[Bug fortran/30409] New: [fortran] missed optimization with pure function arguments

["kargl at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

The attached code demonstrates a missed optimization opportunity that
can have a severe impact on code.  Here are the timings for the 2 loops
on 2 GHz pentium4-m processor.

laptop:kargl[208] gfc4x -o z -O2 a.f90
laptop:kargl[209] ./z
 time 1:   5.0741002E-02
 time 2:    31.28215    

For a pure function the arguments are not changed during execution.  Thus,
in the second loop, the 1/y can be hoisted out of the nested loops.  Currently,
gfortran allocates/deallocates a temp array with every iteration on the 
outer loop, and it does the 1/y array operation on every iteration.


-- 
           Summary: [fortran] missed optimization with pure function
                    arguments
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: fortran
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kargl at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30409