Comment #4 on issue 2306 by [email protected]: Unexpected bounds-check bailout accesing Int8Array
http://code.google.com/p/v8/issues/detail?id=2306
The performance regression comes from a correctness fix for a performance improvement, making it now deopt when a heap object goes into bounds check. It fixes a bug that would just cast the pointer to a Smi and use it as an index into the array. However, this makes it currently overall regress for heap numbers in comparison with the performance of up to mid-july.
I'm working on converting heap numbers back to Smi when used to index into the array, so it won't deopt anymore; and so we'll get the performance back.
Of course it's always useful for you to avoid generating heap numbers in the first place, to improve performance.
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