[intrinsics]: performance before after (String::format)

[Claes Redestad]

On 2019-02-23 01:36, Vicente Romero wrote:
> 
> 
> On 2/22/19 4:59 PM, Alex Buckley wrote:
>> On 2/22/2019 1:46 PM, Vicente Romero wrote:
>>> To complete the picture please find attached the performance results for
>>> Objects.hash for a number of experiments. In general they don't look as
>>> good as the ones for String::format. In general it seems like there is
>>> no much gain unless the number of parameters is large and all the
>>> parameters are constants. This is understandable because the compiler
>>> generates an LDC of the result. In all other cases the performance is
>>> just a bit better or a lot worst.
>>
>>                  Intrinsified  Vanilla  Speedup
>> testHash1IntVariable    42564    42799       1x
>> testHash2IntVariables   41573     9019       5x
>> testHash100IntVariables     4       27       0.15x
>>
>> With a large number of parameters, you might hope that avoiding double 
>> boxing (int -> Integer -> array store) gives us some win, even for 
>> non-constant arguments. But something is happening that kills the 
>> speedup, do you know what it is?
> 
> I'm doing some research on this, my assumption is that HS was able to 
> recognize the old pattern but it has issues with the MH graph being 
> generated now. It could be that some nodes in the graph are more opaque. 
> But this is just my opinion

If I were to guess you're hitting some JIT limit - likely inlining-
related - which cause a miscompilation at some point.. I've been
mulling over whether we in general need to build in heuristics into our
BSMs to generate simpler shapes once the number of arguments grow,
e.g., only specialize for the first N arguments and emit a call to
Objects.hash(Object[]) for the remainder.

What value N and how to gracefully downgrade to a simpler implementation
is implementation dependent, and might even be chosen differently
depending on whether you're optimizing for peak performance or
startup/footprint, as a smaller N could reduce potential for a BSM to
emit combinatorially explosivs MH graphs.

/Claes