On May 1, 2010, at 3:25 PM, Brendan Eich wrote:
> On May 1, 2010, at 1:40 PM, Oliver Hunt wrote:
>
>> On May 1, 2010, at 1:27 PM, Brendan Eich wrote:
>>>> A simple solution that would allow maintaining the
>>>> above mentioned 'invariant' in most cases and allow breaking it when
>>>> necessary would involve a flag passed into the 'get' handler
>>>> indicating that the property is about to be 'called'. What would be
>>>> the negative ramifications of such a solution (asides from the fact
>>>> that it may get abused - like the 'get' accessor in es5 ;)?
>>>
>>> This is a good idea. In all practical implementations, callee expressions
>>> are specialized so the "get" can tell it is computing a callee. This is
>>> done to bind |this| in the call expression of which the callee expression
>>> is a left prefix.
>>
>> [[Get]] in JavaScriptCore does not distinguish the get for a.b vs a.b(), i
>> like to imagine that we count as being a practical implementation ;)
>
> My mistake, I thought JSC did. How do you compute |this| for the call? I
> mean, |this| binds to whatever |a| evaluated to, and it evaluates only once,
> so *something* differs in how you compile a.b in an expression such as (x =
> a.b) or (a.b + c.d) vs. how you compile a.b in a.b().
We copy |this| in advance. For reference here is the bytecode representation
for the function:
function f(){
var a;
a.b;
}
3 m_instructions; 1 parameter(s); 2 callee register(s)
[ 0] enter
[ 1] get_by_id r1, r0, b(@id0)
[ 9] ret undefined(@k0)
(I'm assuming this is sufficiently obvious to not require any real explanation)
Here's
function f() {
var a;
a.b();
}
5 m_instructions; 1 parameter(s); 11 callee register(s)
[ 0] enter
[ 1] mov r2, r0
copy the variable a from r0 to r2
[ 4] get_by_id r1, r2, b(@id0)
resolve a.b -> r1
[ 12] call r1, r1, 1, 11
Call r1 -- this will be in r2, the instruction arguments are somewhat
"interesting" so i won't go into details as to what they mean
[ 17] ret undefined(@k0)
As you can see there's no difference between how we do the property resolution
for an ordinary lookup vs. a function call.
--Oliver
>
> /be
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