spec: use (*[4]int)(x) to convert slice x into array pointer

[rogpeppe]

Currently, it's possible to convert from an array or array pointer to a slice, but
there's no way of reversing 
this.

A possible syntax could be similar to the current notation for type assertions:

ArrayAssertion  = "." "[" Expression ":" Expression
"]" .

where the operands either side of the ":" are constant expressions.

One motivation for doing this is that using an array pointer allows the compiler to
range check constant 
indices at compile time.

a function like this:

func foo(a []int) int
{
    return a[0] + a[1] + a[2] + a[3];
}

could be turned into:

func foo(a []int) int
{
    b := a.[0:4];
    return b[0] + b[1] + b[2] + b[3];
}

allowing the compiler to check all the bounds once only and give compile-time errors
about out of range 
indices.

[robpike]

Comment 1:

Labels changed: added languagechange.

Status changed to Thinking.

[rsc]

Comment 2:

I think this functionality would be nice.
Personally I would rather not assume
that the compiler can subtract arbitrary
expressions (as in b := a.[0:4]) but instead
say explicitly what type I want:
b := (*[4]int)(a[0:4])
The argument against this is that we hoped
introducing x[:] would let us get rid of the
implicit conversion from *[4]int to slice.
Maybe it still does, but we allow the explicit one.
There are certainly compelling cases (mostly
in low-level things like jpeg or sha1 block
processing) where converting a slice to *[N]int
for some N would eliminate many bounds checks
for cheap.

Owner changed to r...@golang.org.

[rogpeppe]

Comment 3:

> b := (*[4]int)(a[0:4])
i thought about this. the problem is that it looks like other type conversions, and
currently no 
type conversion can fail at runtime.
actually, i can't see any reason why we couldn't just use the normal type coercion
syntax:
b := a[0:4]).(*[4]int)

[rsc]

Comment 4:

Yes, that's a disadvantage of (*[4]int)(a[0:4]).
A disadvantage of a[0:4].(*[4]int) is that it takes over
syntax currently reserved for interface values.  At one
point conversion syntax and type guard syntax was 
interchangeable.  It clarified things quite a bit
to require that in x.(T), x must be an interface value
and that in T(x), the conversion must be statically
guaranteed to succeed.
Unfortunately, this particular conversion doesn't fit 
into either of those categories.
We've got enough going on that's it going to be a while
before we do anything with this.

[rogpeppe]

Comment 5:

I just encountered a nice example of when this functionality might be useful. To quote
from a reddit user on why Go "needs" pointer arithmetic: 
"One well-used example is making classes as small as possible for tree nodes or linked
list nodes so you can cram as many of them into L1 cache lines as possible. This is done
by each node having a single pointer to a left sub-node, and the right sub-node being
accessed by the pointer to the left sub-node + 1. This saves the 8-bytes for the
right-node pointer. To do this you have to pre-allocate all the nodes in a vector or
array so they're laid out in memory sequentially, but it's worth it when you need it for
performance. (This also has the added benefit of the prefetchers being able to help
things along performance-wise - at least in the linked list case).''
You can *almost* do this in Go with 
   type node struct {
      value int
      children *[2]node
   }
except that there's no way of getting a *[2]node from the underlying slice.

[gopherbot]

Comment 6 by nmessenger:

If neither syntax is ideal, perhaps a new unslice builtin?
    ary, ok := unslice([n]T, slc)
Though should ary have type [n]T or *[n]T? If n is large and the unslice fails, a large
zeroed array might not be ideal. Anything wrong with this that I'm not seeing? Well,
besides it being another new builtin.

[gopherbot]

Comment 7 by robpike:

This is a big deal because ary would not have static type.

[rsc]

Comment 8:

If we were going to do it - which is far from even up for debate - I think
the syntax x.(*[10]int) works well (x has type []int).  You can't .(T) a slice
type right now, so it would not be overloading anything, and like an
interface type assertion it can fail or be checked at run time.
You can even think of it as []int containing a *[10]int the same way
an interface value holds a concrete type, and you're extracting the
underlying array pointer.
That said, I don't think this is important enough to worry about now.
There is enough else going on.

[rsc]

Comment 9:

Labels changed: added priority-later.

[mdempsky]

Comment 11:

Since this is something that's been bugging me lately too, I thought I'd add a few
random thoughts I had that don't seem to have been mentioned:
Allowing conversions from slices to array-pointers means pointers can now refer to
partially overlapping objects.  I don't believe that's currently possible in the
language.  Slices can already overlap though, so it's not a big change overall.
Like #8 says, []T is sort-of an interface type for *[N]T, so type assertions are
arguably suitable syntax.  Except that cap(x.(*[N]T)) might give a different value than
cap(x), which isn't true for other interfaces/implementor-type relations.  Seems like an
open question whether this inconsistency is worth accepting into the language, and
really since there's already a way to convert a *[N]T into a []T, just the ability to
turn a []T into a *[N]T is the relevant missing feature.
It would be nice if an expression like x[e1:e2] could actually have a static type of
*[e2-e1]T (assuming e1 and e2 are constant expressions), then you could write something
like *dst[16:24] = *src[136:144] and the compiler can verify that the array bounds match
up.  Unfortunately, the expression can't actually be x[e1:e2] since existing code might
rely on cap(x[e1:e2]) == cap(x)-e1, and that would be a backwards incompatible change. 
The x.[e1:e2] syntax suggested originally would solve this issue.
If you want a range like x.[n:n+4], instead of requiring the language to recognize this
pattern somehow, x[n:].[:4] is equivalent and has static indices at the expense of
clunkier notation.  A short-hand notation like x.[n:+4] might be nice to indicate that 4
is a length not an end position, but not strictly necessary and complicates the
language.  (Also +4 here is technically ambiguous here whether it's length 4 or end
position "+4", so again some new notation would be necessary.)

[gopherbot]

Comment 12 by peter.waller:

I just want to note that we have a use case for this at go-gl. More information:
go-gl/gl#111
The underlying OpenGL API only accepts the equivalent of, e.g, a *[4]float32, so it is
nice to have this in the type system on our side. OTOH, a consumer of this API might be
holding a []float32 they want to pass to us. So it would be great to find a solution to
this, as the current solutions are a bit messy or require the use of unsafe.

[rsc]

Comment 13:

Labels changed: added go1.3maybe.

[rsc]

Comment 14:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 15:

Labels changed: added repo-main.

[ncw]

Comment 16:

An alternative which occurred to me was in a function which only indexes a slice with
constants values (eg the JPEG routines or unrolled FFTs which are what I'm working on)
and the slice doesn't change the compiler could bounds check the slice just once at the
start of the function with min(constants) and max(constants).
This would achieve the removal of the bounds checking without a language change.  It
wouldn't allow the compiler to do range checking at compile time though.