Hi again,

To answer the question by myself, I have ran a few tests. It turned
out that specializing Array.blit provides a significant boost, but
doing such an ugly hack was dangerous and unnecessary. I now use the
following standard code:

CAMLprim value caml_float_array_blit(value _src, value _src_off,
                                     value _dst, value _dst_off, value _len) {
  int src_off = Int_val(_src_off) ;
  int dst_off = Int_val(_dst_off) ;
  int len = Int_val(_len) ;
  int i ;
  for (i=0 ; i<len ; i++)
    Store_double_field(_dst,dst_off+i,Double_field(_src,src_off+i)) ;
  return Val_unit ;
}

I got the following timings in seconds for a program doing intensive
blits on float arrays: 0.17 for the ugly hack, 0.19 for the clean C
function, 1.04 for the standard Array.blit.
-- 
David