[stubs] Further optimize Load/StoreGlobalViaContext stubs.

[stubs] Further optimize Load/StoreGlobalViaContext stubs.

a) Don't use separate read/write slots for context globals, but share the
   same slot, which reduces the number of initial misses, and also saves
   some memory for large scripts.
b) No need to pass the name explicitly to the stubs; the runtime can
   extract the name from the ScopeInfo (the extension of the
   ScriptContext) on-demand easily without any performance impact.

R=ishell@chromium.org

[Benedikt Meurer, 2015-07-24 11:29:56]

Hey Igor,

This is the next step we talked about.
Please take a look.

ARM/MIPS people: Please port these changes.

Thanks,
Benedikt

[balazs.kilvady, 2015-07-24 13:59:24]

> ARM/MIPS people: Please port these changes.
We started to work on the MIPS/MIPS64 ports.

[Igor Sheludko, 2015-07-25 16:50:34]

sgtm for b), but I'm not sure about a) because it could cause penalties for
loading from variables that are read-only fields (see 
https://codereview.chromium.org/1259853002/ )

Anyway, I would prefer to have two separate CLs for easy reverting. ))

https://codereview.chromium.org/1250413002/diff/20001/src/flag-definitions.h
File src/flag-definitions.h (right):

https://codereview.chromium.org/1250413002/diff/20001/src/flag-definitions.h#...
src/flag-definitions.h:678: DEFINE_BOOL(global_var_shortcuts, true, "use ic-less
global loads and stores")
Already enabled.

[paul.l..., 2015-07-26 18:58:03]

On 2015/07/25 16:50:34, Igor Sheludko wrote:
> sgtm for b), but I'm not sure about a) because .....

The MIPS port for combined a & b is done by Balázs in
https://codereview.chromium.org/1255953002/

If you do decide to split this up, we can rework it as needed.

[Benedikt Meurer, 2015-07-27 11:00:16]

On 2015/07/26 18:58:03, paul.l... wrote:
> On 2015/07/25 16:50:34, Igor Sheludko wrote:
> > sgtm for b), but I'm not sure about a) because .....
> 
> The MIPS port for combined a & b is done by Balázs in
> https://codereview.chromium.org/1255953002/
> 
> If you do decide to split this up, we can rework it as needed.

No need to split. Thanks for the port.