compiler: no stack allocation for varargs that don't escape

[eliasnaur]

Test:

package main

import "testing"

func TestContentAllocs(t *testing.T) {
	res := testing.Benchmark(func(b *testing.B) {
		for range b.N {
			callVariadicString()
			callVariadicStruct()
		}
	})
	if a := res.AllocsPerOp(); a > 0 {
		t.Errorf("%d allocs, expected 0", a)
	}
}

var sink bool
var sink2 string
var sink3 SomeStruct

type SomeStruct struct {
	v int
}

func callVariadicStruct() {
	variadic(new(SomeStruct))
}

func callVariadicString() {
	str := "hi"
	if sink {
		str = "somethingelse"
	}
	variadic(str)
}

//go:noinline
func variadic(args ...any) {
	for _, a := range args {
		switch a := a.(type) {
		case string:
			sink2 = a
		case *SomeStruct:
			sink3 = *a
		}
	}
}

Tests

$ go test -gcflags=-l      # disable inlining
PASS
ok  	example.com	1.815s
$ tinygo test -opt 2
--- FAIL: TestContentAllocs (2.18s)
    2 allocs, expected 0
FAIL
FAIL	example.com	2.384s

[aykevl]

Note that variadic parameters are basically just syntactic sugar in Go. This would be equivalent:

func callVariadicStruct() {
	variadic([]any{new(SomeStruct)})
}

func callVariadicString() {
	str := "hi"
	if sink {
		str = "somethingelse"
	}
	variadic([]any{str})
}

//go:noinline
func variadic(args []any) {
	for _, a := range args {
		switch a := a.(type) {
		case string:
			sink2 = a
		case *SomeStruct:
			sink3 = *a
		}
	}
}

So the issue here seems to be that while the []any slice can be stack allocated, a pointer inside an interface inside a slice can't be. I don't know how the Go compiler manages to avoid allocations here, because it's basically an allocation inside an allocation.

(Also, sidenote, AFAIK the Go compiler also supports //go:noinline).