Even if with() performance is much better now than in Moz 0.9.4, i'm alost sure that we can do it better. with() is much faster in Mozilla than in Opera or NS4, but still slower than in IE.

Testcase http://alladyn.art.pl/gandalf/MozillaWith/

Reassigning to Kenton; cc'ing Mazie -

Windows 2k SP3, Athlon 750 NS4.8: Direct: 1702 With: 481 Opera 6.03: Direct: 731 With: 721 Mozilla 1.2 trunk (20020909): Direct: 440 With: 391 IE6 SP1: Direct: 410 With: 201

The |a| object is global which triggers a DOM security Check. I have edited his program where |a| is defined locally. Here are the results I get: System: Windows 2000 1.70 GHz Memory 256 MB Global |a| Local |a| IE6 Direct:297 With:156 Direct:219 With:140 Netscape Nav 4.7 Direct:813 With:281 Direct:172 With:297 Optimized JS Shell Direct:125 With:141 Direct:94 With:172 XPCShell Direct:156 With:203 Direct:110 With:187 Mozilla trunk (2002053012)Direct:203 With:203 Direct:109 With:219 Mozilla branch(2002053012)Direct:203 With:204 Direct:109 With:203

Oops. I used an old build. These results are based on today's build. Global |a| Local |a| Netscape Nav 4.7 Direct:813 With:281 Direct:172 With:297 Optimized JS Shell Direct:125 With:141 Direct:94 With:172 XPCShell trunk(2002091806) Direct:141 With:187 Direct:78 With:188 XPCShell branch(2002091806)Direct:140 With:203 Direct:94 With:218 Mozilla trunk (2002091806) Direct:203 With:266 Direct:110 With:203 Mozilla branch(2002091806) Direct:203 With:219 Direct:94 With:203 IE6 Direct:297 With:156 Direct:219 With:140 This seems to confirm the report. With local |a|, Mozilla's direct access is much faster than IE, but the |with| access is slower. In fact, notice that IE's |with| access is faster than its direct access, but with Mozilla it is the reverse. (Even in the JS Shell) Here is a sample test I am running in the JS Shell: test(); function test() { var a={x:0} t0=new Date() while(a.x<200000) { a.x++ } print ("Direct: "+(new Date()-t0)+'\n') a.x=0 t0=new Date() with(a) { while(x<200000) { x++ } } print ("With: "+(new Date()-t0)+'\n') }

Mazie, i unshure, but if i'm understanding Your results, we should create other bug for such big decresment of speed between accesing global variable, and local. Is there anychance to make this difference smaller? (at now it's huge!)

I'm talking with my friend about how scope_chain should look. It's only my imagination based on my experience. I dont know how it works in Mozilla (couldn't find in lxr) and i'm unshure if it'll help You anyway, but here's a link to JS algorythm for this(own vision): http://alladyn.art.pl/gandalf/MozillaWith/JSscope.html Just wanted to be helpfull :)

Zbigniew raises a good question in Comment #6. Let me copy my comments from bug 169531 on this point: ------ You raise a good question about the performance problems involving global variable access in the browser. This is because the Mozilla DOM does a security check every time a global variable is accessed (to guard against malicious code). IE does not do this, so IE is faster on global variable access (but less safe!). Any bug report concerning that issue should be filed against the DOM Level 0 component, not JavaScript Engine. The meta bug 117611 is only for performance questions in the standalone JS Engine, not for DOM issues. Note there is a separate meta bug for the DOM: bug 21762 tracking: poor performance using DHTML ------

Just filled bug 169559.

DOM performance tracking bug is 118933

> In fact, notice that IE's |with| access is faster than its direct access, > but with Mozilla it is the reverse. (Even in the JS Shell) I spoke to Waldemar, who was not surprised when I told him about this. He expected our |with| access to be slower than our direct access. cc'ing him in case he would like to add any comments -

I hope it's not unchangable feature... |with| should be much faster basing on it's whole idea! If it's not, something is crapped.

Zbigniew: Sorry to say, |with| as a language feature is crapped, IMHO -- I regret putting it in the language in the first place. I wonder how IE manages to be faster. Given the benchmark here, from comment #5, perhaps they analyze data flow enough to tell that x always resolves to a.x in the |with| case -- but if so, why can't they do as well for the first, explicitly qualified, a.x case in the test? Ideally, for this little test function, the speed would be the same. But note that |with| in general requires more work, because it introduces dynamic scope issues: in general the compiler can't tell where x will be found in the scope chain, so it has to generate code to search at runtime. That's never the case with a.x, where the only search is the usual prototype-chain lookup for 'x' starting in a. /be

Hmm... I think it's easy. Lets analyze small part of code: document.style.top=100; document.style.left=200; document.style.height=100; document.style.width=300; In this part, in every line, parser has to find "document" in scope, then "style" in "document"'s array, then last part, and rewrite it's value. And this part: with(document.style){ left=100; top=200; height=100; width=300; } Here, "document.style" has to be found only once. Then parser always should start from "document.style" when it's looking for "left"|"top"|"height"|"width". So it's faster, true?

Zbigniew, what you show in comment #14 does not exercise what the test function from comment #5 exercises. Even so, it's not obvious with property caching that the longer document.style.foo form you show in #14 is slower than the with-form you sketch there. Source code costs do not translate to runtime as you might think, always. What's more, no one should write repetitive lengthy property references if he or she cares about performance. Use a variable to hold the next-to-last property's value, and get properties from that object repeatedly (var ds = document.style; ds.foo ...). Anyway, let's focus on the test function that's been measured on IE and Mozilla, so we have a consistent bench-mark, for now. /be

Right. Lets take care about result: "Mozilla branch(2002091806) Direct:94 With:203" from Comment 5. What can we do about this?

Testcase from comment 1 is 404. Any backup? /be

(In reply to comment #17) > Testcase from comment 1 is 404. Any backup? Comment 5? ;-) I believe it's the same code as the original just with a local variable "a". 200000 iterations may not be enough for today's machines, you can use a bigger number by typing "javascript:test(1e6)" in the address bar.

Modern style and standards are against with(); seems like we should waste no complexity on optimizing it.

Well, it's not really in topic, but IMO the only "with" useful implementation is the Python one: http://effbot.org/zone/python-with-statement.htm http://docs.python.org/py3k/reference/compound_stmts.html#the-with-statement