Re: [RFC] LKMM: Add volatile_if()

On Tue, Jun 08, 2021 at 01:22:58PM +0200, Peter Zijlstra wrote:

Works for me; and note how it mirrors how we implemented volatile_if()
in the first place, by doing an expression wrapper.

__builtin_ctrl_depends(expr) would have to:

 - ensure !__builtin_const_p(expr)	(A)

Why would it be an error if __builtin_constant_p(expr)?  In many
programs the compiler can figure out some expression does never change.
Having a control dependency on sometthing like that is not erroneous.

 - imply an acquire compiler fence	(B)
 - ensure cond-branch is emitted	(C)

(C) is almost impossible to do.  This should be reformulated to talk
about the effect of the generated code, instead.

*OR*

 - ensure !__builtin_const_p(expr);		(A)
 - upgrade the load in @expr to load-acquire	(D)

So that will only work if there is exactly one read from memory in expr?
That is problematic.

This needs some work.


Segher