[Bug middle-end/85599] Function need not be evaluated in logical expression

[tkoenig at gcc dot gnu.org]

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

Thomas Koenig <tkoenig at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW
          Component|fortran                     |middle-end
            Summary|invalid optimization:       |Function need not be
                   |function not always         |evaluated in logical
                   |evaluated in logical        |expression
                   |expression                  |

--- Comment #13 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
In the original test case, it is not necessary to evaluate
the function at all. A function evaluation should always be
cheaper than just checking a variable.  Constant propagation
would then eliminate all of the test case, leading only
to the effect of 

flag = .false.

As a more realistic example, one should change

subroutine foo(a,b)
  logical, intent(in) :: a
  logical, intent(out) :: b

  b = func() .and. a
end subroutine foo

to

subroutine foo(a,b)
  logical, intent(in) :: a
  logical, intent(out) :: b

  if (a) then
     b = func()
  else
     b = .false.
  end if

end subroutine

We are passing TRUTH_AND_EXPR to the middle end to allow such
optimizations. Is there something in the semantics of TRUTH_AND_EXPR
(for example needed for other languages) that says that the
function check() needs to be called? Or is this simply some
optimization opportunity that is currently not taken?