Both arrays and interfaces are heap allocated. Slice is just a pointer to a heap allocated array.

Structure could be stack allocated, but any of it's fields would not if there is anything but a number.

A trivial example:

https://segment.com/blog/allocation-efficiency-in-high-perfo...

    func main() {
            x := 42
            fmt.Println(x)
    }

    ./main.go:7: x escapes to heap

So a trivial interface cast leads to allocation.

Looks like you're right about interfaces (full benchmark source code: https://gist.github.com/weberc2/87d2fdc379065a2765d1c9f490ad...)!

    BenchmarkEscapeInterface-4        50000000   33.3 ns/op  8 B/op  1 allocs/op
    BenchmarkEscapeConcreteValue-4    200000000  9.45 ns/op  0 B/op  0 allocs/op
    BenchmarkEscapeConcretePointer-4  100000000  10.0 ns/op  0 B/op  0 allocs/op

But arrays are stack allocated:

    BenchmarkEscapeArray-4  50000000   21.3 ns/op  0 B/op  0 allocs/op

And structs are stack allocated, as are their fields--even fields that are structs, slices, and strings!:

    BenchmarkEscapeStruct-4  100000000  12.8 ns/op  0 B/op  0 allocs/op

The code:

    type Inner struct {
    	Slice  []int
    	String string
    	Int    int
    }
    
    type Struct struct {
    	Int    int
    	String string
    	Nested Inner
    }
    
    func (s Struct) AddThings() int {
    	return s.Int + len(s.String) + len(s.Nested.Slice) + len(s.Nested.String) +
    		s.Nested.Int
    }
    
    func BenchmarkEscapeStruct(b *testing.B) {
    	for i := 0; i < b.N; i++ {
    		s := Struct{
    			Int:    42,
    			String: "Hello",
    			Nested: Inner{
    				Slice:  []int{0, 1, 2},
    				String: "World!",
    				Int:    42,
    			},
    		}
    		_ = s.AddThings()
    	}
    }

I'm sure your strings are not stack allocated, they are statically allocated (and would be statically alocated in any language). Not sure about arrays, but dynamic arrays should be dynamically allocated do, your arrays are static probably. They would be heap allocated, if you would use make.

It doesn't matter whether they're stack allocated or statically allocated; neither is garbage, contrary to the original claim ("Go generates a lot of garbage except when dealing with numeric code"). The subsequent supporting claims ("structs with non-numeric members are heap-allocated", "struct fields that are not numbers are heap allocated", etc) were false (sometimes non-numeric members are heap allocated, but they're often not allocated and never because they're non-numeric and their container is never heap allocated on the basis of the location of the member data).

I think this matter is sufficiently resolved. Go trades GC throughput for latency and it doesn't need compaction to get good cache properties because it generates much less garbage than traditional GC-based language implementations.

>It doesn't matter whether they're stack allocated or statically allocated

It does. Any language could do static allocation, go is not different from java here, the problem is that in any real code nearly all your strings and arrays would be dynamic, thus heap allocated, as well as interfaces. Consider also that allocations in Go are much more expensive than in java or haskell.

We're talking past each other. My claim was that Go doesn't need compaction as badly as other languages because it generates less garbage. You're refuting that with "yeah, well it still generates some garbage!". Yes, strings and arrays will often be dynamic in practice, but an array of structs in Go is 1 allocation (at most); in other many other languages it would be N allocations.

> Consider also that allocations in Go are much more expensive than in java or haskell.

This is true, but unrelated to cache performance, and it's also not a big deal for the same reason--allocations are rarer in Go.

EDIT:

Consider `[]struct{nested []struct{i int}}`. In Go, this is at most 1 allocation for the outer array and one allocation for each nested array. In Python, C#, Haskell, etc, that's something like one allocation for the outer array, one allocation for each object in the array, one allocation for each nested array in each object, and one allocation for each object in each nested array. This is what I mean when I say Go generates less garbage.

>Consider `[]struct{nested []struct{i int}}`.

A typical example, yeah. I've said about structs of ints already, it's not a common type unfortunately anywhere beyond number crunching, in which go sucks anyway.

In haskell you could have unboxed array with unboxed records. Check Vector.Unboxed.

> I've said about structs of ints already

Yeah, but you were wrong (you said other kinds of structs would escape to the heap). The innermost struct could have a string member and a `*HeapData` member; it wouldn't matter. The difference in number of allocations between Go and others would remain the same. The difference isn't driven by the leaves, it's driven by number of nodes in the object graph; the deeper or wider the tree, the better Go performs relative to other GC languages.

> In haskell you could have unboxed array with unboxed records. Check Vector.Unboxed.

For sure, but in Go "unboxed" is the default (i.e., common, idiomatic); in Haskell it's an optimization.