IL.Emit.Tail() produces wrong IL

As tail. is a prefix instruction, the only valid code sequence is tail. call (or calli or callvirt) (see link).

Tail();
Calli(new StandAloneMethodSig(CallingConvention.Cdecl, typeof(void)));

produces

IL_0007: tail.
IL_0009: calli void()

instead of (the expected)

IL_0007: tail. calli void()

(the space after the . is sensitive though).

[gfoidl]

Thanks 👍 for the quick fix!

[ltrzesniewski]

Well that was a pretty stupid oversight: debug builds insert nop instructions between lines of code. Your example compiles to something similar to the following:

IL_0008: tail.
IL_000A: calli     void ()
IL_000F: nop
IL_0010: ret

Which is incorrect because a tail call needs to be immediately followed by a ret.

Non-void calls are even worse:

IL_000B: tail.
IL_000D: calli     int32 (int32)
IL_0012: nop
IL_0013: stloc.0
IL_0014: br.s      IL_0016
IL_0016: ldloc.0
IL_0017: ret

The value goes into a local so the debugger can display it before the function returns, but that gets in the way too.

InlineIL will now validate tail calls and remove the boilerplate code, leaving just ret, but that will fail horribly if you have multiple tail calls in a single method. I’ll decide what to do with this case later. In the meantime, v1.1.1 should fix the issue reported here.