Support untagged object pool entries.

Support untagged object pool entries.

The object pool is now implemented in a new VM class (ObjectPool). This allows to store
arbirtrary immediates in the object pool without generating exta tagging code for immediate values
or addresses that can't be represented as raw smis.

The object pool now consists of up to 3 parts: fixed entries, tagged objects, untagged entries.

When generating code is done, the object pool indices of untagged entries are updated
using the assembler fixups.

The GC only traversed tagged objects. Fixed entries are either VM isolate objects or
stub entry points, so they don't need to be visited.

BUG=

[Florian Schneider, 2015-05-26 08:51:52]

PTAL. x64 only. I would like feedback before porting to all platforms since this
is a pretty large CL.

https://codereview.chromium.org/1147303002/diff/20001/runtime/vm/assembler_x6...
File runtime/vm/assembler_x64.cc (right):

https://codereview.chromium.org/1147303002/diff/20001/runtime/vm/assembler_x6...
runtime/vm/assembler_x64.cc:36:
object_pool_.AddFixedObject(Object::null_object());
I'm considering removing all fixed objects / stubs from the object pools. First,
this saves space in places where those are not used. Second, it simplifies would
simplify this new implementation of the constant pool.

[Florian Schneider, 2015-05-26 08:52:29]

fschneider@google.com changed reviewers:
+ srdjan@google.com, vegorov@google.com

[Florian Schneider, 2015-05-26 08:52:29]

[added reviewers]

[srdjan, 2015-05-26 22:36:24]

srdjan@google.com changed reviewers:
+ iposva@google.com

[srdjan, 2015-05-26 22:36:26]

This looks fine to me, others?

https://codereview.chromium.org/1147303002/diff/20001/runtime/vm/assembler_x6...
File runtime/vm/assembler_x64.cc (right):

https://codereview.chromium.org/1147303002/diff/20001/runtime/vm/assembler_x6...
runtime/vm/assembler_x64.cc:36:
object_pool_.AddFixedObject(Object::null_object());
On 2015/05/26 08:51:51, Florian Schneider wrote:
> I'm considering removing all fixed objects / stubs from the object pools.
First,
> this saves space in places where those are not used. Second, it simplifies
would
> simplify this new implementation of the constant pool.

Where would you load them from?