[Bug c/23845] New: missed strength reduction costs performance

[tprince at computer dot org]

Dictation of strength reduction in source is required for adequate performance
on this target.  The same source "optimization" is counter-productive for ia32.

This is a portion of the diff file between 2 versions of
libgfortran/generated/matmul_r8.c.

              for (n = 0; n < count; n++)
                {
!                 abase_n = abase + n * aystride;
                  sum += abase_n[x] * bbase_y[n];
                  sum1 += abase_n[x+1] * bbase_y[n];
--- 199,208 ----
            {
              rtype_name sum = 0, sum1 = 0;
+             index_type nay = 0;
              for (n = 0; n < count; n++)
                {
!                 /* FIXME:  gcc ought to strength reduce from n * aystride */
!                 abase_n = abase + nay;
!                 nay += aystride;
                  sum += abase_n[x] * bbase_y[n];
                  sum1 += abase_n[x+1] * bbase_y[n];

-- 
           Summary: missed strength reduction costs performance
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tprince at computer dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: ia64-unknown-linux-gnu
  GCC host triplet: ia64-unknown-linux-gnu
GCC target triplet: ia64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23845