Append consecutive slices, same underlying array

[Nick Patavalis]

Hi,

It seems that the built-in append does not check for a situation like
the following, and needlessly copies data, when it could avoid it.

  func appendBytes(a, b []byte) []byte {
          if len(b) == 0 {
                  return a
          }
	  // Shouldn't append() check for something like this?
          if cap(a) >= len(a)+len(b) && &a[:len(a)+1][len(a)] == &b[0] {
                  return a[:len(a)+len(b)]
          }
          return append(a, b...)
  }

If I'm following the breadcrumbs correctly, they lead to the fact that
runtime-memmove() does *not* short-circuit when source and destination
addresses are the same (I may be wrong, though).

Running the benchmark at:

  https://gist.github.com/npat-efault/f055654633f43d0ef771d38657fd499e

for any value of N, demonstrates this.

Rather marginal (and possibly very contrived) case, but I though I'd
point it out...

/npat

[T L]

yes, you are right. You can file an issue at https://github.com/golang/go/issues

[Nick Patavalis]

Opened issue here: https://github.com/golang/go/issues/17530

/npat