Chrome thinks 99,999 is drastically different than 100,000

Question

I just ran into a very interesting issue when someone posted a jsperf benchmark that conflicted with a previous, nearly identical, benchmark I ran.

Chrome does something drastically different between these two lines:

new Array(99999);  // jsperf ~50,000 ops/sec new Array(100000); // jsperf ~1,700,000 ops/sec 

benchmarks: http://jsperf.com/newarrayassign/2

I was wondering if anyone has any clue as to what's going on here!

(To clarify, I'm looking for some low-level details on the V8 internals, such as it's using a different data structure with one vs the other and what those structures are)

Answer 1

Just because this sounded pretty interesting, I searched through the V8 codebase for a static defined as 100000, and I found this kInitialMaxFastElementArray var, which is the subsequently used in the builtin ArrayConstructInitializeElements function function. While I'm not a c programmer and don't know the nitty-gritty here, you can see that it's using an if loop to determine if it's smaller than 100,000, and returning at different points based on that.