[Bug fortran/85599] New: invalid optimization: function not always evaluated in logical expression

[janus at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85599

            Bug ID: 85599
           Summary: invalid optimization: function not always evaluated in
                    logical expression
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janus at gcc dot gnu.org
  Target Milestone: ---

Consider the following test case:


program lazy

   logical :: flag

   flag = .false.
   flag = check() .and. flag      ! 'check' is executed as expected
   flag = flag .and. check()      ! bug: 'check' is not executed

contains

   logical function check()
      integer, save :: i = 1
      print *, "check", i
      i = i + 1
      check = .true.
   end function

end


The problem here is that gfortran executes the function 'check' only once,
although it should be executed twice AFAICS.

Optimizing out the function call might be valid for a PURE function, but for an
impure one with side effects, I don't think this is allowed.

Both ifort and flang execute the function twice. For gfortran, the optimization
level does not seem to influence the result: -O0 and -O3 behave in the same
way.

I tried with gfortran versions 5, 6, 7 and 8, which all exhibit the same buggy
behavior.