compiler: put debug information on package initializer functions

Make sure package initializers show up in backtraces, as they should. In
practice, it doesn't actually break backtraces as these functions are
usually inlined anyway, but it may help to debug an error in
initialization code.

compiler/compiler.go

@@ -815,7 +815,13 @@ func (c *Compiler) parseFuncDecl(f *ir.Function) (*Frame, error) {
 		frame.fn.LLVMFn = llvm.AddFunction(c.mod, name, fnType)
 	}
 
-	if c.Debug && f.Syntax() != nil && len(f.Blocks) != 0 {
+	if c.Debug && f.Synthetic == "package initializer" {
+		difunc, err := c.attachDebugInfoRaw(f, "", 0)
+		if err != nil {
+			return nil, err
+		}
+		frame.difunc = difunc
+	} else if c.Debug && f.Syntax() != nil && len(f.Blocks) != 0 {
 		// Create debug info file if needed.
 		difunc, err := c.attachDebugInfo(f)
 		if err != nil {
@@ -829,9 +835,16 @@ func (c *Compiler) parseFuncDecl(f *ir.Function) (*Frame, error) {
 
 func (c *Compiler) attachDebugInfo(f *ir.Function) (llvm.Metadata, error) {
 	pos := c.ir.Program.Fset.Position(f.Syntax().Pos())
-	if _, ok := c.difiles[pos.Filename]; !ok {
-		dir, file := filepath.Split(pos.Filename)
-		c.difiles[pos.Filename] = c.dibuilder.CreateFile(file, dir[:len(dir)-1])
+	return c.attachDebugInfoRaw(f, pos.Filename, pos.Line)
+}
+
+func (c *Compiler) attachDebugInfoRaw(f *ir.Function, filename string, line int) (llvm.Metadata, error) {
+	if _, ok := c.difiles[filename]; !ok {
+		dir, file := filepath.Split(filename)
+		if dir != "" {
+			dir = dir[:len(dir)-1]
+		}
+		c.difiles[filename] = c.dibuilder.CreateFile(file, dir)
 	}
 
 	// Debug info for this function.
@@ -844,15 +857,15 @@ func (c *Compiler) attachDebugInfo(f *ir.Function) (llvm.Metadata, error) {
 		diparams = append(diparams, ditype)
 	}
 	diFuncType := c.dibuilder.CreateSubroutineType(llvm.DISubroutineType{
-		File:       c.difiles[pos.Filename],
+		File:       c.difiles[filename],
 		Parameters: diparams,
 		Flags:      0, // ?
 	})
-	difunc := c.dibuilder.CreateFunction(c.difiles[pos.Filename], llvm.DIFunction{
+	difunc := c.dibuilder.CreateFunction(c.difiles[filename], llvm.DIFunction{
 		Name:         f.RelString(nil),
 		LinkageName:  f.LinkName(),
-		File:         c.difiles[pos.Filename],
-		Line:         pos.Line,
+		File:         c.difiles[filename],
+		Line:         line,
 		Type:         diFuncType,
 		LocalToUnit:  true,
 		IsDefinition: true,