> Regarding the latter, Plan 9 does not allow floating point
> instructions to be executed within note handling, but erring on the
> side of caution also forbids instructions such as MOVOU (don't ask me)
> which is part of the SSE(2?) extension, but hardly qualifies as a
> floating point instruction.

movou (movdqu in the manual) is a sse2 data movement instruction.
not all sse2 instructions require that sse be turned on (pause, for example),
but movou uses at least one xmm register so is clearly using the sse
unit, thus requiring that it be turned on.

the go runtime memmove uses movou for memmoves between 33 and 128
bytes.  i only see a 10 cycle difference for these cases on my atom machine,
(maximum 13%), so we're not missing out on much here by not using sse.

the real win, or loss for the plan 9 memmove, is in the short memmoves.
but this is a µbenchmark, and it would be more convincing with a real
world test.

- erik

harness; 8.memmovetest
memmove
1	92.42578 cycles/op
2	81.28125 cycles/op
4	56.47266 cycles/op
8	58.32422 cycles/op
16	62.28516 cycles/op
32	70.26563 cycles/op
64	86.32031 cycles/op
128	118.3125 cycles/op
512	323.5078 cycles/op
1024	587.1094 cycles/op
4096	2119.242 cycles/op
131072	133058.5 cycles/op

rt·memmove
1	20.60156 cycles/op
2	20.34375 cycles/op
4	24.46875 cycles/op
8	22.42969 cycles/op
16	27.45703 cycles/op
32	52.82813 cycles/op
64	79.19531 cycles/op
128	129.1289 cycles/op
512	314.4492 cycles/op
1024	569.9648 cycles/op
4096	2132.297 cycles/op
131072	135378.3 cycles/op