Re: [RFC][PATCH 0/5] arch: atomic rework

[Torvald Riegel]

On Wed, 2014-02-26 at 18:43 +0000, Joseph S. Myers wrote:
> On Wed, 26 Feb 2014, Torvald Riegel wrote:
> 
> > On Fri, 2014-02-21 at 22:10 +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".
> > 
> > I think that this last sentence can be kind of misleading, especially
> > when looking at it from an implementation point of view.  How
> > dependencies are handled (ie, preserving the syntactic dependencies vs.
> > emitting barriers) must be part of the ABI, or things like
> > [[carries_dependency]] won't work as expected (or lead to inefficient
> > code).  Thus, in practice, all compiler vendors on a platform would have
> > to agree to a particular handling, which might end up in selecting the
> > easy-but-conservative implementation option (ie, always emitting
> > mo_acquire when the source uses mo_consume).
> 
> Regardless of the ABI, my point is that if a program is valid, it is also 
> valid when all uses of [[carries_dependency]] are removed.  If a function 
> doesn't use [[carries_dependency]], that means "dependencies may or may 
> not be carried through this function".  If a function uses 
> [[carries_dependency]], that means that certain dependencies *are* carried 
> through the function (and the ABI should then specify what this means the 
> caller can rely on, in terms of the architecture's memory model).  (This 
> may or may not be useful, but it's how I understand C++11.)

I agree.  What I tried to point out is that it's not the case that an
*implementation* can just ignore [[carries_dependency]].  So from an
implementation perspective, the attribute does have semantics.

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