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

Paul E. McKenney
2020-07-02T18:00:44Z

On Thu, Jul 02, 2020 at 09:37:26AM +0000, David Laight wrote:
> From: Paul E. McKenney
> > Sent: 01 July 2020 17:06
> ...
> > > Would an asm statement that uses the same 'register' for input and
> > > output but doesn't actually do anything help?
> > > It won't generate any code, but the compiler ought to assume that
> > > it might change the value - so can't do optimisations that track
> > > the value across the call.
> >
> > It might replace the volatile load, but there are optimizations that
> > apply to the downstream code as well.
> >
> > Or are you suggesting periodically pushing the dependent variable
> > through this asm? That might work, but it would be easier and
> > more maintainable to just mark the variable.
>
> Marking the variable requires compiler support.
> Although what 'volatile register int foo;' means might be interesting.
>
> So I was thinking that in the case mentioned earlier you do:
> ptr += LAUNDER(offset & 1);
> to ensure the compiler didn't convert to:
> if (offset & 1) ptr++;
> (Which is probably a pessimisation - the reverse is likely better.)

Indeed, Akshat's prototype follows the "volatile" qualifier in many
ways. https://github.com/AKG001/gcc/

Thanx, Paul