Here's a transcript of the behavior. I don't know for sure that reflection is being done, but the performance penalty (about 1300x) suggests it.
user=> (use 'criterium.core)
nil
user=> (def b (make-array Double/TYPE 1000 1000))
#'user/b
user=> (quick-bench (aget ^"[[D" b 304 175))
WARNING: Final GC required 3.5198021166354323 % of runtime
WARNING: Final GC required 29.172288684474303 % of runtime
Evaluation count : 63558 in 6 samples of 10593 calls.
Execution time mean : 9.457308 µs
Execution time std-deviation : 126.220954 ns
Execution time lower quantile : 9.344450 µs ( 2.5%)
Execution time upper quantile : 9.629202 µs (97.5%)
Overhead used : 2.477107 ns
One workaround is to use multiple agets.
user=> (quick-bench (aget ^"[D" (aget ^"[[D" b 304) 175))
WARNING: Final GC required 40.59820310542545 % of runtime
Evaluation count : 62135436 in 6 samples of 10355906 calls.
Execution time mean : 6.999273 ns
Execution time std-deviation : 0.112703 ns
Execution time lower quantile : 6.817782 ns ( 2.5%)
Execution time upper quantile : 7.113845 ns (97.5%)
Overhead used : 2.477107 ns
*Cause:* The inlined version only applies to arity 2, and otherwise it reflects.
