Re: Optimization of conditional access to globals: thread-unsafe?

[Andrew Haley <aph-gcc at littlepinkcloud dot COM>]

Tomash Brechko writes:
 > On Mon, Oct 22, 2007 at 00:07:50 +0100, Dave Korn wrote:
 > >   Because of the 'as-if' rule.  Since the standard is neutral
 > > with regard to threads, gcc does not have to take them into
 > > account when it decides whether an optimisation would satisfy the
 > > 'as-if' rule.
 > 
 > If this would be true, then the compiler is free to inject the
 > sequence
 > 
 >   mov mem -> reg
 >   mov reg -> mem
 > 
 > just _anywhere_.

That's right.  This isn't a standards conformance issue, rather one of
quality of implementation.

The core problem here seems to be that the "C with threads" memory
model isn't sufficiently well-defined to make a determination
possible.  You're assuming that you have no resposibility to mark
shared memory protected by a mutex as volatile, but I know of nothing
in the C standard that makes such a guarantee.  A prudent programmer
will make conservative assumptions.

Please have a read of [1].  Let us know if anything you have observed
isn't covered in that paper.

Andrew.

[1] Hans-Juergen Boehm. Threads cannot be implemented as a library. In
Proc. of the ACM SIGPLAN 2005 Conf. on Programming Language
Design and Implementation (PLDI), pages 261?268, Chicago, IL, June
2005.