Down the Golang Nil Rabbit Hole

Re:Down the Golang Nil Rabbit Hole by morelisp on Tuesday March 30, 2021 @02:12PM (#26638158)

So again: Allocations of regions of memory representing values, not variables, are what does or does not get promoted. If `values` is an array of strings, do you think the following makes a new string on the heap (that is, a new 16 byte heap allocation, internally pointing to the same variable-sized region of memory)?

   for _, v := range values {
        go func(v string) {
            fmt.Println(v)
            done <- true
        }(v)
    }

No, the 16 bytes will be copied onto the goroutine's stack.

Do you think it's smart enough to do the trivial transformation of the first into the second?

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Re:Down the Golang Nil Rabbit Hole by throwaway894345 on Tuesday March 30, 2021 @05:39PM (#26640895)

> So again: Allocations of regions of memory representing values, not variables, are what does or does not get promoted.

Right, I understand the difference between a variable and a region of memory.

> If `values` is an array of strings, do you think the following makes a new string on the heap (that is, a new 16 byte heap allocation, internally pointing to the same variable-sized region of memory)?

No.

> Do you think it's smart enough to do the trivial transformation of the first into the second?

I guess I'm surprised that the semantics for the `v := v` in a loop thing are "this will get allocated somewhere else such that it's guaranteed not to be stomped on by another loop iteration". For example:

    for _, v := range []int{2, 4, 6} {
        v := v
        mut := sync.Mutex{}
        go func() {
            mut.Lock()
            fmt.Println(v)
            mut.Unlock()
            done <- true
        }()
        go func() {
            mut.Lock()
            v = 1
            mut.Unlock()
            done <- true
        }()
    }

If it allocates `v` on the goroutines' stacks, then it won't catch the mutation, so presumably it allocates on the heap at least in that case. In whatever case, the semantics are surprising to me.

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