On Fri, Jun 04, 2021 at 11:09:55AM -0500, Segher Boessenkool wrote: > I didn't find a description of the expected precise semantics anywhere > in this patch. This however is the most important thing required here! Fair enough; so a control-dependency is a LOAD->STORE memory ordering provided by conditional branches. The conditional branch instruction itself must have a data dependency on a previous LOAD, while the branch itself guards a STORE. Then because speculative STOREs result in out-of-thin-air values, the STORE must not become visible until the branch is decided, which can only be done if the LOAD is complete. We make use of this, and would like the compiler to not ruin this code pattern for us. So we need the STORE to say inside the selection statement, we need the LOAD not be optimized away, and we need the conditional branch to be emitted. Alternatively, we need the LOAD to be upgraded to a LOAD-ACQUIRE (an option on platforms where this is sufficiently cheap). Which will also ensure the STORE happens after. So we can force the LOAD using READ_ONCE() (a volatile cast). We can prohibit hoisting by adding a compiler barrier to the expression. And then we use asm goto() to force emit a conditional branch. Combined this leaves the compiler very little room to mess things up, but it does produce sub-optimal code, and doesn't allow the LOAD-ACQUIRE upgrade Will would like (but he can't always have that anyway due to our other use of asm()). We also have a 'CONTROL DEPENDENCIES' section in Documentation/memory-barriers.txt for further reading.
