likwid

Rust - it looks like a very well designed language that knows what it wants to accomplish. It just happens to want the exact opposite things from what I want, and that's fine. I know I'm in a weird minority (most of the people who think like I do seem to be game engine developers). [1] https://github.com/RRZE-HPC/likwid

ts such that only the smaller alignment has the chance of being fast. First, RESOURCE_STALLS_ANY is picking up some important difference. Slow has lots, Fast has fewer. Breaking these down to see what subtype it is might give an exact answer: https://download.01.org/perfmon/index/skylake.html. It's not RESOURCE_STALLS_SB that changes, but the others don't seem to be predefined. LSD_UOPS is the number of µops delivered by the "Loop Stream Detector", which is the smallest and fastest cache for instructions. It's very small, and only works for tiny loops. Usually this is where you want the µops to stream from, but in this case using it results in a slower final result. IDQ_DSB_UOPS is the number of µops delivered from the "Decode Stream Buffer". This is usually slightly slower than the "Loop Stream Buffer", but in this case using it produces a faster result. This is large enough to hold a reasonably sized function. IDQ_MITE_UOPS is the "legacy" instruction decoder. This is the normal path by which instructions are decoded into µops. It's always used the first time instructions are encountered, but for hot loops and hot functions the results can be cached. Usually you'd like to see less of this, but here using the legacy decoding path gives a faster result. What does this mean? It probably means that something about the alignment combined with the jump defeats what would normally be the fastest µop cache. This isn't that strange in itself, but the oddity is that for some reason, this causes an overall faster result. Why? I'd thought it was something to do with "store forwarding", and it still might be, but I haven't found any counters that directly point to that as being the problem. Instead, I think it might have to do with the processor getting too far ahead on the reads and stores, and then having to redo them once it realizes that the data is out of date. Somehow having fewer instructions available in the reorder buffer (because they are coming in from a slightly slower decoder) improves overall performance. But I don't yet understand the specifics. I'm going to stop for the night, but I'd love to wake up to someone revealing the "true" answer!

l memory accesses you can manage to create a more significant slowdown, but even then you can usually lower this something acceptable by sampling. So yes --- you probably should be using perf far more! Or VTune, or likwid (https://github.com/RRZE-HPC/likwid).