Comment #105 on issue 164 by [email protected]: Wrong order in Object
properties interation
http://code.google.com/p/v8/issues/detail?id=164
The V8 devs are reading this, and discussing it. You have almost convinced
me,
for example. The problem is that JSObjects and JSArrays are implemented
with the
same code, and changing the behavior of one of them means that every access
to any property of an object or array would need an additional check to see
if the object
is a JSObject or JSArray.
Changing both to keep the insertion order would increase the size of all
large
JSArray objects by a factor of 3, and plenty of application code relies on
using
lots of large arrays.
A solution that might have minimal impact is based on the fact that
elements with
numeric indices are stored in one of two ways. A dense set of elements,
like
elements with keys 1,2,3,4,5,6,7,8,9, and 10, are stored as "fast
elements", which
take one word per element, and have optimized fast methods. A non-dense
set of
elements, like elements 2415436, 5403932, and 3060393, are stored as "slow
elements",
a hash table which includes an insertion order field for each element. The
fact that
this insertion order is maintained, for "slow elements", is because V8 does
maintain
the insertion order for almost all object properties, just not those with
numeric
indices. This is done for compatibility reasons - it just does not go the
final step
to include properties with numeric indices.
If JSObjects only used "slow elements", and the for-in loop used insertion
order for
JSObject slow elements, and sorted order for JSArray slow elements (and fast
elements), then JSObjects could maintain insertion order, without much
slowdown.
The only disadvantage would be if people were using JSObjects like arrays.
A
JSObject with elements 1 through 1000 would now be much slower and bigger.
But, again, it is not just synthetic benchmarks that depend on large
arrays, so we
cannot make all arrays in the system 3 times their current size. V8
implements
JSArray and JSObject the same way, and making Arrays and Objects behave
differently
is hard to do without slowing down everything in the system, because every
use of
something would need to check if it is an array or object. If this change
was easy
to make, it would have been done already.
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