James Baker · tech blog

When does Java inline functions?

There are a number of heuristics within Java that decide whether or not a function should be inlined. Some are as follows: If a method has thrown many times, it is inlined. Presumably this is because the try-catch loop can be optimized. If a method A calls a method B many times more frequently (default 20x) than the outer method is called, it is inlined unless its definition is very large (> 335 JVM opcodes).

Why is it so much better when a function can be inlined?

There are two key improvements that inlining can lead to. The first is that it avoids the function call overhead. Calling a function is cheap but not free because various book-keeping must be done (flushing registers to the stack, moving the program counter to the other function, restoring when it completes, etc). Once we’ve inlined a function, these costs disappear.

@ForceInline for thee but not for me?

For the small cases (<=8 elements) I implemented a sorting network using a single vector. As a part of this implementation, I implemented a ‘compare and exchange’ method as follows.

SPECIES_PREFERRED considered harmful?

I primarily tested on my modern desktop, which supports AVX2. At a late stage, I wanted to see how well the code would work with an AVX512 server. I used GCP and requested an Ice Lake architecture processor. The performance crashed; using 512 bit vectors in any part of the code appeared to cause the throughput to drop by a factor of 3.