I asked a collection of knowledgeable people I know about the issue. The
consensus is that the optimization is not permitted in POSIX code but that
it is permitted in pure C code. The basic argument goes like this:
To make POSIX-compliant code even possible, surely optimizations that add
writes to variables must be prohibited. That is -- if POSIX prohibits
writing to a variable in certain cases only the programmer can detect, then
a POSIX-compliant compiler cannot write to a variable except where
explicitly told to do so. Any optimization that *adds* a write to a variable
that would not otherwise occur *must* be prohibited.
Otherwise, it is literally impossible to comply with the POSIX requirement
that concurrent modifications and reads to shared variables take place while
holding a mutex.
The simplest solution is simply to ditch the optimization. If it really
isn't even an optimization, then that's an easy way out.
DS
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