Make ElementsAccessors more tolerant of varying backing store types

Make ElementsAccessors more tolerant of varying backing store types

This avoids bogus calls to Fixed*Array::cast() when FastElements-backed objects are empty (and thus backed by empty_fixed_array).

Committed: https://code.google.com/p/v8/source/detail?r=13071

[adamk, 2012-11-27 01:35:40]

It seems like it might be cleaner to put these empty_fixed_array checks in
elements.cc instead of objects.cc, but the latter is where they are for other
calls (like Get()).

[Toon Verwaest, 2012-11-27 08:34:00]

I would prefer replacing the BackingStore::cast call with
BackingStore::castOrEmptyFixedArray. Currently castOrEmptyFixedArray only exists
for FixedDoubleArray, but you can introduce them for the other backing stores as
well. Either that, or we postpone casting to the explicit type until we are far
enough down the line to know which path we are in; and/or if the array may be
empty.

Shortcutting for the empty_fixed_array is just too error-prone. I just fixed 3
(~one year old) bugs related to prematurely shortcutting on empty arrays.

If other places in the code explicitly check for empty_fixed_array, we should
probably replace them as well at some point.

[adamk, 2012-11-27 18:27:56]

Good feedback. I've updated this patch to pass FixedArrayBase down further in
GetCapacityImpl and GetAttributsImpl.

[Toon Verwaest, 2012-11-28 08:41:35]

lgtm, landing.