Re: Null pointer check elimination

[Diego Novillo <dnovillo at redhat dot com>]

On Saturday 12 November 2005 14:35, Per Bothner wrote:

> The internal SSA form might be something like:
>
> foo()
> {
>     if (test)
>       {
>         int __attribute__((never_null)) *p_1 =
> this_never_returns_NULL(); < ... use p without modifying it ... >	<--
> p_1 never NULL here. }
>     else
>      {
>        int *p_2 = other_fn ();
>        < ... use p without modifying it ... > <-- p_2 may be NULL here.
>      }
>    int *p_3 = PHI(p_1, p_2);
>     ... more uses of p ...;
> }
>
Well, I am not an FE person, so I don't much care how you convey the 
information into the optimizers.

From a user's perspective I see the obvious drawback that you've just 
forced me to edit a potentially large number of source files just to add 
the __attribute__((never_null)).  The other approach just needs a single 
__attribute__ in the function declaration.

Another problem I would have as a user is if you suddenly make 'int *' and 
'int * __attribute__((non_null))' different/incompatible types.  If I have 
a function that takes 'int *' as argument, I don't want to start adding 
tons of type casts to get around compiler warnings/errors.

From an optimizer perspective, I fail to see how would your approach give 
me better information inside VRP.  But since your approach seems to give 
me the same information, I don't much care how the FE implements this.  If 
you decide to have type attributes, I'll use it.  If you mark the 
function, I'll use it with the same effect.