Re: why are we not using const?

["Richard Guenther" <richard dot guenther at gmail dot com>]

On 6/29/06, Kaveh R. Ghazi <ghazi@caipclassic.rutgers.edu> wrote:
>  > Notice that the value of the parameter "b" is never changed in the
>  > function body.  Consequently, if the current optimizers cannot figure
>  > that simple cases out (where "b" is not annotated const), then the
>  > optimizers in deficient in that respect.  That is the point.
>  > -- Gaby
>
> I agree that the compiler should/could be better at optimizing.
>
> However my feeling is that 'const' is more important as documentation
> and enforcement of APIs rather than an optimization hint.  From this
> perspective, what "b" points to not getting changed is more important
> to the caller than whether the function body changes the value of "b"
> itself, since the caller doesn't see the latter.
>
> I'd like to do for tree and rtx what I did for const char *, namely
> constify those tree/rtx functions that aren't supposed to modify their
> arguments.  This would require introducing the const_tree and
> const_rtx typedefs Tristan suggested.

But with C language constructs you cannot assume that an object
passed to a function via a const pointer is not modified.  So, there
is no real "const" regarding to objects pointed to.  Consider

void foo(const int *i)
{
 int *k = (int *)i;
 *k = 0;
}
int bar(void)
{
 int i = 1;
 foo(&i);
 return i;
}

should return 0, not 1.

Richard.

> Something for stage1, obviously.
>
>                 --Kaveh
> --
> Kaveh R. Ghazi                  ghazi@caip.rutgers.edu