From: Evan Green > Sent: 27 June 2023 20:12 > > On Mon, Jun 26, 2023 at 2:42 PM Jessica Clarke <jrtc27@xxxxxxxxxx> wrote: > > > > On 23 Jun 2023, at 23:20, Evan Green <evan@xxxxxxxxxxxx> wrote: > > > > > > Rather than deferring misaligned access speed determinations to a vendor > > > function, let's probe them and find out how fast they are. If we > > > determine that a misaligned word access is faster than N byte accesses, > > > mark the hardware's misaligned access as "fast". > > > > How sure are you that your measurements can be extrapolated and aren’t > > an artefact of the testing process? For example, off the top of my head: > > > > * The first run will potentially be penalised by data cache misses, > > untrained prefetchers, TLB misses, branch predictors, etc. compared > > with later runs. You have one warmup, but who knows how many > > iterations it will take to converge? > > I'd expect the cache penalties to be reasonably covered by a single > warmup. You're right about branch prediction, which is why I tried to > use a large-ish buffer size, minimize the ratio of conditionals to > loads/stores, and do the test for a decent number of iterations (on my > THead, about 1800 and 400 for words and bytes). > > When I ran the test a handful of times, I did see variation on the > order of ~5%. But the comparison of the two numbers doesn't seem to be > anywhere near that margin (THead C906 was ~4x faster doing misaligned > word accesses, others with slow misaligned accesses also reporting > numbers not anywhere close to each other). Isn't the EMULATED case so much slower than anything else that it is even pretty obvious from a single access? (Possibly the 2nd access to avoid 'cold cache'.) One of the things that can perturb measurements is hardware interrupts. That can be mitigated by counting clocks for a few (10 is plenty) iterations of a short request and taking the fastest value. For short hot-cache code sequences you can actually compare the actual clock counts with theoretical minimum values. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
