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>