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.
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.
y
On Aug 28, 2009, at 4:32 PM, Will M Farr <f...@mit.edu> wrote:
Hello all,
I'm running OCaml 3.11.1, and I noticed something strange in some
native code for matrix multiply today. The code was
let mmmul store m1 m2 =
let (ni,nk) = dims m1 and
(nk2,nj) = dims m2 and
(sni,snj) = dims store in
assert(nk=nk2);
assert(ni=sni);
assert(nj=snj);
for i = 0 to ni - 1 do
let row1 = m1.(i) and
srow = store.(i) in
for j = 0 to nj - 1 do
let sum = ref 0.0 in (* Un-boxed float ref? *)
for k = 0 to nk - 1 do
let row2 = Array.unsafe_get m2 k in
let x = Array.unsafe_get row1 k and
y = Array.unsafe_get row2 j in
sum := !sum +. x*.y
done;
Array.unsafe_set srow j !sum
done
done;
store
(I compiled with ocamlopt.) It multiplies the matrices (represented
as arrays of arrays of floats) m1 and m2 together and puts the
result into the matrix store. Profiling the code, I noticed a call
to caml_modify during the execution of this function! Turns out
that the culprit was the float ref "sum". Changing to the following
code (which eliminates the float ref, and uses the <- and .( )
operators instead of unsafe_set and unsafe_get) eliminated that
call, and sped things up tremendously:
let mmmul store m1 m2 =
let (ni,nk) = dims m1 and
(nk2,nj) = dims m2 in
for i = 0 to ni - 1 do
let row1 = m1.(i) and
srow = store.(i) in
for j = 0 to nj - 1 do
srow.(j) <- 0.0;
for k = 0 to nk - 1 do
let row2 = Array.unsafe_get m2 k in
let x = row1.(k) and
y = row2.(j) in
srow.(j) <- srow.(j) +. x*.y
done
done
done;
store
But, I thought that float ref's were automatically unboxed by the
compiler when they didn't escape the local context. Is this a
complier bug, is there a bad interaction with unsafe_get and
unsafe_set, or is there something else going on that I don't
understand? Any enlightenment would be appreciated.
Thanks!
Will
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