cmd/compile: don't set line number to 0 when building SSA

The frontend may emit node with line number missing. In this case,
use the parent line number. Instead of changing every call site of
pushLine, do it in pushLine itself.

Fixes #16214.

Change-Id: I80390550b56e4d690fc770b01ff725b892ffd6dc
Reviewed-on: https://go-review.googlesource.com/24641Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/asm_test.go

@@ -122,3 +122,38 @@ NextVar:
 	}
 	return out
 }
+
+// TestLineNumber checks to make sure the generated assembly has line numbers
+// see issue #16214
+func TestLineNumber(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	dir, err := ioutil.TempDir("", "TestLineNumber")
+	if err != nil {
+		t.Fatalf("could not create directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	src := filepath.Join(dir, "x.go")
+	err = ioutil.WriteFile(src, []byte(issue16214src), 0644)
+	if err != nil {
+		t.Fatalf("could not write file: %v", err)
+	}
+
+	cmd := exec.Command("go", "tool", "compile", "-S", "-o", filepath.Join(dir, "out.o"), src)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("fail to run go tool compile: %v", err)
+	}
+
+	if strings.Contains(string(out), "unknown line number") {
+		t.Errorf("line number missing in assembly:\n%s", out)
+	}
+}
+
+var issue16214src = `
+package main
+
+func Mod32(x uint32) uint32 {
+	return x % 3 // frontend rewrites it as HMUL with 2863311531, the LITERAL node has Lineno 0
+}
+`

src/cmd/compile/internal/gc/ssa.go

@@ -384,6 +384,14 @@ func (s *state) endBlock() *ssa.Block {
 
 // pushLine pushes a line number on the line number stack.
 func (s *state) pushLine(line int32) {
+	if line == 0 {
+		// the frontend may emit node with line number missing,
+		// use the parent line number in this case.
+		line = s.peekLine()
+		if Debug['K'] != 0 {
+			Warn("buildssa: line 0")
+		}
+	}
 	s.line = append(s.line, line)
 }