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
