cmd/compile: add -linkobj flag to allow writing object file in two parts

This flag is experimental and the semantics may change
even after Go 1.7 is released. There are no changes to code
not using the flag.

The first part is for reading by future compiles.
The second part is for reading by the final link step.
Splitting the file this way allows distributed build systems
to ship the compile-input part only to compile steps and
the linker-input part only to linker steps.

The first part is basically just the export data,
and the second part is basically everything else.
The overall files still have the same broad structure,
so that existing tools will work with both halves.
It's just that various pieces are empty in the two halves.

This also copies the two bits of data the linker needed from
export data into the object header proper, so that the linker
doesn't need any export data at all. That eliminates a TODO
that was left for switching to the binary export data.
(Now the linker doesn't need to know about the switch.)

The default is still to write out a combined output file.
Nothing changes unless you pass -linkobj to the compiler.
There is no support in the go command for -linkobj,
since the go command doesn't copy objects around.
The expectation is that other build systems (like bazel, say)
might take advantage of this.

The header adjustment and the option for the split output
was intended as part of the zip archives, but the zip archives
have been cut from Go 1.7. Doing this to the current archives
both unblocks one step in the switch to binary export data
and enables alternate build systems to experiment with the
new flag using the Go 1.7 release.

Change-Id: I8b6eab25b8a22b0a266ba0ac6d31e594f3d117f3
Reviewed-on: https://go-review.googlesource.com/22500
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/doc.go

@@ -61,7 +61,12 @@ Flags:
 		Look for packages in $GOROOT/pkg/$GOOS_$GOARCH_suffix
 		instead of $GOROOT/pkg/$GOOS_$GOARCH.
 	-largemodel
-		Generated code that assumes a large memory model.
+		Generate code that assumes a large memory model.
+	-linkobj file
+		Write linker-specific object to file and compiler-specific
+		object to usual output file (as specified by -o).
+		Without this flag, the -o output is a combination of both
+		linker and compiler input.
 	-memprofile file
 		Write memory profile for the compilation to file.
 	-memprofilerate rate

src/cmd/compile/internal/gc/go.go

@@ -133,6 +133,7 @@ var pragcgobuf string
 var infile string
 
 var outfile string
+var linkobj string
 
 var bout *bio.Writer
 

src/cmd/compile/internal/gc/main.go

@@ -178,6 +178,7 @@ func Main() {
 	flag.StringVar(&flag_installsuffix, "installsuffix", "", "set pkg directory `suffix`")
 	obj.Flagcount("j", "debug runtime-initialized variables", &Debug['j'])
 	obj.Flagcount("l", "disable inlining", &Debug['l'])
+	flag.StringVar(&linkobj, "linkobj", "", "write linker-specific object to `file`")
 	obj.Flagcount("live", "debug liveness analysis", &debuglive)
 	obj.Flagcount("m", "print optimization decisions", &Debug['m'])
 	flag.BoolVar(&flag_msan, "msan", false, "build code compatible with C/C++ memory sanitizer")
@@ -772,7 +773,7 @@ func importfile(f *Val, indent []byte) {
 
 	if p != "empty archive" {
 		if !strings.HasPrefix(p, "go object ") {
-			Yyerror("import %s: not a go object file", file)
+			Yyerror("import %s: not a go object file: %s", file, p)
 			errorexit()
 		}
 

src/cmd/compile/internal/gc/obj.go

@@ -22,7 +22,34 @@ func formathdr(arhdr []byte, name string, size int64) {
 	copy(arhdr[:], fmt.Sprintf("%-16s%-12d%-6d%-6d%-8o%-10d`\n", name, 0, 0, 0, 0644, size))
 }
 
+// These modes say which kind of object file to generate.
+// The default use of the toolchain is to set both bits,
+// generating a combined compiler+linker object, one that
+// serves to describe the package to both the compiler and the linker.
+// In fact the compiler and linker read nearly disjoint sections of
+// that file, though, so in a distributed build setting it can be more
+// efficient to split the output into two files, supplying the compiler
+// object only to future compilations and the linker object only to
+// future links.
+//
+// By default a combined object is written, but if -linkobj is specified
+// on the command line then the default -o output is a compiler object
+// and the -linkobj output is a linker object.
+const (
+	modeCompilerObj = 1 << iota
+	modeLinkerObj
+)
+
 func dumpobj() {
+	if linkobj == "" {
+		dumpobj1(outfile, modeCompilerObj|modeLinkerObj)
+	} else {
+		dumpobj1(outfile, modeCompilerObj)
+		dumpobj1(linkobj, modeLinkerObj)
+	}
+}
+
+func dumpobj1(outfile string, mode int) {
 	var err error
 	bout, err = bio.Create(outfile)
 	if err != nil {
@@ -40,8 +67,27 @@ func dumpobj() {
 		startobj = bout.Offset()
 	}
 
-	fmt.Fprintf(bout, "go object %s %s %s %s\n", obj.Getgoos(), obj.Getgoarch(), obj.Getgoversion(), obj.Expstring())
-	dumpexport()
+	printheader := func() {
+		fmt.Fprintf(bout, "go object %s %s %s %s\n", obj.Getgoos(), obj.Getgoarch(), obj.Getgoversion(), obj.Expstring())
+		if buildid != "" {
+			fmt.Fprintf(bout, "build id %q\n", buildid)
+		}
+		if localpkg.Name == "main" {
+			fmt.Fprintf(bout, "main\n")
+		}
+		if safemode {
+			fmt.Fprintf(bout, "safe\n")
+		} else {
+			fmt.Fprintf(bout, "----\n") // room for some other tool to write "safe"
+		}
+		fmt.Fprintf(bout, "\n") // header ends with blank line
+	}
+
+	printheader()
+
+	if mode&modeCompilerObj != 0 {
+		dumpexport()
+	}
 
 	if writearchive {
 		bout.Flush()
@@ -53,12 +99,20 @@ func dumpobj() {
 		formathdr(arhdr[:], "__.PKGDEF", size)
 		bout.Write(arhdr[:])
 		bout.Flush()
-
 		bout.Seek(startobj+size+(size&1), 0)
+	}
+
+	if mode&modeLinkerObj == 0 {
+		bout.Close()
+		return
+	}
+
+	if writearchive {
+		// start object file
 		arhdr = [ArhdrSize]byte{}
 		bout.Write(arhdr[:])
 		startobj = bout.Offset()
-		fmt.Fprintf(bout, "go object %s %s %s %s\n", obj.Getgoos(), obj.Getgoarch(), obj.Getgoversion(), obj.Expstring())
+		printheader()
 	}
 
 	if pragcgobuf != "" {

src/cmd/link/internal/ld/go.go

@@ -59,72 +59,34 @@ func ldpkg(f *bio.Reader, pkg string, length int64, filename string, whence int)
 	}
 	data := string(bdata)
 
-	// first \n$$ marks beginning of exports - skip rest of line
-	p0 = strings.Index(data, "\n$$")
-	if p0 < 0 {
-		if Debug['u'] != 0 && whence != ArchiveObj {
-			Exitf("cannot find export data in %s", filename)
+	// process header lines
+	isSafe := false
+	isMain := false
+	for data != "" {
+		var line string
+		if i := strings.Index(data, "\n"); i >= 0 {
+			line, data = data[:i], data[i+1:]
+		} else {
+			line, data = data, ""
+		}
+		if line == "safe" {
+			isSafe = true
+		}
+		if line == "main" {
+			isMain = true
+		}
+		if line == "" {
+			break
 		}
-		return
 	}
 
-	// \n$$B marks the beginning of binary export data - don't skip over the B
-	p0 += 3
-	for p0 < len(data) && data[p0] != '\n' && data[p0] != 'B' {
-		p0++
-	}
-
-	// second marks end of exports / beginning of local data
-	p1 = strings.Index(data[p0:], "\n$$\n")
-	if p1 < 0 && whence == Pkgdef {
-		p1 = len(data) - p0
-	}
-	if p1 < 0 {
-		fmt.Fprintf(os.Stderr, "%s: cannot find end of exports in %s\n", os.Args[0], filename)
-		if Debug['u'] != 0 {
-			errorexit()
+	if whence == Pkgdef || whence == FileObj {
+		if pkg == "main" && !isMain {
+			Exitf("%s: not package main", filename)
 		}
-		return
-	}
-	p1 += p0
-
-	for p0 < p1 && data[p0] != 'B' && (data[p0] == ' ' || data[p0] == '\t' || data[p0] == '\n') {
-		p0++
-	}
-	// don't check this section if we have binary (B) export data
-	// TODO fix this eventually
-	if p0 < p1 && data[p0] != 'B' {
-		if !strings.HasPrefix(data[p0:], "package ") {
-			fmt.Fprintf(os.Stderr, "%s: bad package section in %s - %.20s\n", os.Args[0], filename, data[p0:])
-			if Debug['u'] != 0 {
-				errorexit()
-			}
-			return
-		}
-
-		p0 += 8
-		for p0 < p1 && (data[p0] == ' ' || data[p0] == '\t' || data[p0] == '\n') {
-			p0++
-		}
-		pname := p0
-		for p0 < p1 && data[p0] != ' ' && data[p0] != '\t' && data[p0] != '\n' {
-			p0++
-		}
-		if Debug['u'] != 0 && whence != ArchiveObj && (p0+6 > p1 || !strings.HasPrefix(data[p0:], " safe\n")) {
+		if Debug['u'] != 0 && whence != ArchiveObj && !isSafe {
 			Exitf("load of unsafe package %s", filename)
 		}
-
-		name := data[pname:p0]
-		for p0 < p1 && data[p0] != '\n' {
-			p0++
-		}
-		if p0 < p1 {
-			p0++
-		}
-
-		if pkg == "main" && name != "main" {
-			Exitf("%s: not package main (package %s)", filename, name)
-		}
 	}
 
 	// __.PKGDEF has no cgo section - those are in the C compiler-generated object files.
@@ -133,7 +95,7 @@ func ldpkg(f *bio.Reader, pkg string, length int64, filename string, whence int)
 	}
 
 	// look for cgo section
-	p0 = strings.Index(data[p1:], "\n$$  // cgo")
+	p0 = strings.Index(data, "\n$$  // cgo")
 	if p0 >= 0 {
 		p0 += p1
 		i := strings.IndexByte(data[p0+1:], '\n')

src/debug/gosym/pclntab_test.go

@@ -5,6 +5,7 @@
 package gosym
 
 import (
+	"bytes"
 	"debug/elf"
 	"internal/testenv"
 	"io/ioutil"
@@ -42,6 +43,21 @@ func dotest(t *testing.T) {
 	if err := cmd.Run(); err != nil {
 		t.Fatal(err)
 	}
+
+	// stamp .o file as being 'package main' so that go tool link will accept it
+	data, err := ioutil.ReadFile(pclinetestBinary + ".o")
+	if err != nil {
+		t.Fatal(err)
+	}
+	i := bytes.IndexByte(data, '\n')
+	if i < 0 {
+		t.Fatal("bad binary")
+	}
+	data = append(append(data[:i:i], "\nmain"...), data[i:]...)
+	if err := ioutil.WriteFile(pclinetestBinary+".o", data, 0666); err != nil {
+		t.Fatal(err)
+	}
+
 	cmd = exec.Command("go", "tool", "link", "-H", "linux",
 		"-o", pclinetestBinary, pclinetestBinary+".o")
 	cmd.Stdout = os.Stdout

test/linkobj.go

@@ -0,0 +1,155 @@
+// +build !nacl
+// run
+
+// Copyright 2016 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.
+
+// Test the compiler -linkobj flag.
+
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"os/exec"
+	"strings"
+)
+
+var pwd, tmpdir string
+
+func main() {
+	dir, err := ioutil.TempDir("", "go-test-linkobj-")
+	if err != nil {
+		log.Fatal(err)
+	}
+	pwd, err = os.Getwd()
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := os.Chdir(dir); err != nil {
+		os.RemoveAll(dir)
+		log.Fatal(err)
+	}
+	tmpdir = dir
+
+	writeFile("p1.go", `
+		package p1
+		
+		func F() {
+			println("hello from p1")
+		}
+	`)
+	writeFile("p2.go", `
+		package p2
+		
+		import "./p1"
+
+		func F() {
+			p1.F()
+			println("hello from p2")
+		}
+		
+		func main() {}
+	`)
+	writeFile("p3.go", `
+		package main
+
+		import "./p2"
+		
+		func main() {
+			p2.F()
+			println("hello from main")
+		}
+	`)
+
+	// two rounds: once using normal objects, again using .a files (compile -pack).
+	for round := 0; round < 2; round++ {
+		pkg := "-pack=" + fmt.Sprint(round)
+
+		// The compiler expects the files being read to have the right suffix.
+		o := "o"
+		if round == 1 {
+			o = "a"
+		}
+
+		// inlining is disabled to make sure that the link objects contain needed code.
+		run("go", "tool", "compile", pkg, "-D", ".", "-I", ".", "-l", "-o", "p1."+o, "-linkobj", "p1.lo", "p1.go")
+		run("go", "tool", "compile", pkg, "-D", ".", "-I", ".", "-l", "-o", "p2."+o, "-linkobj", "p2.lo", "p2.go")
+		run("go", "tool", "compile", pkg, "-D", ".", "-I", ".", "-l", "-o", "p3."+o, "-linkobj", "p3.lo", "p3.go")
+
+		cp("p1."+o, "p1.oo")
+		cp("p2."+o, "p2.oo")
+		cp("p3."+o, "p3.oo")
+		cp("p1.lo", "p1."+o)
+		cp("p2.lo", "p2."+o)
+		cp("p3.lo", "p3."+o)
+		out := runFail("go", "tool", "link", "p2."+o)
+		if !strings.Contains(out, "not package main") {
+			fatalf("link p2.o failed but not for package main:\n%s", out)
+		}
+
+		run("go", "tool", "link", "-L", ".", "-o", "a.out.exe", "p3."+o)
+		out = run("./a.out.exe")
+		if !strings.Contains(out, "hello from p1\nhello from p2\nhello from main\n") {
+			fatalf("running main, incorrect output:\n%s", out)
+		}
+
+		// ensure that mistaken future round can't use these
+		os.Remove("p1.o")
+		os.Remove("a.out.exe")
+	}
+
+	cleanup()
+}
+
+func run(args ...string) string {
+	out, err := exec.Command(args[0], args[1:]...).CombinedOutput()
+	if err != nil {
+		fatalf("run %v: %s\n%s", args, err, out)
+	}
+	return string(out)
+}
+
+func runFail(args ...string) string {
+	out, err := exec.Command(args[0], args[1:]...).CombinedOutput()
+	if err == nil {
+		fatalf("runFail %v: unexpected success!\n%s", args, err, out)
+	}
+	return string(out)
+}
+
+func cp(src, dst string) {
+	data, err := ioutil.ReadFile(src)
+	if err != nil {
+		fatalf("%v", err)
+	}
+	err = ioutil.WriteFile(dst, data, 0666)
+	if err != nil {
+		fatalf("%v", err)
+	}
+}
+
+func writeFile(name, data string) {
+	err := ioutil.WriteFile(name, []byte(data), 0666)
+	if err != nil {
+		fatalf("%v", err)
+	}
+}
+
+func cleanup() {
+	const debug = false
+	if debug {
+		println("TMPDIR:", tmpdir)
+		return
+	}
+	os.Chdir(pwd) // get out of tmpdir before removing it
+	os.RemoveAll(tmpdir)
+}
+
+func fatalf(format string, args ...interface{}) {
+	cleanup()
+	log.Fatalf(format, args...)
+}