It is a common misconception that the "volatile" keyword is necessary or even helpful to code the implements locks and atomic operations.
What you really want is a "memory barrier" that prevents the compiler from emitting your machine code in the incorrect order, as well as preventing microprocessors that can do out of order execution from reordering your instructions. If you did not have such a memory barrier, then it could easily happen that the payload portion of your critical section is placed outside the region that is protected by your lock. The popular but misunderstood use of the volatile keyword is the result of certain, specific compilers on certain specific instruction set architectures implementing volatile variables by generating memory barrier machine code. It can easily happen that your lock works properly anyway, but even if so, the use of volatile will make your code both slower and larger. I was quite surprised at this fact myself. My bookmark for the page where I learned all about this is on my other computer, but if I remember correctly it was written by an Intel engineer who was one of the developers of an Intel open source multithreaded toolkit. That toolkit did use volatile everywhere at first, but when Intel opened the source and so had to support ISAs other than Intels, they removed all the volatiles and replaced them with explicit memory barriers. Contrary to Ken Thomases' assertion, there are all kinds of reasons to use atomic operations as locking primitives. One is that they cannot result in process context switches; if you are certain that you will release the lock quite quickly, it is a whole lot faster, and uses less memory, to use atomic arithmetic. I'll send you my bill in the mail, Don Quixote -- Don Quixote de la Mancha Dulcinea Technologies Corporation Software of Elegance and Beauty http://www.dulcineatech.com quix...@dulcineatech.com _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
