Re: [PATCH] x86: work around clang IAS bug referencing __force_order

[Arvind Sankar]

On Fri, Aug 21, 2020 at 02:37:48AM +0200, Thomas Gleixner wrote:
> On Thu, Aug 20 2020 at 09:06, Arvind Sankar wrote:
> > I don't think that's an issue, or at least, not one where force_order
> > helps.
> >
> > If the source for foo() is not visible to the compiler, the only reason
> > force_order prevents the reordering is because foo() might have
> > references to it, but equally foo() might have volatile asm, so the
> > reordering isn't possible anyway.
> >
> > If the source is visible, force_order won't prevent any reordering
> > except across references to force_order, but the only references are
> > from the volatile asm's which already prevent reordering.
> >
> > I think force_order can only help with buggy compilers, and for those it
> > should really have been an input-output operand -- it wouldn't currently
> > do anything to prevent cr writes from being reordered.
> 
> Fair enough. Care to provide a patch which has the collected wisdom of
> this thread in the changelog?
> 
> Thanks,
> 
>         tglx

The gcc bug I linked to earlier is only fixed in gcc-6 onwards. Is that
good enough to remove force_order? I can test gcc-4.9 and gcc-5 to check
if it would currently have any impact.

CBL guys, can you confirm that clang also will not reorder volatile asm?

Thanks.