cmd/internal/objfile: correct file table reading for Go object file

Apparently I never actually understood the new file table in Go
object files. The PC value stream actually encodes the file index
in the per-CU table. I thought it was indexing into a per-function
table, which then contains index to the per-CU table. Remove the
extra indirection.

Change-Id: I0aea5629f7b3888ebe3a04fea437aa15ce89519e
Reviewed-on: https://go-review.googlesource.com/c/go/+/262779
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/internal/objfile/goobj.go

@@ -267,13 +267,11 @@ func (f *goobjFile) PCToLine(pc uint64) (string, int, *gosym.Func) {
 		}
 		b := r.BytesAt(r.DataOff(isym), r.DataSize(isym))
 		var info *goobj.FuncInfo
-		lengths := info.ReadFuncInfoLengths(b)
 		pcline := getSymData(info.ReadPcline(b))
 		line := int(pcValue(pcline, pc-addr, f.arch))
 		pcfile := getSymData(info.ReadPcfile(b))
 		fileID := pcValue(pcfile, pc-addr, f.arch)
-		globalFileID := info.ReadFile(b, lengths.FileOff, uint32(fileID))
+		fileName := r.File(int(fileID))
-		fileName := r.File(int(globalFileID))
 		// Note: we provide only the name in the Func structure.
 		// We could provide more if needed.
 		return fileName, line, &gosym.Func{Sym: &gosym.Sym{Name: osym.Name(r)}}

src/cmd/objdump/objdump_test.go

@@ -333,3 +333,41 @@ func TestDisasmGoobj(t *testing.T) {
 		t.Logf("full disassembly:\n%s", text)
 	}
 }
+
+func TestGoobjFileNumber(t *testing.T) {
+	// Test that file table in Go object file is parsed correctly.
+	testenv.MustHaveGoBuild(t)
+
+	t.Parallel()
+
+	tmpdir, err := ioutil.TempDir("", "TestGoobjFileNumber")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	obj := filepath.Join(tmpdir, "p.a")
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-o", obj)
+	cmd.Dir = filepath.Join("testdata/testfilenum")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("build failed: %v\n%s", err, out)
+	}
+
+	cmd = exec.Command(exe, obj)
+	out, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("objdump failed: %v\n%s", err, out)
+	}
+
+	text := string(out)
+	for _, s := range []string{"a.go", "b.go", "c.go"} {
+		if !strings.Contains(text, s) {
+			t.Errorf("output missing '%s'", s)
+		}
+	}
+
+	if t.Failed() {
+		t.Logf("output:\n%s", text)
+	}
+}

src/cmd/objdump/testdata/testfilenum/a.go

@@ -0,0 +1,7 @@
+// Copyright 2020 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.
+
+package p
+
+func A() {}

src/cmd/objdump/testdata/testfilenum/b.go

@@ -0,0 +1,7 @@
+// Copyright 2020 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.
+
+package p
+
+func B() {}

src/cmd/objdump/testdata/testfilenum/c.go

@@ -0,0 +1,7 @@
+// Copyright 2020 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.
+
+package p
+
+func C() {}

src/cmd/objdump/testdata/testfilenum/go.mod

@@ -0,0 +1,3 @@
+module objdumptest
+
+go 1.16