[Bug fortran/68993] MERGE does not evaluate its arguments

[sgk at troutmask dot apl.washington.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68993

--- Comment #6 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
On Mon, Dec 21, 2015 at 08:29:57PM +0000, Joost.VandeVondele at mat dot ethz.ch
wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68993
> 
> --- Comment #5 from Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> ---
> (In reply to Steve Kargl from comment #4)
> > 
> > I would urge anyone trying to be clever to use clear syntax:
> > 
> 
> https://github.com/hfp/libxsmm/commit/cc308fc5debe6151157a4fa9efacc7aa03351283
> 
> is what we used indeed, but it is not quite as concise as one would like.
> 

Interesting project.  I may need to take a closer look at it.

I agree that the suggested syntax is not concise.  In your particular
case, you could write 

             cpa = C_NULL_PTR
             IF (PRESENT(pa)) cpa = C_LOC(pa)

instead of

             IF (PRESENT(pa)) THEN
               cpa = C_LOC(pa)
             ELSE
               cpa = C_NULL_PTR
             END IF

This would reduce your code from 15 lines to 6 lines.  I'll note that
in some testing where I had a similar code structure the former
(ie., 2-line example) was slower than the latter (5-line example).

It is unfortanute that Fortran does not have a ternary operator.
This is a good example where it might be nice to have

cpa = present(pa) ? c_loc(pa) : c_null_ptr

where the TRUE or FALSE choice is only evaluated based on the condition.