Type specialization / recompilation of functions using "arguments"

[Jim Laskey (Oracle)]

Axel,

I’ve created a bug to track.  May be slow to process because of vacations and JDK9 long march.

https://bugs.openjdk.java.net/browse/JDK-8164477

Cheers,

— Jim


> On Aug 19, 2016, at 11:41 AM, Axel Faust <axel.faust.g at googlemail.com> wrote:
> 
> Hello,
> 
> in my ongoing project to implement an alternative scripting integration to
> the ECM platform Alfresco using Nashorn, I recently ran into a performance
> issue due to constructor / function recompilation due to type
> specialization, particularly if the constructor / function in question is
> accessing the "arguments" object.
> 
> I have tried to simplify this with a test and used "jjs
> --log=recompile:finest" to run it.
> 
> Consider the following script:
> 
> function testArguments() { print(JSON.stringify(arguments)); }
> // simple call tests
> testArguments('test1'); // first access => 2x "parameter type
> specialization"
> testArguments('test1'); // identical parameter => no recompilation
> testArguments('test2'); // identical param type, different value =>
> "parameter type specialization"
> testArguments('test3'); // identical param type, different value =>
> "parameter type specialization"
> // iterative call test
> for (var i = 0; i < 1000; i++) { testArguments('test' + i); } // identical
> param type, different values => only one "parameter type specialization"
> 
> 
> Now I don't quite understand why script function "testArguments" needs to
> be recompiled on each call with the same parameter type but different
> value, and how this is not required when executed within a loop structure.
> The latter is interesting on another level, since a test script for my
> Alfresco-Nashorn integration is running an array-forEach loop where the
> called function is being recompiled on every iteration, while replacing the
> for-loop in the test script above with an array-forEach (pre-filled)
> behaves exactly the same as the for-loop with only a single recompilation.
> Additionally, in the integration test script I even see recompilations for
> identical values (in subsequent runs of the same script).
> 
> Now having a single recompilation wouldn't be so bad, but continous
> recompilation hurts quite a bit. Unfortunately this affects quite a central
> piece in my integration project and thus causes significant overhead via
> ClassLoader.defineClass, Class.getDeclaredFields and method handle / call
> site handling operations that result from the recompilation. Each run of my
> integration-specific test script adds ~10 CompiledFunction instances to the
> ScriptFunctionData.code list for the same function (all with apparently
> identical call site types and invokers, at least judging from
> toString-representations). The persistent code cache doesn't seem to be
> used at all as RecompilableScriptFunctionData.getBest calls
> compileTypeSpecialization in a way that disables its use.
> 
> Is this behaviour something that should be considered "expected" or is it a
> "real" performance bug? If it is expected, is there a general
> recommendation not to use functions that access arguments for
> performance-critical code?
> (I tested with JDK 8u71 and JDK 9 ea+125 - behaviour is the same except JDK
> 9 provides deoptimization log output on 1st call)
> 
> Regards
> Axel Faust