On Fri, Feb 21, 2014 at 10:10:54PM +0000, Joseph S. Myers wrote:
> On Fri, 21 Feb 2014, Paul E. McKenney wrote:
>
> > This needs to be as follows:
> >
> > [[carries_dependency]] int getzero(int i [[carries_dependency]])
> > {
> > return i - i;
> > }
> >
> > Otherwise dependencies won't get carried through it.
>
> C11 doesn't have attributes at all (and no specification regarding calls
> and dependencies that I can see). And the way I read the C++11
> specification of carries_dependency is that specifying carries_dependency
> is purely about increasing optimization of the caller: that if it isn't
> specified, then the caller doesn't know what dependencies might be
> carried. "Note: The carries_dependency attribute does not change the
> meaning of the program, but may result in generation of more efficient
> code. - end note".
Good point -- I am so used to them being in gcc that I missed that.
In which case, it seems to me that straight C11 is within its rights
to emit a memory barrier just before control passes into a function
that either it can't see or that it chose to apply dependency-breaking
optimizations to.
Thanx, Paul
