golang中的片不分配任何内存？

Question

This link: http://research.swtch.com/godata

It says (third paragraph of section Slices):

Because slices are multiword structures, not pointers, the slicing operation does not need to allocate memory, not even for the slice header, which can usually be kept on the stack. This representation makes slices about as cheap to use as passing around explicit pointer and length pairs in C. Go originally represented a slice as a pointer to the structure shown above, but doing so meant that every slice operation allocated a new memory object. Even with a fast allocator, that creates a lot of unnecessary work for the garbage collector, and we found that, as was the case with strings above, programs avoided slicing operations in favor of passing explicit indices. Removing the indirection and the allocation made slices cheap enough to avoid passing explicit indices in most cases.

What...? Why does it not allocate any memory? If it is a multiword structure or a pointer? Does it not need to allocate memory? Then it mentions that it was originally a pointer to that slice structure, and it needed to allocate memory for a new object. Why does it not need to do that now? Very confused

Answer 1

To expand on Pravin Mishra's answer:

the slicing operation does not need to allocate memory.

"Slicing operation" refers to things like s1[x:y] and not slice initialization or make([]int, x). For example:

var s1 = []int{0, 1, 2, 3, 4, 5} // <<- allocates (or put on stack)
s2 := s1[1:3]                    // <<- does not (normally) allocate

That is, the second line is similar to:

type SliceHeader struct {
        Data uintptr
        Len  int
        Cap  int
}
…
example := SliceHeader{&s1[1], 2, 5}

Usually local variables like example get put onto the stack. It's just like if this was done instead of using a struct:

var exampleData            uintptr
var exampleLen, exampleCap int

Those example* variables go onto the stack. Only if the code does return &example or otherFunc(&example) or otherwise allows a pointer to this to escape will the compiler be forced to allocate the struct (or slice header) on the heap.

Then it mentions that it was originally a pointer to that slice structure, and it needed to allocate memory for a new object. Why does it not need to do that now?

Imagine that instead of the above you did:

example2 := &SliceHeader{…same…}
// or
example3 := new(SliceHeader)
example3.Data = …
example3.Len = …
example3.Cap = …

i.e. the type is *SliceHeader rather than SliceHeader. This is effectively what slices used to be (pre Go 1.0) according to what you mention.

It also used to be that both example2 and example3 would have to be allocated on the heap. That is the "memory for a new object" being refered to. I think that now escape analysis will try and put both of these onto the stack as long as the pointer(s) are kept local to the function so it's not as big of an issue anymore. Either way though, avoiding one level of indirection is good, it's almost always faster to copy three ints compared to copying a pointer and dereferencing it repeatedly.