Re: [4.3] Invalid code or invalid optimisation?

[Andrew Haley <aph at redhat dot com>]

Dave Korn wrote:

>   Adding a "memory" clobber to the inline asm works around the problem,
> causing 4.3 series to generate the same assembly as head, but I think it's a
> sledgehammer approach.  Am I asking too much of GCC to not sink the store, or
> is 4.3 doing something wrong?  I /think/ that the fact that there's a volatile
> store in ilockcmpexch means the earlier store shouldn't be moved past it, and
> that GCC is perhaps missing that the asm's output operand effectively
> represents a volatile write through *t, but I could be misunderstanding the
> rules about volatile.  Anyone got their language lawyer's hat on at the moment?

You could just look at the standard, y'know.  Volatile stores only block other
volatile stores: they don't block *all* stores.  If you really want a complete
memory barrier, which in a mutex you surely do, then you're going to have to
clobber memory.

Andrew.