[ffi] Implement leaf FFI calls

[sjindel-google]

Add support for FFI leaf calls:

• No need to enter/leave safepoint - avoid few locked instructions
• No need to create exit frame or update Thread state
• No need to block all registers; callee-saved registers can still be live across the call
• Replace existing InvokeMathCFunction with FFICall

[mraleph]

Additional benefit is that you can pass some pointers down directly (of course assuming that native side is not going to store them somewhere), e.g. pass an internal pointer into typed data arrays or strings, because you know that GC can't move them. For example this opens possibility to just call memcpy to quickly copy memory from typed array or a string into some other place.

Some informal benchmarking suggests that replacing functions like dart:math's sqrt and sin with FFI native calls, result in a run time of about 200%, compare to the current implementation.

[dcharkes]

Some informal benchmarking suggests that replacing functions like dart:math's sqrt and sin with FFI native calls, result in a run time of about 200%, compare to the current implementation.

That is without implementing leaf-calls I presume @cskau-g.

It might be worth exploring implementing leaf calls. Both for dart:math, as well as for example exposing the inner pointers of TypedData as <...>* in FFI trampolines (StackOverflow).

Exactly. My previous comment was to further back the need for leaf-calls.

[mkustermann]

As discussed in todays meeting we might want to use optional parameter: DynamicLibrary.lookupFunction<..>(..., isLeaf: true) Pointer<NativeFunction<...>>.asFunction(..., isLeaf: true).