[dev.link] cmd/asm: new -p option, changes to DWARF generation

Adds a new "-p" option to the assembler, for specifying the import
path of the package being compiled. DWARF generation is now conditional
on having a valid package path -- if we don't know the package path,
then don't emit DWARF.

This is intended to lay the groundwork for removing the various
"patchDWARFname" hacks in the linker.

Change-Id: I5f8315c0881791eb8fe1f2ba32f5bb0ae76f6b98
Reviewed-on: https://go-review.googlesource.com/c/go/+/222718
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/asm/internal/flags/flags.go

@@ -23,6 +23,7 @@ var (
 	Dynlink    = flag.Bool("dynlink", false, "support references to Go symbols defined in other shared libraries")
 	AllErrors  = flag.Bool("e", false, "no limit on number of errors reported")
 	SymABIs    = flag.Bool("gensymabis", false, "write symbol ABI information to output file, don't assemble")
+	Importpath = flag.String("p", "", "set expected package import to path")
 )
 
 var (

src/cmd/asm/main.go

@@ -73,7 +73,7 @@ func main() {
 			pList.Firstpc, ok = parser.Parse()
 			// reports errors to parser.Errorf
 			if ok {
-				obj.Flushplist(ctxt, pList, nil, "")
+				obj.Flushplist(ctxt, pList, nil, *flags.Importpath)
 			}
 		}
 		if !ok {

src/cmd/go/internal/work/gc.go

@@ -37,6 +37,17 @@ func (gcToolchain) linker() string {
 	return base.Tool("link")
 }
 
+func pkgPath(a *Action) string {
+	p := a.Package
+	ppath := p.ImportPath
+	if cfg.BuildBuildmode == "plugin" {
+		ppath = pluginPath(a)
+	} else if p.Name == "main" && !p.Internal.ForceLibrary {
+		ppath = "main"
+	}
+	return ppath
+}
+
 func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, output []byte, err error) {
 	p := a.Package
 	objdir := a.Objdir
@@ -47,12 +58,7 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg []byte, s
 		ofile = objdir + out
 	}
 
-	pkgpath := p.ImportPath
+	pkgpath := pkgPath(a)
-	if cfg.BuildBuildmode == "plugin" {
-		pkgpath = pluginPath(a)
-	} else if p.Name == "main" && !p.Internal.ForceLibrary {
-		pkgpath = "main"
-	}
 	gcargs := []string{"-p", pkgpath}
 	if p.Module != nil && p.Module.GoVersion != "" && allowedVersion(p.Module.GoVersion) {
 		gcargs = append(gcargs, "-lang=go"+p.Module.GoVersion)
@@ -240,7 +246,8 @@ func (a *Action) trimpath() string {
 func asmArgs(a *Action, p *load.Package) []interface{} {
 	// Add -I pkg/GOOS_GOARCH so #include "textflag.h" works in .s files.
 	inc := filepath.Join(cfg.GOROOT, "pkg", "include")
-	args := []interface{}{cfg.BuildToolexec, base.Tool("asm"), "-trimpath", a.trimpath(), "-I", a.Objdir, "-I", inc, "-D", "GOOS_" + cfg.Goos, "-D", "GOARCH_" + cfg.Goarch, forcedAsmflags, p.Internal.Asmflags}
+	pkgpath := pkgPath(a)
+	args := []interface{}{cfg.BuildToolexec, base.Tool("asm"), "-p", pkgpath, "-trimpath", a.trimpath(), "-I", a.Objdir, "-I", inc, "-D", "GOOS_" + cfg.Goos, "-D", "GOARCH_" + cfg.Goarch, forcedAsmflags, p.Internal.Asmflags}
 	if p.ImportPath == "runtime" && cfg.Goarch == "386" {
 		for _, arg := range forcedAsmflags {
 			if arg == "-dynlink" {

src/cmd/internal/obj/plist.go

@@ -109,8 +109,10 @@ func Flushplist(ctxt *Link, plist *Plist, newprog ProgAlloc, myimportpath string
 			continue
 		}
 		linkpcln(ctxt, s)
+		if myimportpath != "" {
 			ctxt.populateDWARF(plist.Curfn, s, myimportpath)
 		}
+	}
 }
 
 func (ctxt *Link) InitTextSym(s *LSym, flag int) {