Re: [RFC] LKMM: Add volatile_if()

[Alan Stern <stern@rowland.harvard.edu>]

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:
> 
> > 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