Derived from bug 887016. Currently RegExp object can only be allocated with NewObjectKind==TenuredObject. Allocating short life span RegExp object in Nursery will improve performance. in bug 887016's patch, it improves 10% when calling String.prototype.match/search with pattern string with meta char (like str.match("a+")), that allocates temporary RegExp object. This case seems to be rare, as it doesn't appear in octane/regexp, so this won't block bug 887016 for now. of course there will be more applicable cases :)
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