> How is the compiler going to know which path is going to be taken the most? > There's two main paths in the ring buffer logic. One when an event stays on > the sub-buffer, the other when the event crosses over to a new sub buffer. > As there's 100s of events that happen on the same sub-buffer for every one > time there's a cross over, I optimized the paths that stayed on the > sub-buffer, which caused the time for those events to go from 250ns down to > 150 ns!. That's a 40% speed up. > > I added the unlikely/likely and 'always_inline' and 'noinline' paths to > make sure the "staying on the buffer" path was always the hot path, and > keeping it tight in cache. > > How is a compiler going to know that? It might have some heuristics to try to guess unlikely/likely, but that is not what we are talking about here. How much difference did 'always_inline' and 'noinline' make? Hopefully the likely is enough of a clue it should prefer to inline whatever is in that branch, where as for the unlikely case it can do a function call. But compilers is not my thing, which is why i would reach out to the compiler people and ask them, is it expected to get this wrong, could it be made better? Andrew
