609075 - speed up JSString::flatten a bit

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Luke Wagner [:luke]]

Attachment: SS harness results

The attached patches tweak JSString::flatten/js_ConcatStrings.  I'm measuring a 1-2% (2-4ms) win on SS and .5% (10ms) win on V8 (SS harness -m).  A micro-benchmark under valgrind shows a 38% decrease in instruction count in flatten.

Comment 1

[Luke Wagner [:luke]]

Attachment: JS_CeilingLog2

Comment 2

[Luke Wagner [:luke]]

Attachment: optimize flatten

Comment 3

[Luke Wagner [:luke]]

I had a third patch that took out the "grow the left string in-place if there is room" case, but I remembered what it was for, so no change to js_ConcatStrings.  (It's for cases like

for (...) {
  x = x + "asdf";
  print(x);  // flatten x
}

where x is flatten on each iteration to avoid O(n^2) copying.  I think this case can be removed in bug 608776 by having flatten() do it.)

Comment 4

[Luke Wagner [:luke]]

Comment on attachment 487650 [details] [diff] [review]
optimize flatten

Andreas, I think you reviewed the initial ropes patch.

Comment 5

[Andreas Gal :gal]

Comment on attachment 487650 [details] [diff] [review]
optimize flatten

Nice profiling job Luke. ++lw

Comment 6

[Luke Wagner [:luke]]

http://hg.mozilla.org/tracemonkey/rev/0ba07bd00178
http://hg.mozilla.org/tracemonkey/rev/9e5cd5815d4d

Comment 7

[Robert Sayre]

http://hg.mozilla.org/mozilla-central/rev/0ba07bd00178