Re: [RFC] LKMM: Add volatile_if()

Jakub Jelinek
2021-06-06T18:59:44Z

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.

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