Allen Wirfs-Brock wrote:
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.
I'm telling you what engines do. Not what they might do. I did
assignment analysis in SpiderMonkey for Firefox 3.6, it was helpful in
its day. I think a bunch has been ripped out because modern JITs don't
need it.
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.
You are not responding to the point I made, cited above: " engines don't
do it currently and feel little pressure to do so. Chicken and egg."
Benchmarks won't cut it if there's no organic developer pressure.
/be
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