Optimization of conditional access to globals: thread-unsafe?

[Richard Kenner]

     I thought it was pretty clear in intent: the compiler will emit
     exactly one machine load/store operation for any rvalue
     reference/lvalue assignment (respectively) in the source, at the exact
     sequence point in the generated code corresponding to the location of
     the reference in the source.

The problem is that "one machine load/store operation" and "any rvalue"
aren't precisely-defined terms.  We've had numerous discussions on this
list before about how one might want to define them.