632020 - Module pattern can inhibit bytecode-level constant-folding and dead code elimination

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Jason Orendorff [:jorendorff]]

Paste this into a debug shell:
    dis(); (function () { return 3 * 4; }());  // constant-folding
    dis(); (function () { return 3 * 4; })();  // no constant-folding

Likewise:
    dis(); (function () { if (0) f(); }());   // DCE
    dis(); (function () { if (0) f(); })();   // no DCE

I don't know that it matters much, since both JITs do their own
optimizations. If we're stuck in the interpreter, you can of course observe
the difference.

$ ./js    # note: no -m, no -j
js> var t = Date.now(); (function () { for (var i = 0; i < 10*10*10*10*10*10; i++);}()); print(Date.now()-t);
290
js> var t = Date.now(); (function () { for (var i = 0; i < 10*10*10*10*10*10; i++);})(); print(Date.now()-t);
1107

Comment 2

[André Bargull [:anba]]

Neither `(function(){ ... })()` nor `(function(){ ... }())` is currently constant folded.

`(function(){ ... })()` is ignored in FoldCall because the callable component is parenthesized: https://dxr.mozilla.org/mozilla-central/rev/1f4cf75c894862cf3634d6014d8de9c807a054a7/js/src/frontend/FoldConstants.cpp#1585

The function in `(function(){ ... }())` is marked as parenthesized and then ignored in FoldCall just like the other one: https://dxr.mozilla.org/mozilla-central/rev/1f4cf75c894862cf3634d6014d8de9c807a054a7/js/src/frontend/FullParseHandler.h#799

Comment 4

[Jan de Mooij [:jandem]]

Attachment: Bug 632020 - Allow constant folding to see inside functions when using the module pattern. r?jwalden

Comment 5

[Jeff Walden [:Waldo]]

Comment on attachment 9012141 [details]
Bug 632020 - Allow constant folding to see inside functions when using the module pattern. r?jwalden

Jeff Walden [:Waldo] has approved the revision.

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/abd419a57735
Allow constant folding to see inside functions when using the module pattern. r=jwalden

Comment 7

[Raul Gurzau (:RaulG)]

https://hg.mozilla.org/mozilla-central/rev/abd419a57735