Re: Number of compiler passes

glen herrmannsfeldt wrote:


> I believe for Fortran INTENT(OUT) dummy variables, you must
> write first, but you can then read the value just as any other
> variable. They are not write-only.


I thought about this. But there is no (easy) way at compile time to figure
out if a variable has actually been written to. Think of this example:


int f(out int a) {
        if (rnd(0,10) > 5) {
                a <- 5;
        }
        result <- a; // is this allowed?
}


Note:
* rnd returns a random number
* <- is the assignment operator
* the language has no return statement, but rather a result variable


> Among other reasons, Fortran allows either call by reference
> or call by value result (sometimes called copy-in copy-out).
> In the latter case, INTENT(OUT) might not do the copy in.
> It can also affect the way the optimizer works.


I believe the optimizer could do much with the guarantee that a variable
will never be read.


Anyway, there are ways around this, of course. Just assign to a local
variable first, so you can use the value again.


--
Michiel Helvensteijn

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