RyuJit doesn't support inlining P/Invokes whose return type is a struct

[nattress]

In importer.cpp, Compiler::impCanPInvokeInlineCallSite excludes P/Invokes from being emitted inline if they return a struct. This prevents CoreRT from compiling invoke stubs. The actual external method call ends up as a managed direct call instead.

See, ie: System.Console!mincore.GetLargestConsoleWindowSize

[jkotas]

We need to get onto the compMustInlinePInvokeCalli path for CoreRT.

[nattress]

We can force that in JIT startup by setting CorJitFlag.CORJIT_FLG_IL_STUB for p/invokes. That only works for Calli though. We maybe need something CoreRT-specific in that area of the importer to allow it for direct calls.

[jkotas]

We maybe need something CoreRT-specific

Agree. CorJitFlag.CORJIT_FLG_IL_STUB flag flips number of different things ... not sure whether we want all of them ... .

[christianscheuer]

Is there a workaround for this, to prevent the inlining from taking place? [MethodImpl(MethodImplOptions.NoInlining)] doesn't help unfortunately.

[jkotas]

The proper fix should be like a five line change in RyuJIT. I will try to do it in a next several days.

There is a related problem (#158) that causes structs on Unix to not be always passed in registers according to the platform calling convention. If you have PInvokes that return structs, you are likely to hit that one too.