/src/heap-profiler.cc#old...
> > src/heap-profiler.cc:176:
> heap()->CollectAllGarbage(Heap::kMakeHeapIterableMask,
> > Does this mean that we don't need to call GC anymore in order to make heap
> > iterable? If this is the case we could speed up heap profiler in a few
places.
> 
> Unfortunately no, the iterator now calls the GC in its constructor.

I see, thanks for clarifying.