Java intrinsic functions are mentioned in various places (e.g. here). My understanding is that these are methods that handled with special native code. This seems similar to a JNI method which is also a block of native code.
What is the difference?
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Answer
The JIT knows about intrinsics, so it can inline the relevant machine instruction into the code it’s JITing, and optimize around it as part of a hot loop.
A JNI function is a 100% black box for the compiler, with significant call/return overhead (especially if you use it for just a scalar).
But even if it were just a call to a function like int bitcount(unsigned x){ return __builtin_popcount(x); } that compiled to x86-64 popcnt eax, edi ; ret (x86-64 System V calling convention) the caller (which the JIT compiler is emitting) would still have to assume that all the call-clobbered registers were clobbered. On x86-64, that’s most of the integer registers and all the FP/vector registers. (Just like the cost for an ahead-of-time C++ compiler for calling a black-box function vs. an intrinsic). But I suspect the cost for calling a JNI function includes some extra overhead on top of that.
And of course a call to any unknown function means that variables which were in registers might need to be synced to memory if the JIT compiler can’t prove that nothing else has a reference to them. (Escape analysis.)
Plus, intrinsics mean the JVM understands what the function does, and can optimize through it. e.g. with constant propagation, it knows that popcount(5) = 2 set bits. But with an actual JNI function, it would still have to call it. And every call is a visible side-effect unless there’s some way to declare the function as “pure” so it can CSE.
With heavy inlining, compile time constants are not rare.