[tree-ssa] Removing useless/redundant "const" calls

[Fergus Henderson]

On 12-Jun-2003, Zack Weinberg <zack@codesourcery.com> wrote:
> This seems like an opportune moment to point out that calls to
> noreturn functions shouldn't count as side effects when determining
> whether functions are pure or const.

That is not correct in general.

> Example - if I have a square root function that starts with
> 
>   if (arg < 0) abort ();
> 
> but has no other potential side effects, it should still be considered
> const.

Doing that would violate the C standard, and would potentially create
security holes.  Consider, for example:

	#define ARRAY_SIZE 10
	int array[ARRAY_SIZE];

	void checkbounds(int arg) {
		if (arg < 0) abort();
		if (arg > ARRAY_SIZE) abort();
	}

	int set_array(int posn, int val) {
		checkbounds(posn);
		array[posn] = val;
	}

If the call to checkbounds() gets optimized away, then the resulting
code may be prone to buffer overflow attacks.

-- 
Fergus Henderson <fjh@cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.