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.
