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Re: food for optimizer developers


On 8/10/2010 9:21 PM, Ralf W. Grosse-Kunstleve wrote:
Most of the time is spent in this function...

void
dlasr(
   str_cref side,
   str_cref pivot,
   str_cref direct,
   int const&  m,
   int const&  n,
   arr_cref<double>  c,
   arr_cref<double>  s,
   arr_ref<double, 2>  a,
   int const&  lda)

in this loop:

         FEM_DOSTEP(j, n - 1, 1, -1) {
           ctemp = c(j);
           stemp = s(j);
           if ((ctemp != one) || (stemp != zero)) {
             FEM_DO(i, 1, m) {
               temp = a(i, j + 1);
               a(i, j + 1) = ctemp * temp - stemp * a(i, j);
               a(i, j) = stemp * temp + ctemp * a(i, j);
             }
           }
         }

a(i, j) is implemented as

T* elems_; // member

     T const&
     operator()(
       ssize_t i1,
       ssize_t i2) const
     {
       return elems_[dims_.index_1d(i1, i2)];
     }

with

   ssize_t all[Ndims]; // member
   ssize_t origin[Ndims]; // member

     size_t
     index_1d(
       ssize_t i1,
       ssize_t i2) const
     {
       return
           (i2 - origin[1]) * all[0]
         + (i1 - origin[0]);
     }

The array pointer is buried as elems_ member in the arr_ref<>  class template.
How can I apply __restrict in this case?

Do you mean you are adding an additional level of functions and hoping for efficient in-lining? Your programming style is elusive, and your insistence on top posting will make this thread difficult to deal with.
The conditional inside the loop likely is even more difficult for C++ to optimize than Fortran. As already discussed, if you don't optimize otherwise, you will need __restrict to overcome aliasing concerns among a,c, and s. If you want efficient C++, you will need a lot of hand optimization, and verification of the effect of each level of obscurity which you add. How is this topic appropriate to gcc mail list?


--
Tim Prince


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