Re: [RFC] LKMM: Add volatile_if()

Segher Boessenkool
2021-06-04T16:14:27Z

Hi!

On Fri, Jun 04, 2021 at 12:12:07PM +0200, Peter Zijlstra wrote:
> With optimizing compilers becoming more and more agressive and C so far
> refusing to acknowledge the concept of control-dependencies even while
> we keep growing the amount of reliance on them, things will eventually
> come apart.

Yes, C is still not a portable assembler.

> There have been talks with toolchain people on how to resolve this; one
> suggestion was allowing the volatile qualifier on branch statements like
> 'if', but so far no actual compiler has made any progress on this.

"if" is not a "branch statement".

> --- a/arch/powerpc/include/asm/barrier.h
> +++ b/arch/powerpc/include/asm/barrier.h
> @@ -80,6 +80,19 @@ do { \
> ___p1; \
> })
>
> +#ifndef __ASSEMBLY__
> +/* Guarantee a conditional branch that depends on @cond. */
> +static __always_inline bool volatile_cond(bool cond)
> +{
> + asm_volatile_goto("and. %0,%0,%0; bne %l[l_yes]"
> + : : "r" (cond) : "cc", "memory" : l_yes);
> + return false;
> +l_yes:
> + return true;
> +}
> +#define volatile_cond volatile_cond
> +#endif

"cmpwi" is ever so slightly better than "and.". And you can write "cr0"
instead of "cc" more explicitely (it means the same thing though).

I didn't find a description of the expected precise semantics anywhere
in this patch. This however is the most important thing required here!

Segher