Re: [RFC] LKMM: Add volatile_if()

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

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.

This is an example of a "syntactic" dependency versus a "semantic" 
dependency.  We shouldn't expect syntactic control dependencies to be 
preserved.

As a rule, people don't write non-semantic dependencies on purpose.  But 
they can occur in some situations, thanks to definitions the programmer 
isn't aware of.  One example would be:

(In some obscure header file): #define NUM_FOO 1

(Then in real code): if (READ_ONCE(*x) % NUM_FOO) ...

Alan