Re: [PATCH 00/22] add support for Clang LTO

Paul E. McKenney
2020-07-01T16:03:45Z

On Wed, Jul 01, 2020 at 05:05:12PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 01, 2020 at 07:06:54AM -0700, Paul E. McKenney wrote:
>
> > The current state in the C++ committee is that marking variables
> > carrying dependencies is the way forward. This is of course not what
> > the Linux kernel community does, but it should not be hard to have a
> > -fall-variables-dependent or some such that causes all variables to be
> > treated as if they were marked. Though I was hoping for only pointers.
> > Are they -sure- that they -absolutely- need to carry dependencies
> > through integers???
>
> What's 'need'? :-)

Turning off all dependency-killing optimizations on all pointers is
likely a non-event. Turning off all dependency-killing optimizations
on all integers is not the road to happiness.

So whatever "need" might be, it would need to be rather earthshaking. ;-)
It is probably not -that- hard to convert to pointers, even if they
are indexing multiple arrays.

> I'm thinking __ktime_get_fast_ns() is better off with a dependent load
> than it is with an extra smp_rmb().
>
> Yes we can stick an smp_rmb() in there, but I don't like it. Like I
> wrote earlier, if I wanted a control dependency, I'd have written one.

No argument here.

But it looks like we are going to have to tell the compiler.

Thanx, Paul