[Bug fortran/36842] New: Fortran: Minimize heap allocation of temporary arrays.

[rajiv dot adhikary at amd dot com]

Instead of automatically allocating the temporary array in heap, it would be
wise to perform a few checks to determine a temporary array is actually
required, whether to reserve memory in stack instead etc.

   The code produced by GCC for the following subroutine does the following.
   i.   Start the loop.
   ii.  Malloc memory required to hold Ry(:,n) * Rx(:)
   iii. Perform Ry(:,n)* Rx(:), store result in malloced memory
   iv.  Copy result from malloced memory to Ry(:,n)
   v.   Free malloced memory
   vi.  go to loop start.
   This is very inefficient.

    subroutine malloc_test(Ry, Rx, ny)
    implicit none
      integer(kind=kind(1)), intent(in) :: ny
      real(kind=kind(1.0d0)), dimension(:,:), pointer :: Ry
      real(kind=kind(1.0d0)), dimension(:),  pointer :: Rx
      integer(kind=kind(1)) :: n

      do n = 1,ny
        Ry(:,n) = Ry(:,n) * Rx(:)
      end do
    end subroutine malloc_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


-- 
           Summary: Fortran: Minimize heap allocation of temporary arrays.
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rajiv dot adhikary at amd dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36842