[cpp-threads] Relaxed reload example

[Paul E. McKenney]

On Wed, Aug 01, 2007 at 03:16:49PM -0700, Herb Sutter wrote:
> > DEC Alpha would in fact permit this outcome.
> 
> Why doesn't that surprise me? :-)

;-)

> > I am not aware of any other CPU that permits this outcome.
> 
> I was more specifically wondering what the draft C++MM said about it. At
> least one subquestion would be whether relaxed reads can be replicated
> (by the CPU, or the compiler, etc.). Not sure if there are other ways
> the result could happen.

The current C++MM does in fact allow this outcome.  N2260 proposes
adding mechanism that permits the programmer to disallow this outcome
without incurring the overhead of an explicit fence.  There will be a
new version of this document soon.

This outcome could happen on non-Alpha platforms via value speculation
and other similar compiler optimizations.  The order of events might
be as follows:

T1.	Correctly guess the value of the not-yet-assigned pointer.

T1.	Dereference the guess, pick up uninitialized fields from the
	structure.

T2.	Initialize the fields in the structure.

T2.	Store-release the pointer to the new structure.

T1.	Validate the guessed value for the pointer.  Since they
	match, incorrectly conclude that the dereference operation
	went smoothly.

Can't say that I am a fan of such optimizations, but others disagree.

						Thanx, Paul