[boost] [Lockfree] Tagged pointers

Subject: [boost] [Lockfree] Tagged pointers
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2011-07-27 19:16:23

Tim & others,

I understand that your implementation uses tagged pointers to avoid ABA 
problems.  An issue with tagged pointers is that naively they will be 
2*sizeof(T*) and systems may not have atomic operations that operate on 
things of that size.  Looking at the code, I see that you also have a 
"compressed" tagged pointer that is enabled for some architectures.  
Having found this code, I now vaguely recall some previous mailing list 
discussions about this.

Sometimes it may be possible to claim that users (& reviewers?) of a 
library do not need to know about details of this sort because the 
library "just works", and that if they are curious they can read the 
source.  I don't think this is one of those cases.  The documentation 
should explain in outline how this works, and provide rationale for why 
it is how it is.

IIRC, ARM has 64-bit atomics only from architecture version 6k onwards, 
and my Boost.Atomic ARM code doesn't use them.  This could be fixed.

Regards,  Phil.