More efficient use of space in AllocationSite.

More efficient use of space in AllocationSite.

We can eliminate one word from the object by sharing the pretenuring decision and the found memento count.

R=bmeurer@chromium.org, hpayer@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=18554

[mvstanton, 2014-01-09 17:42:36]

Hi Hannes, hi Benedikt,
Have a quick look.
Thanks,
--Michael

[Hannes Payer (out of office), 2014-01-10 08:44:11]

We would need static asserts in heap to verify that the counters do not overflow
in the worst case.

https://codereview.chromium.org/132063004/diff/1/src/flag-definitions.h
File src/flag-definitions.h (right):

https://codereview.chromium.org/132063004/diff/1/src/flag-definitions.h#newco...
src/flag-definitions.h:223: DEFINE_bool(allocation_site_pretenuring, true,
Shouldn't be part of that CL.

[Benedikt Meurer, 2014-01-10 09:08:27]

As discussed offline, it is easy to overflow the bits and turn the Smi into a
HeapObject, which would crash the GC.

[mvstanton, 2014-01-10 10:16:17]

Thanks for the initial look. Yep, we need 22 bits to count all the mementos we
could theoretically create before another gc. So I've just shared the pretenure
decision field with the memento found count, and allowed memento create count to
have a whole smi to play with. PTAL,
--Michael

[Benedikt Meurer, 2014-01-10 10:31:43]

LGTM

[Hannes Payer (out of office), 2014-01-10 11:17:28]

LGTM, one nit. Please land this CL after the local pretenuring CL.

https://codereview.chromium.org/132063004/diff/130001/src/objects-inl.h
File src/objects-inl.h (right):

https://codereview.chromium.org/132063004/diff/130001/src/objects-inl.h#newco...
src/objects-inl.h:51: #include "objects-visiting.h"
alphabetic order

[mvstanton, 2014-01-13 10:28:10]

Committed patchset #3 manually as r18554 (presubmit successful).