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

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

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

--- Comment #10 from kargl at gcc dot gnu.org ---
(In reply to anlauf from comment #8)
> The suggested optimization needs to take into account that the evaluation
> of the temporary expression might trap, or that allocatable variables are
> not allocated, etc.
> 
> The trap etc. would not occur if the trip count of the loop is zero for the
> non-hoisted variant, so we need to make sure not to generate failing code
> for the hoisted one.
> 
> Similarly, for conditional code in the loop body, like
> 
>   if (cond) then
>      expression1 (..., 1/y)
>   else
>      expression2 (..., 1/z)
>   end if
> 
> where cond protects from traps even for finite trip counts, these conditions
> may also need to be identified, and an appropriate block generated.

I'm not sure what you are worried about here.  If one has

   do i = 1, n
      ... = expression1(..., 1/y)
   end do

then this is equivalent to

   do i = 1, n
      tmp = 1 / y
      ... = expression1(..., tmp)
   end do

which is equivalent to 

   tmp = 1 / y
   do i = 1, n
      ... = expression1(..., tmp)
   end do

I suppose I could do something exceedingly stupid such as

   function expression1(..., xxx)
      common /foo/y
      y = 0
      ...
   end

but this would then lead to invalid Fortran when i = 2 in the
above initial loops as (1/y) is invalid Fortran if y = 0.