Re: spinlocks on HP-UX

Greg Stark <stark(at)mit(dot)edu> writes:
> I was going to say the same thing as Tom that sequence points and
> volatile pointers have nothing at all to do with each other. However
> my brief searching online actually seemed to indicate that in fact the
> compiler isn't supposed to reorder volatile memory accesses across
> sequence points. That seemed to make sense since I couldn't think of
> any other way to rigorously describe the constraints the compiler
> should operate under.

It's a bit confusing. I agree that if the code is written such that
there are two volatile accesses with no intervening sequence point,
the compiler is at liberty to do them in either order; for instance in

foo(*x, *y);

there are no guarantees about which value is fetched first, even if x
and y are volatile-qualified. What's bothering me is that in, say,

*x = 0;
*y = 1;
*z = 2;

if x and z are volatile-qualified but y isn't, I believe the compilers
think they are only required to store into *x before *z, and can reorder
the store to *y around either of the others. So this makes the notion
of a sequence point pretty squishy in itself.

regards, tom lane