Encapsulate megamorphic load/tail-call in hydrogen

To aid vector-based load ic work, we need to be able to handle
the megamorphic load case in hydrogen. A simple approach is to
wrap the probe activity in a hydrogen instruction.

The instruction is novel in that it always tail-calls away.

R=yangguo@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=23772

[mvstanton, 2014-09-05 09:36:11]

Patchset #4 (id:60001) has been deleted

[mvstanton, 2014-09-05 11:46:54]

Patchset #5 (id:100001) has been deleted

[mvstanton, 2014-09-05 11:50:18]

mvstanton@chromium.org changed reviewers:
+ yangguo@chromium.org
- verwaest@chromium.org

[mvstanton, 2014-09-05 11:51:42]

Hi Yang,
Thanks for the look at this one. I've verified perf neutrality across
benchmarks. Thanks again,
--Michael

[Yang, 2014-09-08 08:42:47]

On 2014/09/05 11:51:42, mvstanton wrote:
> Hi Yang,
> Thanks for the look at this one. I've verified perf neutrality across
> benchmarks. Thanks again,
> --Michael

lgtm.

[Yang, 2014-09-08 08:43:01]

... with a comment though.

https://codereview.chromium.org/535873002/diff/120001/src/code-stubs.h
File src/code-stubs.h (right):

https://codereview.chromium.org/535873002/diff/120001/src/code-stubs.h#newcod...
src/code-stubs.h:1990: virtual Major MajorKey() const OVERRIDE { return
MegamorphicLoad; }
Please rebase this CL and use the new DEFINE_HYDROGEN_CODE_STUB macro.

[mvstanton, 2014-09-08 12:35:20]

Updated, thanks very much!
--Michael

https://codereview.chromium.org/535873002/diff/120001/src/code-stubs.h
File src/code-stubs.h (right):

https://codereview.chromium.org/535873002/diff/120001/src/code-stubs.h#newcod...
src/code-stubs.h:1990: virtual Major MajorKey() const OVERRIDE { return
MegamorphicLoad; }
On 2014/09/08 08:43:00, Yang wrote:
> Please rebase this CL and use the new DEFINE_HYDROGEN_CODE_STUB macro.

Done.

[mvstanton, 2014-09-08 12:51:54]

Committed patchset #6 (id:140001) manually as 23772 (presubmit successful).