Comments on Meeting Notes

Allen Wirfs-Brock allen at wirfs-brock.com
Tue Dec 4 14:29:38 PST 2012


On Dec 4, 2012, at 1:28 PM, Brendan Eich wrote:

> Mark S. Miller wrote:
>> ...
> 
>>> * It deoptimizes, e.g. a strict-mode function must be optimized to copy
>>> actual parameter values into arguments if it could use the arguments object.
>> 
>> This one I just don't buy at all. In a strict function f, f.arguments
>> is poisoned. f's arguments would only need to be reified if f
>> statically mentions "arguments" or if f has a statically apparent use
>> of the direct eval operator. For all other cases, no arguments object
>> need be created, and therefore no copying is needed.
> 
> Right, that's what I tried to say by "could use the arguments object". Nothing to do with hated f.arguments.
> 
> So every strict function that uses arguments pays a copying price on entry in a naive implementation. Pushing the copies out till just before there might be an aliasing store is an optimization that could be done in a more sophisticated implementation, but engines don't do it currently and feel little pressure to do so. Chicken and egg.

The timing of copying is only an issue if the function actually assigns to a formal parameter.  Such assignments should be pretty easy to (conservatively) statically check for.

So, you should be able to avoid copying on entry if there is no direct eval and no assignments to formal parameters.  A pretty common case.

If there is a reference to arguments (and no assignment to formals), you should only need to copy if a Put is actually made to the arguments object.  This is also rare and potentially optimizable in various ways.

So, I guess I agree with Mark on this one.  Implementations could do a pretty good job of optimizing strict mode function parameters if they thought it was necessary.  It probably needs to be driven by benchmarks.

Allen




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