Should atomic_xxx() functions reject not-_Atomic() arguments ?

Chris Hall gcc@gmch.uk
Sat Mar 7 13:04:19 GMT 2020


On 06/03/2020 17:45, Jonathan Wakely wrote:
> On Fri, 6 Mar 2020 at 16:17, Chris Hall <gcc@gmch.uk> wrote:
>>> Sure.  But the Standard atomic_fetch_add() takes an _Atomic(xxx)* (as
>>> the first parameter), and for the reasons given, I understand that
>>> uint64_t* is not compatible with _Atomic(uint64_t)*.

> I don't think GCC is treating them as though they are compatible. It
> just accepts a broader range of types than only  _Atomic ones, and
> does the right thing for them all (apart from pointers where the
> arithmetic is wrong).

>> FWIW: clang gets this right, and where the Standard says a parameter
>> must be an _Atomic(foo_t)* [for a standard atomic_xxx()], clang rejects
>> foo_t* arguments.

> It's not clear to me that C actually requires it to be rejected, or if
> it's just undefined (in which case GCC's decision to accept it and do
> the obvious thing is OK).

The subtlety of the distinction between "only defined to accept" and 
"defined to only accept" had escaped me for a moment :-(

Of course, for all the machines I know well enough I can say that there 
is no difference between, inter alia, uint64_t and _Atomic(uint64_t). 
So what I have observed GCC doing is, arguably, helpful.

For all I know, GCC may be equally helpful on machines where uint64_t 
and _Atomic(uint64_t) are not the same -- and issue some form of 
diagnostic.  Hopefully an error, rather than a 
-Wincompatible-pointer-types warning, since a warning would (probably?) 
not prevent the resulting code from crashing and burning.

If I write:

   uint64_t px(uint64_t* p) { return *p ; }

   char m[] = "There's glory for you!" ;

   printf("%lu", px(m+9)) ;

GCC chips in a -Wincompatible-pointer-types (if suitably prompted... 
-Wall ?).

Now, if I make a conscious decision to ignore all machines which may 
crash or otherwise misbehave for unaligned uint64_t reads, I can cast 
the m+9 -- and anything I then get is my fault.

I wonder if, by extension, the atomic_xxx() should (at least) warn when 
the _Atomic() parameters are passed not-_Atomic() arguments ?

As I have said elsewhere, I think the real problem is that the Standard 
fails to define vital things as Implementation Defined, leaving the 
programmer guessing as to what their code is going to do when compiled 
for some machine/system they are not familiar with.

I note 
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4013.html -- 
courtesy of Andrew Haley -- suggests new facilities to check at compile 
time specifically for this case.  IMO there should also be facilities to 
check for "wait-free" -- unless "lock-free" already does that job... but 
that's another story.

Chris



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