Null pointer check elimination

Sat Nov 12 20:33:00 GMT 2005

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.