cmd/compile: Opportunity for de-pessimization in provably const range loops

[thockin]

What does 'go version' print?

go version go1.3 linux/amd64

What steps reproduce the problem?

Using the 2-return 'range' over a slice of structs will copy each struct, even if the
struct is provably not modified in the loop.  This is a premature pessimization that the
compiler could and should eliminate.  Ideally the spec would guarantee this optimization
(a la RVO in C++) so people could count on it and maybe even warn when it is not true.

The for-each construct is much nicer to read and write, but this behavior sort of makes
it a non-starter for large data.  This is discussed in many places on the web, with the
general recommendation of "just iterate on index".  This is unfortunate for
the language.

Alternatively: some syntax to say explicitly "iterate by reference" or by
pointer.

Ian asked me to file this issue.

[ianlancetaylor]

Comment 1:

I don't think any spec or syntax change is needed.  As you say, you can already
guarantee the behaviour you want with more awkward syntax, so I don't think we need to
add another kind of syntax for it.  In general, though, the compiler should generate
better code for this.

Labels changed: added release-none, repo-main, suggested.

[dvyukov]

Comment 2:

Note that for the following loop:
for _, v := range s {
  ...
  foo()
  ...
  use(v)
}
you can't generally prove (w/o global analysis) that it does not modify elements of the
slice. Because foo can be:
func foo() {
  c <- true
}
And the other side of the chan can be:
for i := range s {
  <-c
  s[i] = 0
}

[agnivade]

/cc @rasky for possibility of doing this in the prove pass.

[zdjones]

Could you provide a short example? I can look into whether this would fit into the prove pass.

I don't know much about it, but maybe the copyelim pass would be a possibility?

Would this currenlty be possible earlier on in walkrange (

go/src/cmd/compile/internal/gc/range.go

Line 155 in 9a00e64

func walkrange(n *Node) *Node {

). I'm not sure who that would be.

[rasky]

I don't think this fits prove.