[cfe-commits] r79041 - /cfe/trunk/lib/Sema/SemaDeclAttr.cpp

[Ted Kremenek]

On Aug 14, 2009, at 3:38 PM, Eli Friedman wrote:

> On Fri, Aug 14, 2009 at 3:30 PM, Chris Lattner<clattner at apple.com>  
> wrote:
>>
>> On Aug 14, 2009, at 3:20 PM, Eli Friedman wrote:
>>
>>> On Fri, Aug 14, 2009 at 3:12 PM, Ted Kremenek<kremenek at apple.com>  
>>> wrote:
>>>>
>>>> Unless I'm mistaken, this breaks constructs like the following:
>>>> __attribute((malloc)) void *(*f)();
>>>>
>>>> -Eli
>>>>
>>>> I implemented handling of this case, but I noticed that GCC  
>>>> actually
>>>> rejects
>>>> attribute 'malloc' being applied to function pointers ("warning:  
>>>> 'malloc'
>>>> attribute ignored").  Should we do the same in Clang?  For function
>>>> pointers, the malloc attribute really a property of the pointer  
>>>> type, not
>>>> the declaration, but apparently GCC doesn't even reason about that.
>>>
>>> I think it's better to be self-consistent here over being consistent
>>> with gcc, as long as we don't break compatibility.  Function
>>> attributes are confusing enough without making different attributes
>>> act differently.
>>
>> The problem is that this attribute is really a decl attribute, not  
>> a type
>> attribute.  If I have two functions with the same prototype (but  
>> one is
>> attribute malloc) I should be able to assign them to the same  
>> function
>> pointer, no?  If we made it part of the type system, we'd have to  
>> consider
>> the attribute for assignment compatibility etc.
>
> How is malloc different from noreturn in this regard?

There are two points here.  First, the 'malloc' attribute is a GCC- 
extension, and if that attribute only applies to function declarations  
that is at least self-consistent.  Limiting, but self-consistent.   
Other than generalizing the attribute (which might be nice from a  
programming language theory point of view) there is no code out there  
that uses more functionality on that attribute than GCC provides.

As for 'noreturn', Clang's handling of it is incorrect.  Consider:

$ cat t.c
__attribute((noreturn)) void *(*fptr)();
void *(*gptr)();

void bar() {
   gptr = fptr;
}

$ gcc -fsyntax-only t.c
t.c: In function ‘bar’:
t.c:5: warning: assignment makes qualified function pointer from  
unqualified

$ clang -fsyntax-only t.c
(nothing)

I'm not certain what GCC is doing under the hood, but it appears that  
(unlike Clang) GCC is reasoning about 'noreturn' as a qualifier on the  
type.  That doesn't appear to be the case with 'malloc'.  Perhaps  
that's just how it was implemented in GCC, and perhaps they both  
should be handled as qualifiers.  The upshot is that while Clang eats  
code where 'noreturn' is attached to a function pointer, it doesn't  
handle it correctly.
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