cmd/internal/objabi: record GO$GOARCH setting in object header

The object header string is meant to record the relevant toolchain
configuration, so that we don't import or link object files that are
incompatible with each other. One important part of compatibility
is the sub-architecture version (GOARM for GOARCH=arm, and so on).

Add the sub-architecture info to the object header line so that
binaries cannot be built that have inconsistent sub-architecture
configurations across the build.

This check is only important when the build system makes a mistake.
Builds using the go command don't make this kind of mistake anymore,
but we just debugged a difficult problem inside Google where a custom
build system had built part of a program with GOARM=5 and part of
a program with GOARM=7, resulting in corrupted execution when
signal-based preemption was attempted. Updating the check will avoid
this kind of problem in the future, in any custom build system, or if the
go command makes a mistake.

After this change:

% sed 3q pkg/darwin_amd64/runtime.a
!<arch>
__.PKGDEF       0           0     0     644     30525     `
go object darwin amd64 devel go1.20-102ebe10b7 Wed Aug 17 14:31:01 2022 -0400 GOAMD64=v1 X:regabiwrappers,regabiargs
%

Change-Id: I901e0758f1002dd2c58292dc65e2d06da86e4495
Reviewed-on: https://go-review.googlesource.com/c/go/+/427174
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: rsc

src/cmd/internal/objabi/util.go

@@ -22,5 +22,12 @@ const (
 // or link object files that are incompatible with each other. This
 // string always starts with "go object ".
 func HeaderString() string {
-	return fmt.Sprintf("go object %s %s %s X:%s\n", buildcfg.GOOS, buildcfg.GOARCH, buildcfg.Version, strings.Join(buildcfg.Experiment.Enabled(), ","))
+	archExtra := ""
+	if k, v := buildcfg.GOGOARCH(); k != "" && v != "" {
+		archExtra = " " + k + "=" + v
+	}
+	return fmt.Sprintf("go object %s %s %s%s X:%s\n",
+		buildcfg.GOOS, buildcfg.GOARCH,
+		buildcfg.Version, archExtra,
+		strings.Join(buildcfg.Experiment.Enabled(), ","))
 }

src/internal/buildcfg/cfg.go

@@ -172,6 +172,28 @@ func experimentTags() []string {
 	return list
 }
 
+// GOGOARCH returns the name and value of the GO$GOARCH setting.
+// For example, if GOARCH is "amd64" it might return "GOAMD64", "v2".
+func GOGOARCH() (name, value string) {
+	switch GOARCH {
+	case "386":
+		return "GO386", GO386
+	case "amd64":
+		return "GOAMD64", fmt.Sprintf("v%d", GOAMD64)
+	case "arm":
+		return "GOARM", fmt.Sprintf("%d", GOARM)
+	case "mips", "mipsle":
+		return "GOMIPS", GOMIPS
+	case "mips64", "mips64le":
+		return "GOMIPS64", GOMIPS64
+	case "ppc64", "ppc64le":
+		return "GOPPC64", fmt.Sprintf("power%d", GOPPC64)
+	case "wasm":
+		return "GOWASM", GOWASM.String()
+	}
+	return "", ""
+}
+
 func gogoarchTags() []string {
 	switch GOARCH {
 	case "386":