[cpp-threads] Re: Review comments on N2176 WRT dependency ordering

[Paul E. McKenney]

On Wed, Apr 11, 2007 at 11:54:34PM -0000, Boehm, Hans wrote:
> > -----Original Message-----
> > From: Paul E. McKenney [mailto:paulmck at linux.vnet.ibm.com] 
> > What is your reaction to the notion of dependency defined in 
> > the Itanium and the POWER architecture documents?  Page 384 
> > of my copy of volume 2 of the Itanium Architecture manual 
> > defines a dependency from A to B as follows:
> > 
> > 	"A precedes B in program order and A produces a value that
> > 	B consumes."
> > 
> I think that especially at the programming language level, this
> introduces a large number of unclear corner cases:
> 
> 1) When do library functions propagate dependencies?  Is there a
> dependency from a character inserted into a string to the resulting
> string size?  To the output of search for a certain substring?

When the formal parameters are marked as propogating dependencies.

> 2) What if the type involved has only one corresponding valid value?

C and C++ being what they are, the dependency should remain -- if
the head of the dependency chain was marked, and if any function calls
were also appropriately marked.

> 3) In what sense is a value produced in a switch statement dependent on
> the switch expression?
> Etc.

Via control dependency -- and it may be useful to have a primitive that
terminates a dependency chain, as noted in the paper.

> It may be that this can be addressed.  But I don't think it's easy.  The
> topic is certainly on my list of things to be discussed in Oxford.

If it were easy, it would have been done long ago, right?

I do look forward to the discussion!!!

						Thanx, Paul