On Sunday 30 August 2009 20:43:17 Yaron Minsky wrote:
> Float refs are not unboxed automatically, because refs
> Are polymorphic containers. If you create your own pseudo-ref, i.e., a
> record with a single mutable float field, then I believe you should
> get the behaviour you expect.
I believe you are talking at cross purposes. Will wants his accumulator in a
register. You are referring to float references in data structures being
boxed because records are boxed.
Look at the compiled forms of the inner loops of these dot products, for
example:
let dot a b =
let x = ref 0.0 in
for i=0 to Array.length a - 1 do
x := !x +. a.(i) *. b.(i)
done;
!x
.L101:
.L103: movl caml_young_ptr, %eax
subl $12, %eax
movl %eax, caml_young_ptr
cmpl caml_young_limit, %eax
jb .L104
leal 4(%eax), %eax
movl $2301, -4(%eax)
fldl -4(%edi, %ecx, 4)
fmull -4(%ebx, %ecx, 4)
faddl (%esi)
fstpl (%eax)
movl %eax, %esi
movl %ecx, %eax
addl $2, %ecx
cmpl %edx, %eax
jne .L101
let dot2 a b =
let x = ref 0.0 in
for i=0 to Array.length a - 1 do
x := !x +. a.(i) *. b.(i)
done;
1.0 *. !x
.L107:
fldl -4(%eax, %ecx, 4)
fmull -4(%ebx, %ecx, 4)
faddl 0(%esp)
fstpl 0(%esp)
movl %ecx, %edx
addl $2, %ecx
cmpl %esi, %edx
jne .L107
In the latter case, "x" is unboxed into a register.
> Come to think of it, I wonder if it would be better to implement ref
> on top of a single-cell array, since then everyone would get the float
> unboxing whenever applicable. I imagine there is some runtime overhead
> to this, though.
All-float (including one-float) records are unboxed anyway.
Boxing was discussed in the book OCaml for Scientists and the OCaml Journal
articles about optimization and the SciMark2 benchmark.
--
Dr Jon Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?e
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