BuildN             bool                    // -n flag
	BuildO             string                  // -o flag
	BuildP             = runtime.GOMAXPROCS(0) // -p flag
	BuildPGO           string                  // -pgo flag
	BuildPkgdir        string                  // -pkgdir flag
	BuildRace          bool                    // -race flag
	BuildToolexec      []string                // -toolexec flag
	BuildToolchainName string

	// Now that cmd/go is in charge of the build process, enable GOEXPERIMENT.
	os.Setenv("GOEXPERIMENT", goexperiment)
	// No need to enable PGO for toolchain2.
	goInstall(toolenv(), goBootstrap, append([]string{"-pgo=off"}, toolchain...)...)
	if debug {
		run("", ShowOutput|CheckExit, pathf("%s/compile", tooldir), "-V=full")

			return fmt.Errorf("marshal embedcfg: %v", err)
		}
		embedcfg = js
	}

	// Find PGO profile if needed.
	var pgoProfile string
	for _, a1 := range a.Deps {
		if a1.Mode != "preprocess PGO profile" {
			continue
		}
		if pgoProfile != "" {
			return fmt.Errorf("action contains multiple PGO profile dependencies")
		}
		pgoProfile = a1.built
	}

		all := pkgs
		if !*listDeps {
			all = loadPackageList(pkgs)
		}
		// Update import paths to distinguish the real package p
		// from p recompiled for q.test, or to distinguish between
		// p compiled with different PGO profiles.
		// This must happen only once the build code is done
		// looking at import paths, because it will get very confused
		// if it sees these.
		old := make(map[string]string)
		for _, p := range all {

//		run through go run and go test respectively.
//	-pgo file
//		specify the file path of a profile for profile-guided optimization (PGO).
//		When the special name "auto" is specified, for each main package in the
//		build, the go command selects a file named "default.pgo" in the package's
//		directory if that file exists, and applies it to the (transitive)

			// So far this has only been shown profitable for PGO-hot loop headers.
			// The Hotness values allows distinctions betwen initial blocks that are "hot" or not, and "flow-in" or not.
			// Currently only the initial blocks of loops are tagged in this way;
			// there are no blocks tagged "pgo-hot" that are not also tagged "initial".
			// TODO more heuristics, more architectures.