enumerate allocates for arrays of mutables

[JaredCrean2]

Using enumerate to iterate through array of mutable objects allocates, but using a simple for loop does not:

type MyMutable
  a::Int
end

immutable MyImmutable
  a::Int
end

function sumit(arr)
  val = 0
  for i=1:length(arr)
    val += arr[i].a
  end

  return val
end

function sumit2(arr)
  val = 0
  for (i, obj) in enumerate(arr)
    val += obj.a
  end

  return val
end

nel = 5000
arr = Array(MyMutable, nel)
arr2 = Array(MyImmutable, nel)
for i=1:nel
  arr[i] = MyMutable(i)
  arr2[i] = MyImmutable(i)
end

# warmup
@time sumit(arr)
@time sumit(arr2)
@time sumit2(arr)
@time sumit2(arr2)

println("\nfinal times:")
@time sumit(arr)
println("  mutable explicit for loop @time printed above\n")

@time sumit(arr2)
println("  immutable explicit for loop @time printed above\n")

@time sumit2(arr)
println("  mutable enumerate @time printed above\n")

@time sumit2(arr2)
println("  immutable enumerate @time printed above\n")

results:

final times:
  0.000043 seconds (5 allocations: 176 bytes)
  mutable explicit for loop @time printed above

  0.000009 seconds (5 allocations: 176 bytes)
  immutable explicit for loop @time printed above

  0.000123 seconds (10.01 k allocations: 312.688 KB)
  mutable enumerate @time printed above

  0.000008 seconds (6 allocations: 192 bytes)
  immutable enumerate @time printed above

also, using for obj in arr rather than enumerate does not allocate.

[StefanKarpinski]

This is due to the fact that currently types that contain mutable values must be heap-allocated and values that refer to heap-allocated values must themselves be heap allocated. This is essentially the same as #10899 and #14955 (which, afaict are dups of each other), has the same root cause as #1168 and would be addressed by #11714 or #12205.

@carnaval, what's the status of stack-allocating objects that refer to the heap? Wasn't that one of the major goals of the Green Fairy work?

[yuyichao]

This one doesn't need stack allocation to work. The type inference should be able to elide the allocation. This is one of the left over case in #16021 .

[StefanKarpinski]

Even better that we can solve this without needing to solve those other issues.

[KristofferC]

Dup of #10899 and other similar issues

[KristofferC]

Not a dup according to #23240