[boost] Re: shared_ptr on ppc?

From: Miro Jurisic (macdev_at_[hidden])
Date: 2005-04-06 16:47:06

In article <004e01c53ac9$35901640$6601a8c0_at_pdimov>,
 "Peter Dimov" <pdimov_at_[hidden]> wrote:

> or something like that. Post-release. The current code is good (and risky) 
> enough. :-)

My understanding from having spoken to several Apple engineers whom I consider 
knowledgeable on this topic is that it's a very bad idea to write assembly code 
to perform atomic operations on the PPC. There is a variety of CPU-specific 
idiosyncracies that make it very difficult to write such code correctly. As you 
might imagine, writing such code not quite correctly leads bugs that are very 
hard to track down.

This recommendation is explicit in Apple's documentation at 
<http://developer.apple.com/technotes/tn/tn2006.html>:

"Do not use the PowerPC instructions Load Reserved  (lwarx) and Store 
Conditional  (stwcx) to implement atomicity in your  preemptively threaded 
application. As described in  DTS Technote 1137 Disabling  Interrupts on the 
Traditional Mac OS, these  instructions are non-portable and are tricky to use  
correctly across the full spectrum of PowerPC  implementations."

For this reason, Apple supplies a number of atomic primitives in Mac OS 9 and 
Mac OS X which perform a variety of modify-or-fail primitives; Apple takes it 
upon themselves to make sure that their implementation is correct for the CPU 
architectures that the OS supports, and I can guarantee you that Apple has more 
time to develop and test these across a wide range of PPC hardware than we ever 
will (or should try to).

I cannot understate how important it is that use the Apple APIs. They are 
documented at 
<http://developer.apple.com/documentation/Hardware/DeviceManagers/pci_srvcs/pci_c
ards_drivers/PCI_BOOK.1a3.html>. Do not be concerned about the fact that this 
documentation is for PCI drivers; the same APIs are available in userland with 
#include <CoreServices/CoreServices.h>. 

meeroh