The code is faster because it first compute the base addresses and then 
uses post index accesses. For ARM 64, it also copy two values in one 
instruction. Also, the hole nan (double value) used to initialize the array 
is loaded once before the loop and not inside the loop.

To be able to do that within hydrogen, we would need untagged pointer (a 
priori only valid if the code can't deopt or allocate) and a pointer access 
with post increment.

There is no write barriers in the code because, recently, you push a CL 
which always allocate the arrays in the new generation. Before that CL, I 
had a version with write barriers which gave the same results.

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