debug/plan9obj: implement parsing of Plan 9 a.out executables

It implements parsing of the header and symbol table for both
32-bit and 64-bit Plan 9 binaries. The nm tool was updated to
use this package.

R=rsc, aram
CC=golang-codereviews
https://codereview.appspot.com/49970044

2 files changed, 52 insertions, 0 deletions

diff --git a/src/cmd/nm/nm.go b/src/cmd/nm/nm.go
index fdf6ef673..a4036184e 100644
--- a/src/cmd/nm/nm.go
+++ b/src/cmd/nm/nm.go
@@ -105,6 +105,10 @@ var parsers = []struct {
 	{[]byte("\xCE\xFA\xED\xFE"), machoSymbols},
 	{[]byte("\xCF\xFA\xED\xFE"), machoSymbols},
 	{[]byte("MZ"), peSymbols},
+	{[]byte("\x00\x00\x01\xEB"), plan9Symbols}, // 386
+	{[]byte("\x00\x00\x04\x07"), plan9Symbols}, // mips
+	{[]byte("\x00\x00\x06\x47"), plan9Symbols}, // arm
+	{[]byte("\x00\x00\x8A\x97"), plan9Symbols}, // amd64
 }
 
 func nm(file string) {
diff --git a/src/cmd/nm/plan9obj.go b/src/cmd/nm/plan9obj.go
new file mode 100644
index 000000000..006c66ebf
--- /dev/null
+++ b/src/cmd/nm/plan9obj.go
@@ -0,0 +1,48 @@
+// Copyright 2014 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.
+
+// Parsing of Plan 9 a.out executables.
+
+package main
+
+import (
+	"debug/plan9obj"
+	"os"
+	"sort"
+)
+
+func plan9Symbols(f *os.File) []Sym {
+	p, err := plan9obj.NewFile(f)
+	if err != nil {
+		errorf("parsing %s: %v", f.Name(), err)
+		return nil
+	}
+
+	plan9Syms, err := p.Symbols()
+	if err != nil {
+		errorf("parsing %s: %v", f.Name(), err)
+		return nil
+	}
+
+	// Build sorted list of addresses of all symbols.
+	// We infer the size of a symbol by looking at where the next symbol begins.
+	var addrs []uint64
+	for _, s := range plan9Syms {
+		addrs = append(addrs, s.Value)
+	}
+	sort.Sort(uint64s(addrs))
+
+	var syms []Sym
+
+	for _, s := range plan9Syms {
+		sym := Sym{Addr: s.Value, Name: s.Name, Code: rune(s.Type)}
+		i := sort.Search(len(addrs), func(x int) bool { return addrs[x] > s.Value })
+		if i < len(addrs) {
+			sym.Size = int64(addrs[i] - s.Value)
+		}
+		syms = append(syms, sym)
+	}
+
+	return syms
+}