RFR(XS): JDK-8010941: MinJumpTableSize is set to 18, investigate if that's still optimal

[Aleksey Shipilev]

On 05/21/2013 05:58 PM, Niclas Adlertz wrote:
>> At very least:
>> * mean includes warmup time
> As I see it, and please correct me if I'm wrong, warmup time should only be the first iteration of NUMBER_OF_TEST_EXECUTIONS (when doing 90000000 iterations (NUMBER_OF_ITERATIONS) and the on stack replacement triggers).
> Ignoring the first test_time from the first iteration of NUMBER_OF_TEST_EXECUTIONS should be enough?

You can't see beforehand how long is the warmup. Hence, it is
insufficient to throw away first iteration. You should instead make a
few iterations, identify the steady state (= where performance settled),
and derive the metrics from there.

>> * store to $dead enables us to compute only the last iteration, and
>> remove all others
> What if I add to the dead value in each iteration of NUMBER_OF_ITERATIONS; 

Seems to be working to evade DCE. Another problem: different loop
unrolling can skew the results, because the loop can be effectively
pipelined (also, if we were not using doubles, we might have collapsed
the calculations even more, but thanks to double non-associativity, it
is not the issue here).

Also, I begin to wonder if after the multiply_by_power_of_ten inlining
start to affect how far we unroll the loop, since the jump tables are
starting to be larger.

> Please see the updated version of Test.java 
> http://cr.openjdk.java.net/~adlertz/JDK-8010941/update/Test.java

Again, these are only the surface issues, I recommend you to migrate to
JMH and stop guessing where your benchmark can be wrong :) It is a good
exercise anyway.

-Aleksey.