cmd/internal/objabi: centralize GOEXPERIMENT parsing

objabi parses GOEXPERIMENT, but most of the consumers look at the raw
GOEXPERIMENT string that objabi gets from the environment. Centralize
this logic by only exposing the parsed GOEXPERIMENT value from objabi.
This sets us up for the next few changes. It also has the nice but
mostly useless property that the order of experiment names will be
canonicalized in build cache hashes.

After this, the only remaining place that looks at raw GOEXPERIMENT is
cmd/dist, which we'll fix in the next CL.

For #40724.

Change-Id: Idb150f848e17c184fae91372ca8b361591472f51
Reviewed-on: https://go-review.googlesource.com/c/go/+/302049
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/go/internal/cfg/cfg.go

@@ -258,7 +258,6 @@ var (
 	GOMIPS64 = envOr("GOMIPS64", objabi.GOMIPS64)
 	GOPPC64  = envOr("GOPPC64", fmt.Sprintf("%s%d", "power", objabi.GOPPC64))
 	GOWASM   = envOr("GOWASM", fmt.Sprint(objabi.GOWASM))
-	GOEXPERIMENT = envOr("GOEXPERIMENT", objabi.GOEXPERIMENT)
 
 	GOPROXY    = envOr("GOPROXY", "https://proxy.golang.org,direct")
 	GOSUMDB    = envOr("GOSUMDB", "sum.golang.org")

src/cmd/go/internal/work/exec.go

@@ -8,7 +8,6 @@ package work
 
 import (
 	"bytes"
-	"cmd/go/internal/fsys"
 	"context"
 	"encoding/json"
 	"errors"
@@ -31,10 +30,12 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/cache"
 	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
 	"cmd/go/internal/load"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/str"
 	"cmd/go/internal/trace"
+	"cmd/internal/objabi"
 )
 
 // actionList returns the list of actions in the dag rooted at root
@@ -276,8 +277,8 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 		key, val := cfg.GetArchEnv()
 		fmt.Fprintf(h, "%s=%s\n", key, val)
 
-		if exp := cfg.GOEXPERIMENT; exp != "" {
-			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", exp)
+		if objabi.GOEXPERIMENT != "" {
+			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", objabi.GOEXPERIMENT)
 		}
 
 		// TODO(rsc): Convince compiler team not to add more magic environment variables,
@@ -1250,8 +1251,8 @@ func (b *Builder) printLinkerConfig(h io.Writer, p *load.Package) {
 		key, val := cfg.GetArchEnv()
 		fmt.Fprintf(h, "%s=%s\n", key, val)
 
-		if exp := cfg.GOEXPERIMENT; exp != "" {
-			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", exp)
+		if objabi.GOEXPERIMENT != "" {
+			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", objabi.GOEXPERIMENT)
 		}
 
 		// The linker writes source file paths that say GOROOT_FINAL, but

src/cmd/go/internal/work/init.go

@@ -52,16 +52,11 @@ func BuildInit() {
 	// build tag with the same name but prefixed by "goexperiment." which can be
 	// used for compiling alternative files for the experiment. This allows
 	// changes for the experiment, like extra struct fields in the runtime,
-	// without affecting the base non-experiment code at all. [2:] strips the
-	// leading "X:" from objabi.Expstring().
-	exp := objabi.Expstring()[2:]
-	if exp != "none" {
-		experiments := strings.Split(exp, ",")
-		for _, expt := range experiments {
+	// without affecting the base non-experiment code at all.
+	for _, expt := range strings.Split(objabi.GOEXPERIMENT, ",") {
 		cfg.BuildContext.BuildTags = append(cfg.BuildContext.BuildTags, "goexperiment."+expt)
 	}
 }
-}
 
 func instrumentInit() {
 	if !cfg.BuildRace && !cfg.BuildMSan {

src/cmd/internal/objabi/flag.go

@@ -95,7 +95,7 @@ func (versionFlag) Set(s string) error {
 	// to distinguish go1.10.2 with an experiment
 	// from go1.10.2 without an experiment.
 	p := Expstring()
-	if p == DefaultExpstring() {
+	if p == defaultExpstring {
 		p = ""
 	}
 	sep := ""

src/cmd/internal/objabi/util.go

@@ -24,7 +24,6 @@ var (
 	GOROOT   = envOr("GOROOT", defaultGOROOT)
 	GOARCH   = envOr("GOARCH", defaultGOARCH)
 	GOOS     = envOr("GOOS", defaultGOOS)
-	GOEXPERIMENT = envOr("GOEXPERIMENT", defaultGOEXPERIMENT)
 	GO386    = envOr("GO386", defaultGO386)
 	GOARM    = goarm()
 	GOMIPS   = gomips()
@@ -33,6 +32,12 @@ var (
 	GOWASM   = gowasm()
 	GO_LDSO  = defaultGO_LDSO
 	Version  = version
+
+	// GOEXPERIMENT is a comma-separated list of enabled
+	// experiments. This is derived from the GOEXPERIMENT
+	// environment variable if set, or the value of GOEXPERIMENT
+	// when make.bash was run if not.
+	GOEXPERIMENT string // Set by package init
 )
 
 const (
@@ -125,7 +130,12 @@ func Getgoextlinkenabled() string {
 }
 
 func init() {
-	for _, f := range strings.Split(GOEXPERIMENT, ",") {
+	// Capture "default" experiments.
+	defaultExpstring = Expstring()
+
+	goexperiment := envOr("GOEXPERIMENT", defaultGOEXPERIMENT)
+
+	for _, f := range strings.Split(goexperiment, ",") {
 		if f != "" {
 			addexp(f)
 		}
@@ -135,6 +145,9 @@ func init() {
 	if GOARCH != "amd64" {
 		Regabi_enabled = 0
 	}
+
+	// Set GOEXPERIMENT to the parsed and canonicalized set of experiments.
+	GOEXPERIMENT = expList()
 }
 
 // Note: must agree with runtime.framepointer_enabled.
@@ -171,7 +184,6 @@ var (
 // Toolchain experiments.
 // These are controlled by the GOEXPERIMENT environment
 // variable recorded when the toolchain is built.
-// This list is also known to cmd/gc.
 var exper = []struct {
 	name string
 	val  *int
@@ -182,21 +194,29 @@ var exper = []struct {
 	{"regabi", &Regabi_enabled},
 }
 
-var defaultExpstring = Expstring()
+var defaultExpstring string
 
-func DefaultExpstring() string {
-	return defaultExpstring
-}
-
-func Expstring() string {
-	buf := "X"
+// expList returns the list of enabled GOEXPERIMENTS as a
+// commas-separated list.
+func expList() string {
+	buf := ""
 	for i := range exper {
 		if *exper[i].val != 0 {
 			buf += "," + exper[i].name
 		}
 	}
-	if buf == "X" {
-		buf += ",none"
+	if len(buf) == 0 {
+		return ""
 	}
-	return "X:" + buf[2:]
+	return buf[1:]
+}
+
+// Expstring returns the GOEXPERIMENT string that should appear in Go
+// version signatures. This always starts with "X:".
+func Expstring() string {
+	list := expList()
+	if list == "" {
+		return "X:none"
+	}
+	return "X:" + list
 }