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Optimizing Float Ref's
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Date: -- (:)
From: Will M Farr <farr@M...>
Subject: Optimizing Float Ref's
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