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bmeurer authored
Functions that take mandatory parameters plus a number of optional parameters, that need to be materialized as an Array are quite common. The simplest possible case of this is essentially: function foo(mandatory, ...args) { return args; } Babel translates this to something like: function foo(mandatory) { "use strict"; for (var _len = arguments.length, args = Array(_len > 1 ? _len - 1 : 0), _key = 1; _key < _len; _key++) { args[_key - 1] = arguments[_key]; } return args; } The key to great performance here is to make sure that we don't materialize the (unmapped) arguments object in this case, plus that we have some kind of fast-path for the Array constructor and the initialization loop. This microbenchmark ensures that we have decent performance even in the case where the assignment to args is polymorphic, i.e. the arguments have seen different elements kinds, starting with FAST_HOLEY_ELEMENTS and then FAST_HOLEY_SMI_ELEMENTS. R=yangguo@chromium.org BUG=v8:6262 Review-Url: https://codereview.chromium.org/2823343004 Cr-Commit-Position: refs/heads/master@{#44709}
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