3 files changed, 134 insertions, 16 deletions

diff --git a/src/cmd/link/internal/ld/dwarf.go b/src/cmd/link/internal/ld/dwarf.go
index 8df03d74f1..6d330061ab 100644
--- a/src/cmd/link/internal/ld/dwarf.go
+++ b/src/cmd/link/internal/ld/dwarf.go
@@ -1264,22 +1264,6 @@ func (d *dwctxt2) writelines(unit *sym.CompilationUnit, ls loader.Sym) {
 		}
 	}
 
-	// Issue 38192: the DWARF standard specifies that when you issue
-	// an end-sequence op, the PC value should be one past the last
-	// text address in the translation unit, so apply a delta to the
-	// text address before the end sequence op. If this isn't done,
-	// GDB will assign a line number of zero the last row in the line
-	// table, which we don't want. The 1 + ptrsize amount is somewhat
-	// arbitrary, this is chosen to be consistent with the way LLVM
-	// emits its end sequence ops.
-	lsu.AddUint8(dwarf.DW_LNS_advance_pc)
-	dwarf.Uleb128put(d, lsDwsym, int64(1+d.arch.PtrSize))
-
-	// Emit an end-sequence at the end of the unit.
-	lsu.AddUint8(0) // start extended opcode
-	dwarf.Uleb128put(d, lsDwsym, 1)
-	lsu.AddUint8(dwarf.DW_LNE_end_sequence)
-
 	if d.linkctxt.HeadType == objabi.Haix {
 		saveDwsectCUSize(".debug_line", unit.Lib.Pkg, uint64(lsu.Size()-unitLengthOffset))
 	}
diff --git a/src/cmd/link/internal/ld/dwarf_test.go b/src/cmd/link/internal/ld/dwarf_test.go
index fb9c45b07d..f3dd53792a 100644
--- a/src/cmd/link/internal/ld/dwarf_test.go
+++ b/src/cmd/link/internal/ld/dwarf_test.go
@@ -1479,3 +1479,122 @@ func TestIssue38192(t *testing.T) {
 		t.Logf("row %d: A=%x F=%s L=%d\n", i, r.Address, r.File.Name, r.Line)
 	}
 }
+
+func TestIssue39757(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; no DWARF symbol table in executables")
+	}
+
+	// In this bug the DWARF line table contents for the last couple of
+	// instructions in a function were incorrect (bad file/line). This
+	// test verifies that all of the line table rows for a function
+	// of interest have the same file (no "autogenerated").
+	//
+	// Note: the function in this test was written with an eye towards
+	// ensuring that there are no inlined routines from other packages
+	// (which could introduce other source files into the DWARF); it's
+	// possible that at some point things could evolve in the
+	// compiler/runtime in ways that aren't happening now, so this
+	// might be something to check for if it does start failing.
+
+	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", "issue39757")
+	f := gobuildTestdata(t, tmpdir, pdir, DefaultOpt)
+
+	syms, err := f.Symbols()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var addr uint64
+	for _, sym := range syms {
+		if sym.Name == "main.main" {
+			addr = sym.Addr
+			break
+		}
+	}
+	if addr == 0 {
+		t.Fatal("cannot find main.main in symbols")
+	}
+
+	// Open the resulting binary and examine the DWARF it contains.
+	// Look for the function of interest ("main.main")
+	// and verify that all line table entries show the same source
+	// file.
+	dw, err := f.DWARF()
+	if err != nil {
+		t.Fatalf("error parsing DWARF: %v", err)
+	}
+	rdr := dw.Reader()
+	ex := examiner{}
+	if err := ex.populate(rdr); err != nil {
+		t.Fatalf("error reading DWARF: %v", err)
+	}
+
+	// Locate the main.main DIE
+	mains := ex.Named("main.main")
+	if len(mains) == 0 {
+		t.Fatalf("unable to locate DIE for main.main")
+	}
+	if len(mains) != 1 {
+		t.Fatalf("more than one main.main DIE")
+	}
+	maindie := mains[0]
+
+	// Collect the start/end PC for main.main
+	lowpc := maindie.Val(dwarf.AttrLowpc).(uint64)
+	highpc := maindie.Val(dwarf.AttrHighpc).(uint64)
+
+	// Now read the line table for the 'main' compilation unit.
+	mainIdx := ex.idxFromOffset(maindie.Offset)
+	cuentry := ex.Parent(mainIdx)
+	if cuentry == nil {
+		t.Fatalf("main.main DIE appears orphaned")
+	}
+	lnrdr, lerr := dw.LineReader(cuentry)
+	if lerr != nil {
+		t.Fatalf("error creating DWARF line reader: %v", err)
+	}
+	if lnrdr == nil {
+		t.Fatalf("no line table for main.main compilation unit")
+	}
+	rows := []dwarf.LineEntry{}
+	mainrows := 0
+	var lne dwarf.LineEntry
+	for {
+		err := lnrdr.Next(&lne)
+		if err == io.EOF {
+			break
+		}
+		rows = append(rows, lne)
+		if err != nil {
+			t.Fatalf("error reading next DWARF line: %v", err)
+		}
+		if lne.Address < lowpc || lne.Address > highpc {
+			continue
+		}
+		if !strings.HasSuffix(lne.File.Name, "issue39757main.go") {
+			t.Errorf("found row with file=%s (not issue39757main.go)", lne.File.Name)
+		}
+		mainrows++
+	}
+	f.Close()
+
+	// Make sure we saw a few rows.
+	if mainrows < 3 {
+		t.Errorf("not enough line table rows for main.main (got %d, wanted > 3", mainrows)
+		for i, r := range rows {
+			t.Logf("row %d: A=%x F=%s L=%d\n", i, r.Address, r.File.Name, r.Line)
+		}
+	}
+}
diff --git a/src/cmd/link/internal/ld/testdata/issue39757/issue39757main.go b/src/cmd/link/internal/ld/testdata/issue39757/issue39757main.go
new file mode 100644
index 0000000000..76e0ea1b08
--- /dev/null
+++ b/src/cmd/link/internal/ld/testdata/issue39757/issue39757main.go
@@ -0,0 +1,15 @@
+// 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
+
+var G int
+
+func main() {
+	if G != 101 {
+		println("not 101")
+	} else {
+		println("well now that's interesting")
+	}
+}