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
