Overhead of cgo function calls

In some other project I found out that cgo has a significant overhead in calling functions in C. This is a simple benchmark to demonstrate that.

Why function pointers.

An observant reader will note that I'm calling my functions weirdly. This is to defeat the compiler. At the first attempt I did this the overhead of Go function calls was almost zero and that's because the compiler helpfully inlined everything which defeated the whole purpose of the benchmark.

Even with go:noinline the compiler still happily inlines empty functions. Not sure what can be done about it. Making the functions do something requires a non-trivial amount of "something" to make them not inline and the old tricks (like switch{}) don't seem to work anymore. defer still works, but then the benchmark tests defer which is also a bit in Go that isn't very fast.

This is the best I can do and it isn't great, but it shows the magnitude difference at least.

results (1.8)

BenchmarkCFuncall-4    	20000000	        72.2 ns/op
BenchmarkGoFuncall-4   	1000000000	         2.03 ns/op

old results (from 1.6)

BenchmarkCFuncall-4 	10000000	       184 ns/op
BenchmarkGoFuncall-4	1000000000	         2.00 ns/op