The
SunSpider JavaScript performance benchmark, devised by the developers of the WebKit browser engine, is used and quoted widely as a measure of browser scripting performance. A surprising result was recently noticed by a Mozilla developer, Rob Sayre, looking at Internet Explorer 9's performance in this test. On one of the many subtests it performs, Internet Explorer 9 was finishing the subtest almost instantly.
In and of itself, that's not necessarily very interesting; several of the subtests in SunSpider are near-instant in the browser. However, it
piqued the developer's curiosity. He made some
minor changes to the test—changes that don't alter the result of the calculation the test performs and that, naively at least, should be treated as equivalent—and saw Internet Explorer 9
slow down considerably. He
filed a bug against Internet Explorer.
Sayre's bug report was conservative—he suggested that an optimization that Internet Explorer 9's Chakra JavaScript engine was performing was fragile, and was easily disabled by minor alterations to the code that it should disregard. Coverage earlier today of the same issue was less guarded: Internet Explorer 9 was accused of
cheating in the test. The allegation is that Microsoft has built a specific optimization into Chakra that detects, and bypasses, the specific code in SunSpider, but which has no other purpose. In other words, the optimization will not do anything to improve the browser's performance in any other scenario.
