cmd/go: work out VCS information once per repository
We need VCS status information for each main package we load.
If two main packages are under the same VCS repository,
we can reuse that information to avoid duplicating work.
For instance, the kubernetes holds 51 main packages in its root module,
meaning that "go list ./..." repeated the same git calls 51 times.
Instead, use a global par.Cache to deduplicate that work.
Below are the numbers on kubernetes
5eb584d1cb6917,
via "benchcmd -n 8 KubernetesListPackages go list ./...":
name old time/op new time/op delta
KubernetesListPackages 8.91s ± 0% 3.33s ± 1% -62.61% (p=0.000 n=7+8)
name old user-time/op new user-time/op delta
KubernetesListPackages 11.2s ± 1% 8.1s ± 2% -27.50% (p=0.000 n=7+8)
name old sys-time/op new sys-time/op delta
KubernetesListPackages 8.02s ± 0% 1.67s ± 6% -79.21% (p=0.001 n=6+8)
name old peak-RSS-bytes new peak-RSS-bytes delta
KubernetesListPackages 127MB ± 2% 123MB ± 7% ~ (p=0.328 n=8+8)
Fixes #49582.
Change-Id: Ib7ef5dc7a35c83a11e209441f5d6f3b8da068259
Reviewed-on: https://go-review.googlesource.com/c/go/+/365394
Trust: Daniel Martí <mvdan@mvdan.cc>
Trust: Dominik Honnef <dominik@honnef.co>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>