2 ms·

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 yo

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?

throwaway894345 6y ago
> 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.

throwaway894345 6y ago

> 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.