ConstantDynamic in unvisited catch causes 74X slowdown

[Remi Forax]

Hi,
if i'm not wrong in the code you generate the catch block can be reached either by the normal case (no goto in the normal case) and if there is an exception,
usually JITs bail out when they detect this kind of patterns.

There is a simple way to generate efficient bytecodes, generate exactly the same code as javac :)
In your case, you want to generate an equivalent of a try/finally, so the invokedynamic/ldc constantdynamic should be generated twice, one in the normal case and one in the exceptional case with a goto after the normal case to jump over the exception block.

regards,
Rémi

----- Original Message -----
> From: "eirbjo" <eirbjo at gmail.com>
> To: "hotspot compiler" <hotspot-compiler-dev at openjdk.java.net>
> Sent: Tuesday, January 25, 2022 8:19:27 PM
> Subject: ConstantDynamic in unvisited catch causes 74X slowdown

> Hi,
> 
> I'm looking at a JMH benchmark which shows a 74X slowdown when a
> single invokeDynamic instruction is replaced by an equivalent
> ConstantDynamic LDC instruction.
> 
> An interesting side note is that the LDC/invokeDynamic instructions live
> inside a catch block which wraps the method being run.  So the
> LDC/invokeDynamic instructions are actually never executed when running the
> benchmark (because no exception was thrown/catched).
> 
> I was expecting ConstantDynamic to be as fast as (or faster than)
> invokeDynamic, so this one is a bit of a puzzle. Does it make sense that
> simply having a ConstantDynamic instruction present in the code prevents
> some Hotspot optimizations from kicking in?
> 
> I've made a Maven project [1] with a minimal reproducer [2] for the issue.
> (Minimal does not mean small, since there is a good amount of code for
> setting up the instrumentation and class loading magic)
> 
> With Java 17.0.1 on my Intel Macbook pro, the JMH benchmark runs at ~350
> ops/s using ConstantDynamic, and around 27000 ops/s when switching to
> invokeDynamic.
> 
> What is going on here?
> 
> [1] https://github.com/eirbjo/constant-dynamic-cliff
> [2]
> https://github.com/eirbjo/constant-dynamic-cliff/blob/main/src/test/java/com/eirbjo/cpc/ConstantDynamicCliff.java
> 
> Thanks,
> Eirik.