cmd/asm: require -p flag

CL 391014 requires the compiler to be invoked with the -p flag, to
specify the package path. Later, CL 394217 makes the compiler to
produce an unlinkable object file, so "go tool compile x.go" can
still be used on the command line. This CL does the same for the
assembler, requiring -p, otherwise generating an unlinkable object.

No special case for the main package, as the main package cannot
be only assembly code, and there is no way to tell if it is the
main package from an assembly file.

Now we guarantee that we always have an expanded package path in
the object file. A later CL will delete the name expansion code
in the linker.

Change-Id: I8c10661aaea2ff794614924ead958d80e7e2487d
Reviewed-on: https://go-review.googlesource.com/c/go/+/404298
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -6,6 +6,7 @@
 package flags
 
 import (
+	"cmd/internal/obj"
 	"cmd/internal/objabi"
 	"flag"
 	"fmt"
@@ -23,7 +24,7 @@ var (
 	Linkshared       = flag.Bool("linkshared", false, "generate code that will be linked against Go 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")
+	Importpath       = flag.String("p", obj.UnlinkablePkg, "set expected package import to path")
 	Spectre          = flag.String("spectre", "", "enable spectre mitigations in `list` (all, ret)")
 	CompilingRuntime = flag.Bool("compiling-runtime", false, "source to be compiled is part of the Go runtime")
 )

src/cmd/internal/goobj/objfile.go

@@ -281,10 +281,10 @@ const SymSize = stringRefSize + 2 + 1 + 1 + 1 + 4 + 4
 const SymABIstatic = ^uint16(0)
 
 const (
-	ObjFlagShared            = 1 << iota // this object is built with -shared
+	ObjFlagShared       = 1 << iota // this object is built with -shared
-	ObjFlagNeedNameExpansion             // the linker needs to expand `"".` to package path in symbol names
+	_                               // was ObjFlagNeedNameExpansion
-	ObjFlagFromAssembly                  // object is from asm src, not go
+	ObjFlagFromAssembly             // object is from asm src, not go
-	ObjFlagUnlinkable                    // unlinkable package (linker will emit an error)
+	ObjFlagUnlinkable               // unlinkable package (linker will emit an error)
 )
 
 // Sym.Flag
@@ -873,6 +873,6 @@ func (r *Reader) Flags() uint32 {
 }
 
 func (r *Reader) Shared() bool            { return r.Flags()&ObjFlagShared != 0 }
-func (r *Reader) NeedNameExpansion() bool { return r.Flags()&ObjFlagNeedNameExpansion != 0 }
+func (r *Reader) NeedNameExpansion() bool { return false } // TODO: delete
 func (r *Reader) FromAssembly() bool      { return r.Flags()&ObjFlagFromAssembly != 0 }
 func (r *Reader) Unlinkable() bool        { return r.Flags()&ObjFlagUnlinkable != 0 }

src/cmd/internal/obj/objfile.go

@@ -51,7 +51,7 @@ func WriteObjFile(ctxt *Link, b *bio.Writer) {
 		flags |= goobj.ObjFlagUnlinkable
 	}
 	if w.pkgpath == "" {
-		flags |= goobj.ObjFlagNeedNameExpansion
+		log.Fatal("empty package path")
 	}
 	if ctxt.IsAsm {
 		flags |= goobj.ObjFlagFromAssembly

src/cmd/link/link_test.go

@@ -235,9 +235,9 @@ void foo() {
 	cflags := strings.Fields(runGo("env", "GOGCCFLAGS"))
 
 	// Compile, assemble and pack the Go and C code.
-	runGo("tool", "asm", "-gensymabis", "-o", "symabis", "x.s")
+	runGo("tool", "asm", "-p=main", "-gensymabis", "-o", "symabis", "x.s")
 	runGo("tool", "compile", "-symabis", "symabis", "-p=main", "-o", "x1.o", "main.go")
-	runGo("tool", "asm", "-o", "x2.o", "x.s")
+	runGo("tool", "asm", "-p=main", "-o", "x2.o", "x.s")
 	run(cc, append(cflags, "-c", "-o", "x3.o", "x.c")...)
 	runGo("tool", "pack", "c", "x.a", "x1.o", "x2.o", "x3.o")
 

test/run.go

@@ -1024,7 +1024,7 @@ func (t *test) run() {
 				t.err = fmt.Errorf("write empty go_asm.h: %s", err)
 				return
 			}
-			cmd := []string{goTool(), "tool", "asm", "-gensymabis", "-o", "symabis"}
+			cmd := []string{goTool(), "tool", "asm", "-p=main", "-gensymabis", "-o", "symabis"}
 			cmd = append(cmd, asms...)
 			_, err = runcmd(cmd...)
 			if err != nil {
@@ -1045,7 +1045,7 @@ func (t *test) run() {
 		}
 		objs = append(objs, "go.o")
 		if len(asms) > 0 {
-			cmd = []string{goTool(), "tool", "asm", "-e", "-I", ".", "-o", "asm.o"}
+			cmd = []string{goTool(), "tool", "asm", "-p=main", "-e", "-I", ".", "-o", "asm.o"}
 			cmd = append(cmd, asms...)
 			_, err = runcmd(cmd...)
 			if err != nil {