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

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

Tomash Brechko writes:

 > 
 > Several people already suggested to use volatile for shared data.
 > Yes, it will help because we know it will disable all access
 > optimizations, including thread-unaware ones.  But I don't want to
 > disable _all_ optimizations, I rather vote for thread-aware
 > optimizations.

But your plan would disable optimizations even when it isn't necessary
to do so.  Only a small part of the data in a multi-threaded program
are shared.

 > There is no requirement in POSIX to make all shared data volatile.
 > As the article referenced in the thread explains, there is no
 > agreement between POSIX and C/C++ wrt memory access.  But should it
 > be fixed in the compiler (as article suggests), or should every
 > shared data in every threaded program be defined volatile, just for
 > the case?  I never seen latter approach in any Open Source project
 > (though didn't look for it specifically), and many of them are
 > considered quite portable.
 > 
 > Again, we are not discussing some particular code sample, and how it
 > might be fixed, but the problem in general.  Should GCC do
 > thread-unsafe optimizations, or not?

We do understand what you're saying, and simply repeating the same
thing doesn't help.

I think we should wait to see what the C++ working group comes up with
and consider implementing that, rather than some ad-hoc gcc-specific
proposal.

There's some discussion here:

http://www.artima.com/cppsource/threads_meeting.html

and here:

http://www.hpl.hp.com/personal/Hans_Boehm/c++mm/

Andrew.