cmd/go: add duration on each -x of go build to know which pkg that slow

[kokizzu]

What version of Go are you using (go version)?

$ go version
go version go1.14 linux/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/kyz/.cache/go-build"
GOENV="/home/kyz/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/kyz/MEGA/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build483691684=/tmp/go-build -gno-record-gcc-switches"

What did you do?

compile a project

What did you expect to see?

compiled fast

What did you see instead?

took 13s-ish each time code changed using SSD and i7-4770HQ

it would be nice if we could see time of each -x flag to see which part or pkg causes slowdown (especially more detailed on linking part)

[dmitshur]

/cc @bcmills @matloob @jayconrod

[bcmills]

See #15736 (comment).

We could perhaps also emit timing information, although I'm not sure it's all that useful independent of the action graph: since steps can and do occur in parallel, the step that takes the longest is not necessarily the bottleneck. (That said, the bottleneck is almost always linking the final executable, often with the system linker.)

[bcmills]

@kokizzu, does -debug-actiongraph address your use-case? Otherwise, it's not at all clear to me how this would help for builds with non-trivial parallelism.

[kokizzu]

nice, never know there was this flag, thanks.