Avoid exposing compiler-allocated temporaries to the debugger

Avoid exposing compiler-allocated temporaries to the debugger

R=yangguo@chromium.org
BUG=

Committed: https://code.google.com/p/v8/source/detail?r=20899

[wingo, 2014-04-22 13:17:13]

PTAL :)  "LocalIsSynthetic" might not be the right name.

[Yang, 2014-04-23 06:58:06]

On 2014/04/22 13:17:13, wingo wrote:
> PTAL :)  "LocalIsSynthetic" might not be the right name.

LGTM. I think the name is fine as long as it's sufficiently commented.

[Sven Panne, 2014-04-23 07:20:19]

On 2014/04/23 06:58:06, Yang wrote:
> On 2014/04/22 13:17:13, wingo wrote:
> > PTAL :)  "LocalIsSynthetic" might not be the right name.
> 
> LGTM. I think the name is fine as long as it's sufficiently commented.

Using the term "synthetic" is a common term for this kind of stuff and perfectly
fine. It is e.g. used in Java's implementation of inner classes, see
http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.8. The
difference is that in the Java class file format the actual name does not
matter, instead of that a separate flag is used. Can we introduce a similar flag
in our case?

[wingo, 2014-04-23 08:11:47]

On 2014/04/23 07:20:19, Sven Panne wrote:
> Using the term "synthetic" is a common term for this kind of stuff and
perfectly
> fine. It is e.g. used in Java's implementation of inner classes, see
> http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.8. The
> difference is that in the Java class file format the actual name does not
> matter, instead of that a separate flag is used. Can we introduce a similar
flag
> in our case?

Such a flag already exists for context-allocated locals but not for
stack-allocated locals (the VariableMode).  We can introduce a flag for stack
locals as well, but it would take up space in ScopeInfo and be redundant with
the dot-prefix which we already have because of eval() and other run-time scope
lookups.  That could be worked around but it seems best (and certainly easiest
;) just to take advantage of the dot-prefix.

[wingo, 2014-04-23 08:58:51]

Committed patchset #1 manually as r20899 (presubmit successful).