cmd/link: new DWARF line table test case

Add a test case for an issue with how Go emits DWARF line tables,
specifically relating to the line table "end sequence" operator.

Updates #38192.

Change-Id: I878b262e6ca6c550c0e460c3d5a1969ac4a2c31b
Reviewed-on: https://go-review.googlesource.com/c/go/+/235917
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

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

@@ -1368,3 +1368,114 @@ func main() {
 		rdr.SkipChildren()
 	}
 }
+
+func TestIssue38192(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; no DWARF symbol table in executables")
+	}
+
+	// Build a test program that contains a translation unit whose
+	// text (from am assembly source) contains only a single instruction.
+	tmpdir, err := ioutil.TempDir("", "TestIssue38192")
+	if err != nil {
+		t.Fatalf("could not create directory: %v", err)
+	}
+	defer os.RemoveAll(tmpdir)
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("where am I? %v", err)
+	}
+	pdir := filepath.Join(wd, "testdata", "issue38192")
+	f := gobuildTestdata(t, tmpdir, pdir, DefaultOpt)
+
+	// Open the resulting binary and examine the DWARF it contains.
+	// Look for the function of interest ("main.singleInstruction")
+	// and verify that the line table has an entry not just for the
+	// single instruction but also a dummy instruction following it,
+	// so as to test that whoever is emitting the DWARF doesn't
+	// emit an end-sequence op immediately after the last instruction
+	// in the translation unit.
+	//
+	// NB: another way to write this test would have been to run the
+	// resulting executable under GDB, set a breakpoint in
+	// "main.singleInstruction", then verify that GDB displays the
+	// correct line/file information.  Given the headache and flakiness
+	// associated with GDB-based tests these days, a direct read of
+	// the line table seems more desirable.
+	rows := []dwarf.LineEntry{}
+	dw, err := f.DWARF()
+	if err != nil {
+		t.Fatalf("error parsing DWARF: %v", err)
+	}
+	rdr := dw.Reader()
+	for {
+		e, err := rdr.Next()
+		if err != nil {
+			t.Fatalf("error reading DWARF: %v", err)
+		}
+		if e == nil {
+			break
+		}
+		if e.Tag != dwarf.TagCompileUnit {
+			continue
+		}
+		// NB: there can be multiple compile units named "main".
+		name := e.Val(dwarf.AttrName).(string)
+		if name != "main" {
+			continue
+		}
+		lnrdr, err := dw.LineReader(e)
+		if err != nil {
+			t.Fatalf("error creating DWARF line reader: %v", err)
+		}
+		if lnrdr != nil {
+			var lne dwarf.LineEntry
+			for {
+				err := lnrdr.Next(&lne)
+				if err == io.EOF {
+					break
+				}
+				if err != nil {
+					t.Fatalf("error reading next DWARF line: %v", err)
+				}
+				if !strings.HasSuffix(lne.File.Name, "ld/testdata/issue38192/oneline.s") {
+					continue
+				}
+				rows = append(rows, lne)
+			}
+		}
+		rdr.SkipChildren()
+	}
+	f.Close()
+
+	// Make sure that:
+	// - main.singleInstruction appears in the line table
+	// - more than one PC value appears the line table for
+	//   that compilation unit.
+	// - at least one row has the correct line number (8)
+	pcs := make(map[uint64]bool)
+	line8seen := false
+	for _, r := range rows {
+		pcs[r.Address] = true
+		if r.Line == 8 {
+			line8seen = true
+		}
+	}
+	failed := false
+	if len(pcs) < 2 {
+		failed = true
+		t.Errorf("not enough line table rows for main.singleInstruction (got %d, wanted > 1", len(pcs))
+	}
+	if !line8seen {
+		failed = true
+		t.Errorf("line table does not contain correct line for main.singleInstruction")
+	}
+	if !failed {
+		return
+	}
+	for i, r := range rows {
+		t.Logf("row %d: A=%x F=%s L=%d\n", i, r.Address, r.File.Name, r.Line)
+	}
+}

src/cmd/link/internal/ld/testdata/issue38192/main.go

@@ -0,0 +1,11 @@
+// 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 main
+
+func singleInstruction()
+
+func main() {
+	singleInstruction()
+}

src/cmd/link/internal/ld/testdata/issue38192/oneline.s

@@ -0,0 +1,8 @@
+// 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.
+
+#include "textflag.h"
+
+TEXT ·singleInstruction(SB),NOSPLIT,$0
+        RET