Re: [RFC] LKMM: Add volatile_if()

Paul E. McKenney
2021-06-06T19:15:43Z

On Sun, Jun 06, 2021 at 08:59:22PM +0200, Jakub Jelinek wrote:
> On Sat, Jun 05, 2021 at 08:41:00PM -0700, Linus Torvalds wrote:
> > Something like this *does* seem to work:
> >
> > #define ____barrier(id) __asm__ __volatile__("#" #id: : :"memory")
> > #define __barrier(id) ____barrier(id)
> > #define barrier() __barrier(__COUNTER__)
> >
> > which is "interesting" or "disgusting" depending on how you happen to feel.
>
> I think just
> #define barrier() __asm__ __volatile__("" : : "i" (__COUNTER__) : "memory")
> should be enough (or "X" instead of "i" if some arch uses -fpic and will not
> accept small constants in PIC code), for CSE gcc compares that the asm template
> string and all arguments are the same.

This does seem to do the trick: https://godbolt.org/z/K5j3bYqGT

So thank you for that!

Thanx, Paul

> As for volatile, that is implicit on asm without any output operands and
> it is about whether the inline asm can be DCEd, not whether it can be CSEd.
>
> Jakub
>