1866499 - camelCase() is 2.8x faster in V8 than SM

Status: NEW

(Core :: JavaScript Engine: JIT, defect, P3)

Description

[Jeff Muizelaar [:jrmuizel]]

// Matches dashed string for camelizing
var rmsPrefix = /^-ms-/,
	rdashAlpha = /-([a-z])/g;

// Used by camelCase as callback to replace()
function fcamelCase( _all, letter ) {
	return letter.toUpperCase();
}

function camelCase( string ) {
	return string.replace( rmsPrefix, "ms-" ).replace( rdashAlpha, fcamelCase );
}

Making Firefox as fast as Chrome on this function would reduce its time by 1.1% on TodoMVC-Backbone.

camelCase is called 714 times during TodoMVC-Backbone. It is called with these strings:
"focusout"
"events"
"display"

The time in the profiles does not include any time spent in fcamelCase (but there are no matches)

FF: https://share.firefox.dev/47qFM8Z
Chrome: https://share.firefox.dev/49KmQ73

Comment 1

[Jan de Mooij [:jandem]]

This has both functional (with a JS callback function) and non-functional replace calls.

I tried to rewrite the non-functional case in C++ in bug 1859521. It showed some small improvements on Speedometer 3 and it was definitely faster than the Baseline code we're executing here, but it was slower than the Ion-compiled version.

Here we spend more time in RegExpGlobalReplaceOptFunc and that will have to stay as self-hosted code because calling into JS is slow from C++. The simplest option for now might be to try to get some of these self-hosted functions into Ion faster.

Comment 2

[Jan de Mooij [:jandem]]

RegExpGlobalReplaceOptFunc gets a maximum warm up count value of ~1482 on Backbone, just short of the Ion threshold.