runtime: don't acquirem() in vgetrandom unless necessary

I noticed in pprof that acquirem() was a bit of a hotspot. It turns out
that we can use the same trick that runtime.rand() does, and only
acquirem if we're doing something non-nosplit -- in this case, getting a
new state -- but otherwise just do getg().m, which is safe because we're
inside runtime and don't call split functions.

cpu: 11th Gen Intel(R) Core(TM) i7-11850H @ 2.50GHz
                     │   sec/op    │   sec/op     vs base               │
ParallelGetRandom-16   2.651n ± 4%   2.416n ± 7%  -8.87% (p=0.001 n=10)
                     │     B/s      │     B/s       vs base               │
ParallelGetRandom-16   1.406Gi ± 4%   1.542Gi ± 6%  +9.72% (p=0.001 n=10)

Change-Id: Iae075f4e298b923e499cd01adfabacab725a8684
Reviewed-on: https://go-review.googlesource.com/c/go/+/616738
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

0 files changed, 0 insertions, 0 deletions