Improve our simple elimination of hole checks.

Improve our simple elimination of hole checks.

Currently we avoid checking for the hole value after array loads, if the
result is only used by instructions that definitely deoptimize in case
of the hole value (HChange instructions).

This change performs the same procedure for loading from deleteable/read-only
global variable where we can also avoid the check in the same cases.
Committed: http://code.google.com/p/v8/source/detail?r=9453

[Kevin Millikin (Chromium), 2011-09-27 12:01:04]

LGTM, but I have one suggestion for improvement below.

http://codereview.chromium.org/8054008/diff/1/src/hydrogen-instructions.h
File src/hydrogen-instructions.h (right):

http://codereview.chromium.org/8054008/diff/1/src/hydrogen-instructions.h#new...
src/hydrogen-instructions.h:3204: HLoadGlobalCell(Handle<JSGlobalPropertyCell>
cell, bool deleteable_or_const)
I'm not thrilled with the bool.  The terminology is also confusing by now.  We
have a mix of "(not) read only" / "(not) don't delete" in older code and "(not)
writable" / "(not) configurable" in newer code.

One suggestion is to store the PropertyAttributes from the lookup:

new(zone()) HLoadGlobalCell(cell, lookup.GetAttributes())

And then use them in RequiresHoleCheck().

That should take up the same amount of space and be more flexible.

[fschneider, 2011-09-27 12:36:18]

Done. Landing.