diff --git a/src/cmd/compile/fmt_test.go b/src/cmd/compile/fmt_test.go
index 6d18d495c3..17716f82f2 100644
--- a/src/cmd/compile/fmt_test.go
+++ b/src/cmd/compile/fmt_test.go
@@ -229,7 +229,7 @@ func TestFormats(t *testing.T) {
 			}
 		}
 		if mismatch {
-			t.Errorf("knownFormats is out of date; please run with -v to regenerate")
+			t.Errorf("knownFormats is out of date; please 'go test -v fmt_test.go > foo', then extract new definition of knownFormats from foo")
 		}
 	}
 
@@ -683,6 +683,7 @@ var knownFormats = map[string]string{
 	"int32 %d":                                        "",
 	"int32 %v":                                        "",
 	"int32 %x":                                        "",
+	"int64 %.5d":                                      "",
 	"int64 %+d":                                       "",
 	"int64 %-10d":                                     "",
 	"int64 %X":                                        "",
diff --git a/src/cmd/compile/internal/gc/ssa.go b/src/cmd/compile/internal/gc/ssa.go
index c633ee4c93..69ed613412 100644
--- a/src/cmd/compile/internal/gc/ssa.go
+++ b/src/cmd/compile/internal/gc/ssa.go
@@ -4606,22 +4606,31 @@ func genssa(f *ssa.Func, pp *Progs) {
 			var buf bytes.Buffer
 			buf.WriteString("<code>")
 			buf.WriteString("<dl class=\"ssa-gen\">")
+			filename := ""
 			for p := pp.Text; p != nil; p = p.Link {
+				// Don't spam every line with the file name, which is often huge.
+				// Only print changes.
+				if f := p.FileName(); f != filename {
+					filename = f
+					buf.WriteString("<dt class=\"ssa-prog-src\"></dt><dd class=\"ssa-prog\">")
+					buf.WriteString(html.EscapeString("# " + filename))
+					buf.WriteString("</dd>")
+				}
+
 				buf.WriteString("<dt class=\"ssa-prog-src\">")
 				if v, ok := progToValue[p]; ok {
 					buf.WriteString(v.HTML())
 				} else if b, ok := progToBlock[p]; ok {
-					buf.WriteString(b.HTML())
+					buf.WriteString("<b>" + b.HTML() + "</b>")
 				}
 				buf.WriteString("</dt>")
 				buf.WriteString("<dd class=\"ssa-prog\">")
-				buf.WriteString(html.EscapeString(p.String()))
+				buf.WriteString(fmt.Sprintf("%.5d <span class=\"line-number\">(%s)</span> %s", p.Pc, p.LineNumber(), html.EscapeString(p.InstructionString())))
 				buf.WriteString("</dd>")
-				buf.WriteString("</li>")
 			}
 			buf.WriteString("</dl>")
 			buf.WriteString("</code>")
-			f.HTMLWriter.WriteColumn("genssa", buf.String())
+			f.HTMLWriter.WriteColumn("genssa", "ssa-prog", buf.String())
 			// pp.Text.Ctxt.LineHist.PrintFilenameOnly = saved
 		}
 	}
diff --git a/src/cmd/compile/internal/ssa/html.go b/src/cmd/compile/internal/ssa/html.go
index bb87378bdd..47f37f2337 100644
--- a/src/cmd/compile/internal/ssa/html.go
+++ b/src/cmd/compile/internal/ssa/html.go
@@ -66,6 +66,11 @@ th, td {
     padding: 5px;
 }
 
+td.ssa-prog {
+    width: 600px;
+    word-wrap: break-word;
+}
+
 li {
     list-style-type: none;
 }
@@ -121,6 +126,11 @@ dd.ssa-prog {
     font-style: italic;
 }
 
+.line-number {
+    font-style: italic;
+    font-size: 11px;
+}
+
 .highlight-yellow         { background-color: yellow; }
 .highlight-aquamarine     { background-color: aquamarine; }
 .highlight-coral          { background-color: coral; }
@@ -310,17 +320,21 @@ func (w *HTMLWriter) WriteFunc(title string, f *Func) {
 	if w == nil {
 		return // avoid generating HTML just to discard it
 	}
-	w.WriteColumn(title, f.HTML())
+	w.WriteColumn(title, "", f.HTML())
 	// TODO: Add visual representation of f's CFG.
 }
 
 // WriteColumn writes raw HTML in a column headed by title.
 // It is intended for pre- and post-compilation log output.
-func (w *HTMLWriter) WriteColumn(title string, html string) {
+func (w *HTMLWriter) WriteColumn(title, class, html string) {
 	if w == nil {
 		return
 	}
-	w.WriteString("<td>")
+	if class == "" {
+		w.WriteString("<td>")
+	} else {
+		w.WriteString("<td class=\"" + class + "\">")
+	}
 	w.WriteString("<h2>" + title + "</h2>")
 	w.WriteString(html)
 	w.WriteString("</td>")
@@ -353,7 +367,14 @@ func (v *Value) LongHTML() string {
 	// We already have visual noise in the form of punctuation
 	// maybe we could replace some of that with formatting.
 	s := fmt.Sprintf("<span class=\"%s ssa-long-value\">", v.String())
-	s += fmt.Sprintf("%s = %s", v.HTML(), v.Op.String())
+
+	linenumber := "<span class=\"line-number\">(?)</span>"
+	if v.Pos.IsKnown() {
+		linenumber = fmt.Sprintf("<span class=\"line-number\">(%d)</span>", v.Pos.Line())
+	}
+
+	s += fmt.Sprintf("%s %s = %s", v.HTML(), linenumber, v.Op.String())
+
 	s += " &lt;" + html.EscapeString(v.Type.String()) + "&gt;"
 	s += html.EscapeString(v.auxString())
 	for _, a := range v.Args {
@@ -375,6 +396,7 @@ func (v *Value) LongHTML() string {
 	if len(names) != 0 {
 		s += " (" + strings.Join(names, ", ") + ")"
 	}
+
 	s += "</span>"
 	return s
 }
@@ -409,6 +431,11 @@ func (b *Block) LongHTML() string {
 	case BranchLikely:
 		s += " (likely)"
 	}
+	if b.Pos.IsKnown() {
+		// TODO does not begin to deal with the full complexity of line numbers.
+		// Maybe we want a string/slice instead, of outer-inner when inlining.
+		s += fmt.Sprintf(" (line %d)", b.Pos.Line())
+	}
 	return s
 }
 
diff --git a/src/cmd/internal/obj/util.go b/src/cmd/internal/obj/util.go
index bf2d209d7f..67c74c2f89 100644
--- a/src/cmd/internal/obj/util.go
+++ b/src/cmd/internal/obj/util.go
@@ -13,10 +13,34 @@ import (
 
 const REG_NONE = 0
 
+// Line returns a string containing the filename and line number for p
 func (p *Prog) Line() string {
 	return p.Ctxt.OutermostPos(p.Pos).Format(false)
 }
 
+// LineNumber returns a string containing the line number for p's position
+func (p *Prog) LineNumber() string {
+	pos := p.Ctxt.OutermostPos(p.Pos)
+	if !pos.IsKnown() {
+		return "?"
+	}
+	return fmt.Sprintf("%d", pos.Line())
+}
+
+// FileName returns a string containing the filename for p's position
+func (p *Prog) FileName() string {
+	// TODO LineNumber and FileName cases don't handle full generality of positions,
+	// but because these are currently used only for GOSSAFUNC debugging output, that
+	// is okay.  The intent is that "LineNumber()" yields the rapidly varying part,
+	// while "FileName()" yields the longer and slightly more constant material.
+	pos := p.Ctxt.OutermostPos(p.Pos)
+	if !pos.IsKnown() {
+		return "<unknown file name>"
+	}
+
+	return pos.Filename()
+}
+
 var armCondCode = []string{
 	".EQ",
 	".NE",
@@ -72,6 +96,18 @@ func (p *Prog) String() string {
 	if p == nil {
 		return "<nil Prog>"
 	}
+	if p.Ctxt == nil {
+		return "<Prog without ctxt>"
+	}
+	return fmt.Sprintf("%.5d (%v)\t%s", p.Pc, p.Line(), p.InstructionString())
+}
+
+// InstructionString returns a string representation of the instruction without preceding
+// program counter or file and line number.
+func (p *Prog) InstructionString() string {
+	if p == nil {
+		return "<nil Prog>"
+	}
 
 	if p.Ctxt == nil {
 		return "<Prog without ctxt>"
@@ -81,7 +117,7 @@ func (p *Prog) String() string {
 
 	var buf bytes.Buffer
 
-	fmt.Fprintf(&buf, "%.5d (%v)\t%v%s", p.Pc, p.Line(), p.As, sc)
+	fmt.Fprintf(&buf, "%v%s", p.As, sc)
 	sep := "\t"
 
 	if p.From.Type != TYPE_NONE {