I regularly read the assembly output of the OCaml compiler I'm using and there are very few surprises. The mapping from source to generated assembly is quite straightforward. You couldn't say the same for Haskell, though. So it depends on which FP language you're using.
Does OCaml give you enough tools to optimize around things like CPU-cache and memory management costs? It's one thing to know what kind of assembly is going to be produced by a block of code, but it's another thing to be able to get the machine to do exactly what you want.
If “to get the machine to do exactly what you want” is an important goal, I'd recommend C or C++. Those are well known tools for those purposes. FP languages are rather about getting more ideas from math into how you structure your code.
I don’t think you need to go that low level. I find it much easier to optimize around things like memory layout, allocation patterns and cache effects even in Java and C# compared to (essentially) any functional language.
I think this is just a feature of imperative languages over functional ones. Functional languages are excellent for many things, but not for this stuff.
Where should I look to learn about doing those things in Java? Last time I tried looking for guidance I mostly found people saying it's not worth the effort since the JVM JIT will do better.
