internal/goexperiment,cmd: consolidate GOEXPERIMENTs into a new package

Currently there's knowledge about the list of GOEXPERIMENTs in a few
different places. This CL introduces a new package and consolidates
the list into one place: the internal/goexperiment.Flags struct type.

This package gives us a central place to document the experiments as
well as the GOEXPERIMENT environment variable itself. It will also
give us a place to put built-time constants derived from the enabled
experiments.

Now the objabi package constructs experiment names by reflecting over
this struct type rather than having a separate list of these names
(this is similar to how the compiler handles command-line flags and
debug options). We also expose a better-typed API to the toolchain for
propagating enabled experiments.

Change-Id: I06e026712b59fe2bd7cd11a869aedb48ffe5a4b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/307817
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>

src/cmd/asm/internal/lex/input.go

@@ -46,31 +46,18 @@ func NewInput(name string) *Input {
 func predefine(defines flags.MultiFlag) map[string]*Macro {
 	macros := make(map[string]*Macro)
 
-	// Set macros for various GOEXPERIMENTs so we can easily
+	// Set macros for GOEXPERIMENTs so we can easily switch
-	// switch runtime assembly code based on them.
+	// runtime assembly code based on them.
 	if *flags.CompilingRuntime {
-		set := func(name string) {
+		for _, exp := range objabi.EnabledExperiments() {
+			// Define macro.
+			name := "GOEXPERIMENT_" + exp
 			macros[name] = &Macro{
 				name:   name,
 				args:   nil,
 				tokens: Tokenize("1"),
 			}
 		}
-		if objabi.Experiment.RegabiWrappers {
-			set("GOEXPERIMENT_regabiwrappers")
-		}
-		if objabi.Experiment.RegabiG {
-			set("GOEXPERIMENT_regabig")
-		}
-		if objabi.Experiment.RegabiReflect {
-			set("GOEXPERIMENT_regabireflect")
-		}
-		if objabi.Experiment.RegabiDefer {
-			set("GOEXPERIMENT_regabidefer")
-		}
-		if objabi.Experiment.RegabiArgs {
-			set("GOEXPERIMENT_regabiargs")
-		}
 	}
 
 	for _, name := range defines {

src/cmd/dist/buildtool.go

@@ -58,6 +58,7 @@ var bootstrapDirs = []string{
 	"debug/macho",
 	"debug/pe",
 	"go/constant",
+	"internal/goexperiment",
 	"internal/goversion",
 	"internal/race",
 	"internal/unsafeheader",

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

@@ -277,8 +277,8 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 		key, val := cfg.GetArchEnv()
 		fmt.Fprintf(h, "%s=%s\n", key, val)
 
-		if objabi.GOEXPERIMENT != "" {
+		if goexperiment := objabi.GOEXPERIMENT(); goexperiment != "" {
-			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", objabi.GOEXPERIMENT)
+			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", goexperiment)
 		}
 
 		// TODO(rsc): Convince compiler team not to add more magic environment variables,
@@ -1251,8 +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 objabi.GOEXPERIMENT != "" {
+		if goexperiment := objabi.GOEXPERIMENT(); goexperiment != "" {
-			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", objabi.GOEXPERIMENT)
+			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", goexperiment)
 		}
 
 		// The linker writes source file paths that say GOROOT_FINAL, but

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

@@ -18,7 +18,6 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
-	"strings"
 )
 
 func BuildInit() {
@@ -53,7 +52,7 @@ func BuildInit() {
 	// 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.
-	for _, expt := range strings.Split(objabi.GOEXPERIMENT, ",") {
+	for _, expt := range objabi.EnabledExperiments() {
 		cfg.BuildContext.BuildTags = append(cfg.BuildContext.BuildTags, "goexperiment."+expt)
 	}
 }

src/cmd/internal/objabi/exp.go

@@ -0,0 +1,132 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package objabi
+
+import (
+	"fmt"
+	"os"
+	"reflect"
+	"strings"
+
+	"internal/goexperiment"
+)
+
+// Experiment contains the toolchain experiments enabled for the
+// current build.
+//
+// (This is not necessarily the set of experiments the compiler itself
+// was built with.)
+var Experiment goexperiment.Flags
+
+var defaultExpstring string // Set by package init
+
+// FramePointerEnabled enables the use of platform conventions for
+// saving frame pointers.
+//
+// This used to be an experiment, but now it's always enabled on
+// platforms that support it.
+//
+// Note: must agree with runtime.framepointer_enabled.
+var FramePointerEnabled = GOARCH == "amd64" || GOARCH == "arm64"
+
+func init() {
+	// Capture "default" experiments.
+	defaultExpstring = Expstring()
+
+	goexperiment := envOr("GOEXPERIMENT", defaultGOEXPERIMENT)
+
+	// GOEXPERIMENT=none overrides all experiments enabled at dist time.
+	if goexperiment != "none" {
+		// Create a map of known experiment names.
+		names := make(map[string]reflect.Value)
+		rv := reflect.ValueOf(&Experiment).Elem()
+		rt := rv.Type()
+		for i := 0; i < rt.NumField(); i++ {
+			field := rv.Field(i)
+			names[strings.ToLower(rt.Field(i).Name)] = field
+		}
+
+		// Parse names.
+		for _, f := range strings.Split(goexperiment, ",") {
+			if f == "" {
+				continue
+			}
+			val := true
+			if strings.HasPrefix(f, "no") {
+				f, val = f[2:], false
+			}
+			field, ok := names[f]
+			if !ok {
+				fmt.Printf("unknown experiment %s\n", f)
+				os.Exit(2)
+			}
+			field.SetBool(val)
+		}
+	}
+
+	// regabi is only supported on amd64.
+	if GOARCH != "amd64" {
+		Experiment.Regabi = false
+		Experiment.RegabiWrappers = false
+		Experiment.RegabiG = false
+		Experiment.RegabiReflect = false
+		Experiment.RegabiDefer = false
+		Experiment.RegabiArgs = false
+	}
+	// Setting regabi sets working sub-experiments.
+	if Experiment.Regabi {
+		Experiment.RegabiWrappers = true
+		Experiment.RegabiG = true
+		Experiment.RegabiReflect = true
+		Experiment.RegabiDefer = true
+		// Not ready yet:
+		//Experiment.RegabiArgs = true
+	}
+	// Check regabi dependencies.
+	if Experiment.RegabiG && !Experiment.RegabiWrappers {
+		panic("GOEXPERIMENT regabig requires regabiwrappers")
+	}
+	if Experiment.RegabiArgs && !(Experiment.RegabiWrappers && Experiment.RegabiG && Experiment.RegabiReflect && Experiment.RegabiDefer) {
+		panic("GOEXPERIMENT regabiargs requires regabiwrappers,regabig,regabireflect,regabidefer")
+	}
+}
+
+// expList returns the list of enabled GOEXPERIMENTs names.
+func expList(flags *goexperiment.Flags) []string {
+	var list []string
+	rv := reflect.ValueOf(&Experiment).Elem()
+	rt := rv.Type()
+	for i := 0; i < rt.NumField(); i++ {
+		val := rv.Field(i).Bool()
+		if val {
+			field := rt.Field(i)
+			list = append(list, strings.ToLower(field.Name))
+		}
+	}
+	return list
+}
+
+// Expstring returns the GOEXPERIMENT string that should appear in Go
+// version signatures. This always starts with "X:".
+func Expstring() string {
+	list := expList(&Experiment)
+	if len(list) == 0 {
+		return "X:none"
+	}
+	return "X:" + strings.Join(list, ",")
+}
+
+// GOEXPERIMENT returns 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.
+func GOEXPERIMENT() string {
+	return strings.Join(expList(&Experiment), ",")
+}
+
+// EnabledExperiments returns a list of enabled experiments, as
+// lower-cased experiment names.
+func EnabledExperiments() []string {
+	return expList(&Experiment)
+}

src/cmd/internal/objabi/util.go

@@ -5,7 +5,6 @@
 package objabi
 
 import (
-	"fmt"
 	"log"
 	"os"
 	"strings"
@@ -32,12 +31,6 @@ 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 (
@@ -128,175 +121,3 @@ func gowasm() (f gowasmFeatures) {
 func Getgoextlinkenabled() string {
 	return envOr("GO_EXTLINK_ENABLED", defaultGO_EXTLINK_ENABLED)
 }
-
-func init() {
-	// Capture "default" experiments.
-	defaultExpstring = Expstring()
-
-	goexperiment := envOr("GOEXPERIMENT", defaultGOEXPERIMENT)
-
-	// GOEXPERIMENT=none overrides all experiments enabled at dist time.
-	if goexperiment != "none" {
-		for _, f := range strings.Split(goexperiment, ",") {
-			if f != "" {
-				addexp(f)
-			}
-		}
-	}
-
-	// regabi is only supported on amd64.
-	if GOARCH != "amd64" {
-		Experiment.regabi = false
-		Experiment.RegabiWrappers = false
-		Experiment.RegabiG = false
-		Experiment.RegabiReflect = false
-		Experiment.RegabiDefer = false
-		Experiment.RegabiArgs = false
-	}
-	// Setting regabi sets working sub-experiments.
-	if Experiment.regabi {
-		Experiment.RegabiWrappers = true
-		Experiment.RegabiG = true
-		Experiment.RegabiReflect = true
-		Experiment.RegabiDefer = true
-		// Not ready yet:
-		//Experiment.RegabiArgs = true
-	}
-	// Check regabi dependencies.
-	if Experiment.RegabiG && !Experiment.RegabiWrappers {
-		panic("GOEXPERIMENT regabig requires regabiwrappers")
-	}
-	if Experiment.RegabiArgs && !(Experiment.RegabiWrappers && Experiment.RegabiG && Experiment.RegabiReflect && Experiment.RegabiDefer) {
-		panic("GOEXPERIMENT regabiargs requires regabiwrappers,regabig,regabireflect,regabidefer")
-	}
-
-	// Set GOEXPERIMENT to the parsed and canonicalized set of experiments.
-	GOEXPERIMENT = expList()
-}
-
-// FramePointerEnabled enables the use of platform conventions for
-// saving frame pointers.
-//
-// This used to be an experiment, but now it's always enabled on
-// platforms that support it.
-//
-// Note: must agree with runtime.framepointer_enabled.
-var FramePointerEnabled = GOARCH == "amd64" || GOARCH == "arm64"
-
-func addexp(s string) {
-	// We could do general integer parsing here, but there's no need yet.
-	v, vb := 1, true
-	name := s
-	if len(name) > 2 && name[:2] == "no" {
-		v, vb = 0, false
-		name = name[2:]
-	}
-	for i := 0; i < len(exper); i++ {
-		if exper[i].name == name {
-			switch val := exper[i].val.(type) {
-			case *int:
-				*val = v
-			case *bool:
-				*val = vb
-			default:
-				panic("bad GOEXPERIMENT type for " + s)
-			}
-			return
-		}
-	}
-
-	fmt.Printf("unknown experiment %s\n", s)
-	os.Exit(2)
-}
-
-// Experiment contains flags for GOEXPERIMENTs.
-var Experiment = ExpFlags{}
-
-type ExpFlags struct {
-	FieldTrack        bool
-	PreemptibleLoops  bool
-	StaticLockRanking bool
-
-	// regabi is split into several sub-experiments that can be
-	// enabled individually. GOEXPERIMENT=regabi implies the
-	// subset that are currently "working". Not all combinations work.
-	regabi bool
-	// RegabiWrappers enables ABI wrappers for calling between
-	// ABI0 and ABIInternal functions. Without this, the ABIs are
-	// assumed to be identical so cross-ABI calls are direct.
-	RegabiWrappers bool
-	// RegabiG enables dedicated G and zero registers in
-	// ABIInternal.
-	//
-	// Requires wrappers because it makes the ABIs incompatible.
-	RegabiG bool
-	// RegabiReflect enables the register-passing paths in
-	// reflection calls. This is also gated by intArgRegs in
-	// reflect and runtime (which are disabled by default) so it
-	// can be used in targeted tests.
-	RegabiReflect bool
-	// RegabiDefer enables desugaring defer and go calls
-	// into argument-less closures.
-	RegabiDefer bool
-	// RegabiArgs enables register arguments/results in all
-	// compiled Go functions.
-	//
-	// Requires wrappers (to do ABI translation), g (because
-	// runtime assembly that's been ported to ABIInternal uses the
-	// G register), reflect (so reflection calls use registers),
-	// and defer (because the runtime doesn't support passing
-	// register arguments to defer/go).
-	RegabiArgs bool
-}
-
-// Toolchain experiments.
-// These are controlled by the GOEXPERIMENT environment
-// variable recorded when the toolchain is built.
-var exper = []struct {
-	name string
-	val  interface{} // Must be *int or *bool
-}{
-	{"fieldtrack", &Experiment.FieldTrack},
-	{"preemptibleloops", &Experiment.PreemptibleLoops},
-	{"staticlockranking", &Experiment.StaticLockRanking},
-	{"regabi", &Experiment.regabi},
-	{"regabiwrappers", &Experiment.RegabiWrappers},
-	{"regabig", &Experiment.RegabiG},
-	{"regabireflect", &Experiment.RegabiReflect},
-	{"regabidefer", &Experiment.RegabiDefer},
-	{"regabiargs", &Experiment.RegabiArgs},
-}
-
-var defaultExpstring string
-
-// expList returns the list of enabled GOEXPERIMENTS as a
-// commas-separated list.
-func expList() string {
-	buf := ""
-	for i := range exper {
-		switch val := exper[i].val.(type) {
-		case *int:
-			if *val != 0 {
-				buf += "," + exper[i].name
-			}
-		case *bool:
-			if *val {
-				buf += "," + exper[i].name
-			}
-		}
-	}
-	if len(buf) == 0 {
-		return ""
-	}
-	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
-}

src/internal/goexperiment/flags.go

@@ -0,0 +1,72 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package goexperiment implements support for toolchain experiments.
+//
+// Toolchain experiments are controlled by the GOEXPERIMENT
+// environment variable. GOEXPERIMENT is a comma-separated list of
+// experiment names. GOEXPERIMENT can be set at make.bash time, which
+// sets the default experiments for binaries built with the tool
+// chain; or it can be set at build time. GOEXPERIMENT can also be set
+// to "none", which disables any experiments that were enabled at
+// make.bash time.
+//
+// Experiments are exposed to the build in the following ways:
+//
+// - Build tag goexperiment.x is set if experiment x (lower case) is
+// enabled.
+//
+// - In runtime assembly, the macro GOEXPERIMENT_x is defined if
+// experiment x (lower case) is enabled.
+//
+// - TODO(austin): More to come.
+//
+// In the toolchain, the set of experiments enabled for the current
+// build should be accessed via objabi.Experiment.
+//
+// For the set of experiments supported by the current toolchain, see
+// go doc internal/experiment.Flags.
+package goexperiment
+
+// Flags is the set of experiments that can be enabled or disabled in
+// the current toolchain.
+//
+// When specified in the GOEXPERIMENT environment variable or as build
+// tags, experiments use the strings.ToLower of their field name.
+type Flags struct {
+	FieldTrack        bool
+	PreemptibleLoops  bool
+	StaticLockRanking bool
+
+	// Regabi is split into several sub-experiments that can be
+	// enabled individually. GOEXPERIMENT=regabi implies the
+	// subset that are currently "working". Not all combinations work.
+	Regabi bool
+	// RegabiWrappers enables ABI wrappers for calling between
+	// ABI0 and ABIInternal functions. Without this, the ABIs are
+	// assumed to be identical so cross-ABI calls are direct.
+	RegabiWrappers bool
+	// RegabiG enables dedicated G and zero registers in
+	// ABIInternal.
+	//
+	// Requires wrappers because it makes the ABIs incompatible.
+	RegabiG bool
+	// RegabiReflect enables the register-passing paths in
+	// reflection calls. This is also gated by intArgRegs in
+	// reflect and runtime (which are disabled by default) so it
+	// can be used in targeted tests.
+	RegabiReflect bool
+	// RegabiDefer enables desugaring defer and go calls
+	// into argument-less closures.
+	RegabiDefer bool
+	// RegabiArgs enables register arguments/results in all
+	// compiled Go functions.
+	//
+	// Requires wrappers (to do ABI translation), g (because
+	// runtime assembly that's been ported to ABIInternal uses the
+	// G register), reflect (so reflection calls use registers),
+	// and defer (because the runtime doesn't support passing
+	// register arguments to defer/go).
+	RegabiArgs bool
+}