Null pointer check elimination

Sat Nov 12 19:35:00 GMT 2005

Diego Novillo wrote:
 > int *this_never_returns_NULL (void) __attribute__ 
((never_returns_null));
 >
 > We would not want to pin the never-null attribute to 'int *':
Well, of course. That's why we're talking about a type *variant*.
To emphasise: this is a type variant *in the gcc internals* - i.e.
using a distinct POINTER_TYPE node. To what extent this is reflected
at the source level is up to front-ends.
Thus:
int *this_never_returns_NULL(void) __attribute__((never_returns_null));
could be viewed as syntactic sugar for:
(int __attribute__((never_null)) *) this_never_returns_NULL(void);
[Note I'm unsure about __attribute__ and declaration syntax, so it
is possible this wouldn't work. But that's a syntax issue, so it ignore
it in the following.]
 >
 > foo()
 > {
 > int *p = this_never_returns_NULL ();
 > < ... use p without modifying it ... > <-- p is never NULL here.	
 > p = other_fn ();
 > < ... use p without modifying it ... >	<-- p may be NULL here.
 > }
We could allow, if desired:
foo()
{
 int __attribute__((never_null)) *p = this_never_returns_NULL ();
 < ... use p without modifying it ... > <-- p is never NULL here.	
 p = other_fn (); /* error - caught by the compiler */
}
Consider:
foo()
{
 int *p;
 if (test)
 {
 p = this_never_returns_NULL ();
 < ... use p without modifying it ... >	<-- p never NULL here.
 }
 else
 {	
 p = other_fn ();
 < ... use p without modifying it ... >	<-- p may be NULL here.
 }
 ... more uses of p ...;
}
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 ...;
}
-- 
	--Per Bothner
per@bothner.com http://per.bothner.com/