diff --git a/src/cmd/go/build.go b/src/cmd/go/build.go
index ba9aa672b60..bee0f2a76db 100644
--- a/src/cmd/go/build.go
+++ b/src/cmd/go/build.go
@@ -106,6 +106,8 @@ type builder struct {
 	vflag       bool                 // the -v flag
 	arch        string               // e.g., "6"
 	goroot      string               // the $GOROOT
+	goarch      string               // the $GOARCH
+	goos        string               // the $GOOS
 	actionCache map[cacheKey]*action // a cache of already-constructed actions
 }
 
@@ -147,8 +149,10 @@ func (b *builder) init(aflag, nflag, vflag bool) {
 	b.vflag = vflag
 	b.actionCache = make(map[cacheKey]*action)
 	b.goroot = runtime.GOROOT()
+	b.goarch = build.DefaultContext.GOARCH
+	b.goos = build.DefaultContext.GOOS
 
-	b.arch, err = build.ArchChar(build.DefaultContext.GOARCH)
+	b.arch, err = build.ArchChar(b.goarch)
 	if err != nil {
 		fatalf("%s", err)
 	}
@@ -236,7 +240,7 @@ func (b *builder) action(mode buildMode, depMode buildMode, p *Package) *action
 		}
 		if p.Standard {
 			switch p.ImportPath {
-			case "runtime", "runtime/cgo":
+			case "runtime/cgo":
 				// Too complex - can't build.
 				a.f = (*builder).nop
 				return a
@@ -362,17 +366,60 @@ func (b *builder) build(a *action) error {
 	// compile Go
 	if len(gofiles) > 0 {
 		out := "_go_.6"
-		if err := b.gc(a.p.Dir, obj+out, a.p.ImportPath, inc, gofiles); err != nil {
+		gcargs := []string{"-p", a.p.ImportPath}
+		if a.p.Standard && a.p.ImportPath == "runtime" {
+			// runtime compiles with a special 6g flag to emit
+			// additional reflect type data.
+			gcargs = append(gcargs, "-+")
+		}
+		if err := b.gc(a.p.Dir, obj+out, gcargs, inc, gofiles); err != nil {
 			return err
 		}
 		objects = append(objects, out)
 	}
 
-	// assemble .s files
-	if len(a.p.SFiles) > 0 {
-		for _, sfile := range a.p.SFiles {
-			out := sfile[:len(sfile)-len(".s")] + "." + b.arch
-			if err := b.asm(a.p.Dir, obj+out, sfile); err != nil {
+	// copy .h files named for goos or goarch or goos_goarch
+	// to names using GOOS and GOARCH.
+	// For example, defs_linux_amd64.h becomes defs_GOOS_GOARCH.h.
+	_goos_goarch := "_" + b.goos + "_" + b.goarch + ".h"
+	_goos := "_" + b.goos + ".h"
+	_goarch := "_" + b.goarch + ".h"
+	for _, file := range a.p.HFiles {
+		switch {
+		case strings.HasSuffix(file, _goos_goarch):
+			targ := file[:len(file)-len(_goos_goarch)] + "_GOOS_GOARCH.h"
+			if err := b.copyFile(obj+targ, filepath.Join(a.p.Dir, file), 0666); err != nil {
+				return err
+			}
+		case strings.HasSuffix(file, _goarch):
+			targ := file[:len(file)-len(_goarch)] + "_GOARCH.h"
+			if err := b.copyFile(obj+targ, filepath.Join(a.p.Dir, file), 0666); err != nil {
+				return err
+			}
+		case strings.HasSuffix(file, _goos):
+			targ := file[:len(file)-len(_goos)] + "_GOOS.h"
+			if err := b.copyFile(obj+targ, filepath.Join(a.p.Dir, file), 0666); err != nil {
+				return err
+			}
+		}
+	}
+
+	// in a cgo package, the .c files are compiled with gcc during b.cgo above.
+	// in a non-cgo package, the .c files are compiled with 5c/6c/8c.
+	// The same convention applies for .s files.
+	if len(a.p.CgoFiles) == 0 {
+		for _, file := range a.p.CFiles {
+			out := file[:len(file)-len(".c")] + "." + b.arch
+			if err := b.cc(a.p.Dir, obj+out, file); err != nil {
+				return err
+			}
+			objects = append(objects, out)
+		}
+
+		// assemble .s files
+		for _, file := range a.p.SFiles {
+			out := file[:len(file)-len(".s")] + "." + b.arch
+			if err := b.asm(a.p.Dir, obj+out, file); err != nil {
 				return err
 			}
 			objects = append(objects, out)
@@ -510,8 +557,10 @@ func (b *builder) mkdir(dir string) error {
 
 // gc runs the Go compiler in a specific directory on a set of files
 // to generate the named output file. 
-func (b *builder) gc(dir, ofile, importPath string, importArgs []string, gofiles []string) error {
-	args := append([]string{b.arch + "g", "-o", ofile, "-p", importPath}, importArgs...)
+func (b *builder) gc(dir, ofile string, gcargs, importArgs []string, gofiles []string) error {
+	args := []string{b.arch + "g", "-o", ofile}
+	args = append(args, gcargs...)
+	args = append(args, importArgs...)
 	args = append(args, gofiles...)
 	return b.run(dir, args...)
 }
@@ -519,7 +568,7 @@ func (b *builder) gc(dir, ofile, importPath string, importArgs []string, gofiles
 // asm runs the assembler in a specific directory on a specific file
 // to generate the named output file. 
 func (b *builder) asm(dir, ofile, sfile string) error {
-	return b.run(dir, b.arch+"a", "-o", ofile, sfile)
+	return b.run(dir, b.arch+"a", "-o", ofile, "-DGOOS_"+b.goos, "-DGOARCH_"+b.goarch, sfile)
 }
 
 // gopack runs the assembler in a specific directory to create
@@ -538,8 +587,8 @@ func (b *builder) ld(dir, out string, importArgs []string, mainpkg string) error
 // to produce an output file.
 func (b *builder) cc(dir, ofile, cfile string) error {
 	inc := filepath.Join(runtime.GOROOT(), "pkg",
-		fmt.Sprintf("%s_%s", build.DefaultContext.GOOS, build.DefaultContext.GOARCH))
-	return b.run(dir, b.arch+"c", "-FVW", "-I", inc, "-o", ofile, cfile)
+		fmt.Sprintf("%s_%s", b.goos, b.goarch))
+	return b.run(dir, b.arch+"c", "-FVw", "-I", inc, "-o", ofile, "-DGOOS_"+b.goos, "-DGOARCH_"+b.goarch, cfile)
 }
 
 // gcc runs the gcc C compiler to create an object from a single C file.
diff --git a/src/cmd/go/list.go b/src/cmd/go/list.go
index 4d8a3609b06..bd75fd6cc4e 100644
--- a/src/cmd/go/list.go
+++ b/src/cmd/go/list.go
@@ -37,6 +37,7 @@ being passed to the template is:
         // Source files
         GoFiles  []string // .go source files (excluding CgoFiles)
         CFiles   []string // .c source files
+        HFiles []string // .h source files
         SFiles   []string // .s source files
         CgoFiles []string // .go sources files that import "C"
 
diff --git a/src/cmd/go/main.go b/src/cmd/go/main.go
index 97db5abce86..79bcd66746f 100644
--- a/src/cmd/go/main.go
+++ b/src/cmd/go/main.go
@@ -239,7 +239,6 @@ func run(cmdline ...string) {
 func allPackages() []string {
 	have := make(map[string]bool)
 	var pkgs []string
-	runtime := filepath.Join(build.Path[0].SrcDir(), "runtime") + string(filepath.Separator)
 	for _, t := range build.Path {
 		src := t.SrcDir() + string(filepath.Separator)
 		filepath.Walk(src, func(path string, fi os.FileInfo, err error) error {
@@ -251,13 +250,6 @@ func allPackages() []string {
 			if strings.HasSuffix(path, string(filepath.Separator)+"testdata") {
 				return filepath.SkipDir
 			}
-			// Avoid runtime subdirectories.
-			if strings.HasPrefix(path, runtime) {
-				switch path {
-				case runtime + "darwin", runtime + "freebsd", runtime + "linux", runtime + "netbsd", runtime + "openbsd", runtime + "windows":
-					return filepath.SkipDir
-				}
-			}
 
 			_, err = build.ScanDir(path)
 			if err != nil {
diff --git a/src/cmd/go/pkg.go b/src/cmd/go/pkg.go
index e9fb3bf3c03..dcb9afa4723 100644
--- a/src/cmd/go/pkg.go
+++ b/src/cmd/go/pkg.go
@@ -29,6 +29,7 @@ type Package struct {
 	// Source files
 	GoFiles  []string // .go source files (excluding CgoFiles)
 	CFiles   []string // .c source files
+	HFiles   []string // .h source files
 	SFiles   []string // .s source files
 	CgoFiles []string // .go sources files that import "C"
 
@@ -120,6 +121,7 @@ func scanPackage(ctxt *build.Context, t *build.Tree, arg, importPath, dir string
 		Imports:    info.Imports,
 		GoFiles:    info.GoFiles,
 		CFiles:     info.CFiles,
+		HFiles:     info.HFiles,
 		SFiles:     info.SFiles,
 		CgoFiles:   info.CgoFiles,
 		Standard:   t.Goroot && !strings.Contains(importPath, "."),
diff --git a/src/pkg/go/build/build_test.go b/src/pkg/go/build/build_test.go
index e22a49aa3d6..e86cfc012ed 100644
--- a/src/pkg/go/build/build_test.go
+++ b/src/pkg/go/build/build_test.go
@@ -48,6 +48,7 @@ var buildPkgs = []struct {
 		&DirInfo{
 			CgoFiles:    []string{"cgotest.go"},
 			CFiles:      []string{"cgotest.c"},
+			HFiles:      []string{"cgotest.h"},
 			Imports:     []string{"C", "unsafe"},
 			TestImports: []string{},
 			Package:     "cgotest",
diff --git a/src/pkg/go/build/dir.go b/src/pkg/go/build/dir.go
index 2c89224fd4b..29d7c4c7d32 100644
--- a/src/pkg/go/build/dir.go
+++ b/src/pkg/go/build/dir.go
@@ -96,8 +96,9 @@ type DirInfo struct {
 
 	// Source files
 	GoFiles  []string // .go files in dir (excluding CgoFiles)
+	HFiles   []string // .h files in dir
 	CFiles   []string // .c files in dir
-	SFiles   []string // .s files in dir
+	SFiles   []string // .s (and, when using cgo, .S files in dir)
 	CgoFiles []string // .go files that import "C"
 
 	// Cgo directives
@@ -135,6 +136,7 @@ func (ctxt *Context) ScanDir(dir string) (info *DirInfo, err error) {
 		return nil, err
 	}
 
+	var Sfiles []string // files with ".S" (capital S)
 	var di DirInfo
 	imported := make(map[string]bool)
 	testImported := make(map[string]bool)
@@ -154,7 +156,7 @@ func (ctxt *Context) ScanDir(dir string) (info *DirInfo, err error) {
 
 		ext := path.Ext(name)
 		switch ext {
-		case ".go", ".c", ".s":
+		case ".go", ".c", ".s", ".h", ".S":
 			// tentatively okay
 		default:
 			// skip
@@ -175,9 +177,15 @@ func (ctxt *Context) ScanDir(dir string) (info *DirInfo, err error) {
 		case ".c":
 			di.CFiles = append(di.CFiles, name)
 			continue
+		case ".h":
+			di.HFiles = append(di.HFiles, name)
+			continue
 		case ".s":
 			di.SFiles = append(di.SFiles, name)
 			continue
+		case ".S":
+			Sfiles = append(Sfiles, name)
+			continue
 		}
 
 		pf, err := parser.ParseFile(fset, filename, data, parser.ImportsOnly|parser.ParseComments)
@@ -282,6 +290,15 @@ func (ctxt *Context) ScanDir(dir string) (info *DirInfo, err error) {
 		di.TestImports[i] = p
 		i++
 	}
+
+	// add the .S files only if we are using cgo
+	// (which means gcc will compile them).
+	// The standard assemblers expect .s files.
+	if len(di.CgoFiles) > 0 {
+		di.SFiles = append(di.SFiles, Sfiles...)
+		sort.Strings(di.SFiles)
+	}
+
 	// File name lists are sorted because ReadDir sorts.
 	sort.Strings(di.Imports)
 	sort.Strings(di.TestImports)