For calculation involving multidimensional array multiplication followed by a sum along first dimension,
    GCC performs the steps separately - the element-by-element array multiplication is completed first.
    Function gfortran_sum_r8 is called next to calculate the sum.
    A better process would be to keep an accumulator updated as the element-by-element array multiplication
    is carried out. This has following benefits:
    i. gfortran_sum_r8 call is eliminated.
    ii. there is no longer a need for temporary array to hold array multiplication result.

    subroutine sum_test(Rx,Ry,Rz,nx,ny)
    implicit none
      integer(kind=kind(1)), intent(in) :: nx,ny
      real(kind=kind(1.0d0)), dimension(nx,ny), intent(in) :: Rx,Ry
      real(kind=kind(1.0d0)), dimension(ny), intent(out) :: Rz

      Rz = sum(Rx * Ry, 1)
    end subroutine sum_test


Other relevant information:
1. Compile flags: -O3 -ffast-math -m64 -march=amdfam10

2. gfortran version: gfortran -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: /tmp/src/gcc-4.3.0/configure --prefix=/opt/amd/gcc-4.3.0 --enable-languages=c,c++,fortran --enable-stage1-checking --with-as=/opt/amd/gcc-4.3.0/bin/as --with-ld=/opt/amd/gcc-4.3.0/bin/ld --with-mpfr=/tmp/install/mpfr-2.3.0 --with-gmp=/tmp/install/gmp-4.2.2
Thread model: posix
gcc version 4.3.1 20080312 (prerelease) (GCC)

3. model name: AMD Phenom(tm) 8650 Triple-Core Processor
4. flags     : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow constant_tsc pni cx16 popcnt lahf_lm cmp_legacy svm extapic cr8_legacy altmovcr8 abm sse4a misalignsse 3dnowprefetch osvw