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by mike_hearn 820 days ago
Things like that often will be, which is why you generally need to use the JMH harness to do microbenchmarking on the JVM. It uses internal APIs to stop the compiler treating results as dead and eliminating them.

In this case it doesn't happen because to see that the entire operation is dead requires the compiler to inline payload into main, but he says he disabled inlining for that method specifically so it wouldn't happen. Recall that the goal is to see the assembly for a block of code in isolation, not demo what the JVM can do when given free reign.
1 comments
comonoid 820 days ago
Well, inlining is not necessary, good old interprocedure analysis would resolve it too.
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lewurm 820 days ago
Sure, but neither javac (the Java to bytecode compiler) or HotSpot are doing that. The former tries to preserve as much as possible, and for the latter interprocedure analysis is too costly at run-time.
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anonymous-panda 819 days ago
Could javac do the analysis and record it in the bytecode for HotSpot to optimize? Or is this kind of hybrid teamwork not done?
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mike_hearn 819 days ago
It is done, but for this case the problem is partial compilation. For this you'd need methods to be tagged as pure, but that assumption needs to propagate and it could be violated by a library being upgraded.
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