Null pointer check elimination

Sat Nov 12 21:11:00 GMT 2005

Diego Novillo wrote:
>>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)).

I'm clearly not explaining myself ...
Of course I'm not proposing that. My point was that my proposal
*allows* explicit setting of the __attribute__((never_null)). But
more likely it's the *compiler* (optimizer) that sets the attribute.
> The other approach just needs a single 
> __attribute__ in the function declaration.

You missed this
 > 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);
I.e. just a single attribute in the function declaration.
The phrase "syntactic sugar" means that people can write the former,
and it's equivalent to the latter.
> 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.

Bad example. The 'int *' type is more general than
'int*__attribute__((non_null))', so assigning an expression that
has type 'int*__attribute__((non_null))' to a variable or parameter
that has type 'int*' is always safe, and shouldn't warn.
As to the other way round, that is a user interface issue, orthognal to
the semantic issue. But if a variable is explicitly declared
'int * __attribute__((non_null))' and you pass it an 'int *' expression
then a warning is probably in order, since you're doing something dangerous.
One more point: If someone writes:
 int *x = this_never_returns_NULL();
Then the type of x is 'int *' and that of this_never_returns_NULL()
is 'int __attribute__((never_null)) *'. This is fine and safe.
However, ideally the compiler should realize that x actually has
the more specific type 'int __attribute__((never_null)) *', and
should use that information to generate better code. It seems
this should be straightforward using SSA, as I sketched in my
previous message.
-- 
	--Per Bothner
per@bothner.com http://per.bothner.com/