------- Additional Comments From dberlin at gcc dot gnu dot org 2005-05-22
22:24 -------
Subject: Re: missed optimization due with
const function and pulling out of loops
> on the other hand, we should not let the definition make the concept
> useless. Being able to make
The definition actually matches what other compilers call "isolated" (no
access to global variables) combined with the property called
"side-effect free" (calling multiple times with same parameters is same
as calling once).
We could of course, split these concepts if we wanted to.
>
> int something(int i)
> {
> static int a[100];
>
> if (a[i] == 0)
> a[i] = somewhat_slow_computation;
> return a[i];
> }
>
> const is fairly useful.
>
> Anyway, moving possibly non-executed const function may cause also
> other problems. Consider
>
> int my_fancy_divide(int x, int z) attribute(const)
> {
> return x / z;
> }
>
> while (...)
> {
> if (z != 0)
> x = my_fancy_divide (x,z)
> }
Uh, this may be const, but you can't move these out anyway because the
value of the parameters has changed, so i'm not sure what you are going
for.
> Thus you would also have to require the const function to be total.
> Making const still more and more useless.
>
>
const has a very specific definition already. Moving get_type2 out of
the loop is consistent with that definition.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21712
