cmd/compile: skip empty init function in fninit

Fixes #34869

Change-Id: I21bc60b9a5d1204dade1cceed6cddccf5b537b0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/200958
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -73,6 +73,14 @@ func fninit(n []*Node) {
 	// Record user init functions.
 	for i := 0; i < renameinitgen; i++ {
 		s := lookupN("init.", i)
+		fn := asNode(s.Def).Name.Defn
+		// Skip init functions with empty bodies.
+		// noder.go doesn't allow external init functions, and
+		// order.go has already removed any OEMPTY nodes, so
+		// checking Len() == 0 is sufficient here.
+		if fn.Nbody.Len() == 0 {
+			continue
+		}
 		fns = append(fns, s.Linksym())
 	}
 

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

@@ -3812,6 +3812,33 @@ func checkreturn(fn *Node) {
 
 func deadcode(fn *Node) {
 	deadcodeslice(fn.Nbody)
+	deadcodefn(fn)
+}
+
+func deadcodefn(fn *Node) {
+	if fn.Nbody.Len() == 0 {
+		return
+	}
+
+	for _, n := range fn.Nbody.Slice() {
+		if n.Ninit.Len() > 0 {
+			return
+		}
+		switch n.Op {
+		case OIF:
+			if !Isconst(n.Left, CTBOOL) || n.Nbody.Len() > 0 || n.Rlist.Len() > 0 {
+				return
+			}
+		case OFOR:
+			if !Isconst(n.Left, CTBOOL) || n.Left.Bool() {
+				return
+			}
+		default:
+			return
+		}
+	}
+
+	fn.Nbody.Set([]*Node{nod(OEMPTY, nil, nil)})
 }
 
 func deadcodeslice(nn Nodes) {