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

David Laight
2020-07-02T09:37:31Z

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.)

David

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