On Fri, Jun 04, 2021 at 05:42:28PM +0200, Peter Zijlstra wrote:

On Fri, Jun 04, 2021 at 05:22:04PM +0200, Peter Zijlstra wrote:

quoted

On Fri, Jun 04, 2021 at 04:13:57PM +0100, Will Deacon wrote:

quoted

In fact, maybe it's actually necessary to bundle the load and branch
together. I looked at some of the examples of compilers breaking control
dependencies from memory-barriers.txt and the "boolean short-circuit"
example seems to defeat volatile_if:

void foo(int *x, int *y)
{
        volatile_if (READ_ONCE(*x) || 1 > 0)
                WRITE_ONCE(*y, 42);
}  

Yeah, I'm not too bothered about this. Broken is broken.

If this were a compiler feature, the above would be a compile error. But
alas, we're not there yet :/ and the best we get to say at this point
is: don't do that then.

Ha! Fixed it for you:

#define volatile_if(cond) if (({ bool __t = (cond); BUILD_BUG_ON(__builtin_constant_p(__t)); volatile_cond(__t); }))

That won't help with more complicated examples, such as:

	volatile_if (READ_ONCE(*x) * 0 + READ_ONCE(*y))

Alan