[Bug fortran/57071] New: Optimize (-1)**k to 1 - 2 * mod(K, 2)

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57071



             Bug #: 57071

           Summary: Optimize  (-1)**k  to 1 - 2 * mod(K, 2)

    Classification: Unclassified

           Product: gcc

           Version: 4.9.0

            Status: UNCONFIRMED

          Keywords: missed-optimization

          Severity: normal

          Priority: P3

         Component: fortran

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: bur...@gcc.gnu.org

                CC: tkoe...@gcc.gnu.org





Motivated by

https://groups.google.com/forum/?fromgroups=#!topic/comp.lang.fortran/60jYF5XEY3c



The code (k is an integer):

  (-1)**k

and

  (-1.0)**k

can be rather common in numeric code. The former is converted into

  _gfortran_pow_i4_i4 (-1, *k);

the latter is

  __builtin_powif (-1.0e+0, *k);



However, for (-1)**k, the result is simply 1 is k is even and -1 if it is odd,

or in other words:



  1 - 2 * mod(K, 2)





integer function f(k)

  f = (-1)**k  ! Or: (-1.0)**k

end