cmd/go: update ToolTags based on GOARCH value

The build.Context ToolTags value is set based on the set of enabled
experiments, which in turn depends on GOARCH. Before this CL the set
of experiments was being set based on GOARCH in the environment.
That is normally fine, but fails with cmd/go when somebody has run
"go env -w GOARCH=val"; in that case cmd/go changes its GOARCH value
after initialization. The new GOARCH value was affect the set of
enabled experiments, which can affect the ToolTags value. With this
CL, we update ToolTags in cmd/go based on the GOARCH value it is using.

This is a pretty ugly fix. We should do something cleaner for 1.18.

Fixes #46815

Change-Id: Ie9416781a168248813c3da8afdc257acdd3fef7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/329930
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -77,6 +77,14 @@ func defaultContext() build.Context {
 	ctxt.GOOS = envOr("GOOS", ctxt.GOOS)
 	ctxt.GOARCH = envOr("GOARCH", ctxt.GOARCH)
 
+	// The experiments flags are based on GOARCH, so they may
+	// need to change.  TODO: This should be cleaned up.
+	buildcfg.UpdateExperiments(ctxt.GOARCH)
+	ctxt.ToolTags = nil
+	for _, exp := range buildcfg.EnabledExperiments() {
+		ctxt.ToolTags = append(ctxt.ToolTags, "goexperiment."+exp)
+	}
+
 	// The go/build rule for whether cgo is enabled is:
 	//	1. If $CGO_ENABLED is set, respect it.
 	//	2. Otherwise, if this is a cross-compile, disable cgo.

src/cmd/go/testdata/script/env_cross_build.txt

@@ -0,0 +1,29 @@
+# Test that the corect default GOEXPERIMENT is used when cross
+# building with GOENV (#46815).
+
+# Unset variables set by the TestScript harness. Users typically won't
+# explicitly configure these, and #46815 doesn't repro if they are.
+env GOOS=
+env GOARCH=
+env GOEXPERIMENT=
+
+env GOENV=windows-amd64
+go build internal/abi
+
+env GOENV=ios-arm64
+go build internal/abi
+
+env GOENV=linux-mips
+go build internal/abi
+
+-- windows-amd64 --
+GOOS=windows
+GOARCH=amd64
+
+-- ios-arm64 --
+GOOS=ios
+GOARCH=arm64
+
+-- linux-mips --
+GOOS=linux
+GOARCH=mips

src/internal/buildcfg/exp.go

@@ -18,7 +18,7 @@ import (
 //
 // (This is not necessarily the set of experiments the compiler itself
 // was built with.)
-var Experiment goexperiment.Flags = parseExperiments()
+var Experiment goexperiment.Flags = parseExperiments(GOARCH)
 
 var regabiSupported = GOARCH == "amd64" && (GOOS == "android" || GOOS == "linux" || GOOS == "darwin" || GOOS == "windows")
 
@@ -42,7 +42,7 @@ var experimentBaseline = goexperiment.Flags{
 // Note: must agree with runtime.framepointer_enabled.
 var FramePointerEnabled = GOARCH == "amd64" || GOARCH == "arm64"
 
-func parseExperiments() goexperiment.Flags {
+func parseExperiments(goarch string) goexperiment.Flags {
 	// Start with the statically enabled set of experiments.
 	flags := experimentBaseline
 
@@ -99,7 +99,7 @@ func parseExperiments() goexperiment.Flags {
 	}
 
 	// regabi is only supported on amd64.
-	if GOARCH != "amd64" {
+	if goarch != "amd64" {
 		flags.RegabiWrappers = false
 		flags.RegabiG = false
 		flags.RegabiReflect = false
@@ -165,3 +165,10 @@ func EnabledExperiments() []string {
 func AllExperiments() []string {
 	return expList(&Experiment, nil, true)
 }
+
+// UpdateExperiments updates the Experiment global based on a new GOARCH value.
+// This is only required for cmd/go, which can change GOARCH after
+// program startup due to use of "go env -w".
+func UpdateExperiments(goarch string) {
+	Experiment = parseExperiments(goarch)
+}