From: Geert Uytterhoeven > Sent: 14 September 2023 08:33 ... > > > rzfive: > > > cpu0: Ratio of byte access time to unaligned word access is > > > 1.05, unaligned accesses are fast > > > > Hrm, I'm a little surprised to be seeing this number come out so close > > to 1. If you reboot a few times, what kind of variance do you get on > > this? > > Rock-solid at 1.05 (even with increased resolution: 1.05853 on 3 tries) Would that match zero overhead unless the access crosses a cache line boundary? (I can't remember whether the test is using increasing addresses.) ... > > > vexriscv/orangecrab: > > > > > > cpu0: Ratio of byte access time to unaligned word access is > > > 0.00, unaligned accesses are slow > > cpu0: Ratio of byte access time to unaligned word access is 0.00417, > unaligned accesses are slow > > > > I am a bit surprised by the near-zero values. Are these expected? > > > > This could be expected, if firmware is trapping the unaligned accesses > > and coming out >100x slower than a native access. If you're interested > > in getting a little more resolution, you could try to print a few more > > decimal places with something like (sorry gmail mangles the whitespace > > on this): I'd expect one of three possible values: - 1.0x: Basically zero cost except for cache line/page boundaries. - ~2: Hardware does two reads and merges the values. - >100: Trap fixed up in software. I'd think the '2' case could be considered fast. You only need to time one access to see if it was a fault. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
