Messages in this thread | | | Date | Tue, 8 Jun 2021 10:28:51 -0500 | From | Segher Boessenkool <> | Subject | Re: [RFC] LKMM: Add volatile_if() |
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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
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