4 ms·

The FFM downcalls in OpenJDK compile down to argument shuffling + a CALL instruction (in "critical" linker mode), i.e. the same machine code gcc/clang would gen

by pron 2y ago

The FFM downcalls in OpenJDK compile down to argument shuffling + a CALL instruction (in "critical" linker mode), i.e. the same machine code gcc/clang would generate for a call from a C program.

neonsunset 2y ago
This is what it is compiled to in .NET[0] today more or less[1]. What does OpenJDK compile these to? (edit: misread as could compile. Hmm, I wonder how much the difference will there be in average FFI cost with newer APIs vs direct calls) [0] Objects that need pinning are pinned(by toggling a bit in object header), byrefs are pinned by simply storing them on the stack, arguments that need marshalling involve calling corresponding marshalling code. That code can allocate intermediate data on heap, on stack or call NativeMemory.Alloc/.Free C-style. [1] Overhead can be further reduced by 1. annotating FFI calls with [SuppressGCTransition] which saves on possible arguments stack spills and GC helper call, replacing the call with a single flag check and optional call into GC in epilog, 2. in NativeAOT, p/invokes can be "direct" which saves on initialization checks and indirections (though they are reduced in JIT as it can bake data directly into codegen after static init has finished on recompilation). This has a tradeoff as system's dynamic loader will be used at application startup instead of regular lazy initialization and 3. direct p/invokes can be upgraded to static linking, which transforms them into direct calls identical to regular C calls save for the same GC flag check in post-condition. This comes with compiling .NET executables and libraries into a single statically linked binary (well, statically linked for the native dependencies the user has opted into linking this way).
int_19h 2y ago
What does it do if you need to pass a struct that contains another struct?
- pron 2y ago
  The same, but you need to define the layout appropriately. The JEP covers the basics: https://openjdk.org/jeps/454 https://openjdk.org/jeps/454. As I explained in another comment [1], we didn't want to trade off performance or limit the runtime, so the API for describing native layouts is more elaborate. [1]: https://news.ycombinator.com/item?id=40303242 https://news.ycombinator.com/item?id=40303242

by pron 2y ago

The FFM downcalls in OpenJDK compile down to argument shuffling + a CALL instruction (in "critical" linker mode), i.e. the same machine code gcc/clang would generate for a call from a C program.

neonsunset 2y ago

This is what it is compiled to in .NET[0] today more or less[1]. What does OpenJDK compile these to? (edit: misread as could compile. Hmm, I wonder how much the difference will there be in average FFI cost with newer APIs vs direct calls) [0] Objects that need pinning are pinned(by toggling a bit in object header), byrefs are pinned by simply storing them on the stack, arguments that need marshalling involve calling corresponding marshalling code. That code can allocate intermediate data on heap, on stack or call NativeMemory.Alloc/.Free C-style. [1] Overhead can be further reduced by 1. annotating FFI calls with [SuppressGCTransition] which saves on possible arguments stack spills and GC helper call, replacing the call with a single flag check and optional call into GC in epilog, 2. in NativeAOT, p/invokes can be "direct" which saves on initialization checks and indirections (though they are reduced in JIT as it can bake data directly into codegen after static init has finished on recompilation). This has a tradeoff as system's dynamic loader will be used at application startup instead of regular lazy initialization and 3. direct p/invokes can be upgraded to static linking, which transforms them into direct calls identical to regular C calls save for the same GC flag check in post-condition. This comes with compiling .NET executables and libraries into a single statically linked binary (well, statically linked for the native dependencies the user has opted into linking this way).

int_19h 2y ago

What does it do if you need to pass a struct that contains another struct?

pron 2y ago
The same, but you need to define the layout appropriately. The JEP covers the basics: https://openjdk.org/jeps/454 https://openjdk.org/jeps/454. As I explained in another comment [1], we didn't want to trade off performance or limit the runtime, so the API for describing native layouts is more elaborate. [1]: https://news.ycombinator.com/item?id=40303242 https://news.ycombinator.com/item?id=40303242