WIP: PoC of using a bit of generics and less reflection

[Zyko0]

Not meant as a merge candidate, just a PoC

Context: ebitengine#purego (discord server)

This is a proof of concept of a generic alternative for RegisterFunc as functions ranging from RegisterFunc0_0 (0 input, 0 output) to RegisterFunc9_1 (9 inputs, 1 output).
The goal is to remove a good amount of reflection where it's not needed, but also to remove the need of reflect.MakeFunc which returns a heavy function that has plenty of overhead at runtime.

There are tests and benchmarks for a few functions of the libc in func_test.go

Difference for strlen:

Syscall9:       11762629 - 100.5 ns/op - 32 B/op  - 1 allocs/op
RegisterFunc:    2411634 - 490.4 ns/op - 120 B/op - 6 allocs/op
RegisterFunc11:  7690965 - 157.0 ns/op - 176 B/op - 2 allocs/op

Notes:

• This approach might generate heavier go binaries due to the (abuse of) generics
• The syscallStack types are really weird in their implementations, but can be swapped for something else implementing the same interface
• purego.Symbol should probably be named differently or not be exposed this way
• There are lots of type cast and type assertions (unchecked) but they should be safe because the type is always known
• I couldn't implement some arguments or return value's types, so there's still runtime reflection in the generated function for those as a fallback
• Not all RegisterFunc have been implemented yet
• newSyscallStack() causes an allocation everytime in each runtime function as it escapes, couldn't find a way to remove it so far
• /!\ Only tested on Windows
• /!\ It needs way more tests to assess that it's okay
• /!\ I got rid of all the runtime.KeepAlive stuff because I couldn't find a proper way to test their need

Benchmark results on my machine (Windows)

goos: windows
goarch: amd64
pkg: github.com/ebitengine/purego
cpu: AMD Ryzen 7 3800X 8-Core Processor
Benchmark_NewCallBack/RegisterFunc(original)-16         	 1304842	       918.7 ns/op	     328 B/op	      12 allocs/op
Benchmark_NewCallBack/RegisterFunc9_1(new)-16           	 3216594	       373.6 ns/op	     144 B/op	       1 allocs/op
Benchmark_qsort/RegisterFunc(original)-16               	  599888	      2022 ns/op	     264 B/op	       6 allocs/op
Benchmark_qsort/RegisterFunc1_0(new)-16                 	  666532	      1788 ns/op	     296 B/op	       4 allocs/op
Benchmark_strlen/RegisterFunc(original)-16              	 2451050	       491.5 ns/op	     120 B/op	       6 allocs/op
Benchmark_strlen/RegisterFunc1_1(new)-16                	 7641933	       158.3 ns/op	     176 B/op	       2 allocs/op
Benchmark_strlen/SyscallN-16                            	 8331891	       143.2 ns/op	     112 B/op	       2 allocs/op
Benchmark_cos/RegisterFunc(original)-16                 	 3273643	       367.5 ns/op	      64 B/op	       4 allocs/op
Benchmark_cos/RegisterFunc1_1(new)-16                   	 9373315	       126.0 ns/op	     144 B/op	       1 allocs/op
Benchmark_cos/Go-16                                     	137118896	         8.760 ns/op	       0 B/op	       0 allocs/op
Benchmark_isupper/RegisterFunc(original)-16             	 3233931	       371.6 ns/op	      56 B/op	       4 allocs/op
Benchmark_isupper/RegisterFunc1_1(new)-16               	10167706	       120.4 ns/op	     144 B/op	       1 allocs/op

[MatejMagat305]

is this plan or only example and hypotetical idea?

[TotallyGamerJet]

is this plan or only example and hypotetical idea?

It is just a suggestion to improve performance. There is currently no plans to integrate the ideas in this PR. However that could change in the future