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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