> For example, manual loop unrolling is seldom a win in Java

Just like practically everywhere else.

Besides, it increases code size, and can decrease performance just for that reason, L1 code cache spills.

Intel CPUs have increasingly improved loop buffers as well. If the loop qualifies, unrolling has no positive effect at all.

Low end microcontrollers are another matter, that's where unrolling can still sometimes yield significant wins.

I think the biggest issue I had with high performance Java is lack of memory alignment control and (at least last time I checked) vector intrinsics. JNI time...