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

[Florian Weimer <fw at deneb dot enyo dot de>]

* Andrew Haley:

> 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.

Sprinkling volatile all over the place is looks like the wrong answer.
It disables many optimizations, so you could probably use a simpler
compiler which doesn't perform the problematic optimizations in the
first place.

Not creating spurious stores seems to be a saner approach.  Hans Boehm's
concerns still apply, of course, but with knowledge of the architecture
and GCC's existing support of optimization barriers, programmers
probably have enough control to produce what they need.