cmd/compile: do not use statictmp for zeroing

Also fixes #18687.

Change-Id: I7c6d47c71e632adf4c16937a29074621f771844c
Reviewed-on: https://go-review.googlesource.com/35261
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -417,9 +417,6 @@ func ordercall(n *Node, order *Order) {
 // cases they are also typically registerizable, so not much harm done.
 // And this only applies to the multiple-assignment form.
 // We could do a more precise analysis if needed, like in walk.go.
-//
-// Ordermapassign also inserts these temporaries if needed for
-// calling writebarrierfat with a pointer to n->right.
 func ordermapassign(n *Node, order *Order) {
 	switch n.Op {
 	default:
@@ -428,17 +425,6 @@ func ordermapassign(n *Node, order *Order) {
 	case OAS:
 		order.out = append(order.out, n)
 
-		// We call writebarrierfat only for values > 4 pointers long. See walk.go.
-		// TODO(mdempsky): writebarrierfat doesn't exist anymore, but removing that
-		// logic causes net/http's tests to become flaky; see CL 21242.
-		if needwritebarrier(n.Left, n.Right) && n.Left.Type.Width > int64(4*Widthptr) && n.Right != nil && !isaddrokay(n.Right) {
-			m := n.Left
-			n.Left = ordertemp(m.Type, order, false)
-			a := nod(OAS, m, n.Left)
-			a = typecheck(a, Etop)
-			order.out = append(order.out, a)
-		}
-
 	case OAS2, OAS2DOTTYPE, OAS2MAPR, OAS2FUNC:
 		var post []*Node
 		var m *Node

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

@@ -2097,6 +2097,15 @@ func (s *state) expr(n *Node) *ssa.Value {
 	case OAPPEND:
 		return s.append(n, false)
 
+	case OSTRUCTLIT, OARRAYLIT:
+		// All literals with nonzero fields have already been
+		// rewritten during walk. Any that remain are just T{}
+		// or equivalents. Use the zero value.
+		if !iszero(n) {
+			Fatalf("literal with nonzero value in SSA: %v", n)
+		}
+		return s.zeroVal(n.Type)
+
 	default:
 		s.Fatalf("unhandled expr %v", n.Op)
 		return nil

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

@@ -1572,6 +1572,9 @@ opswitch:
 		n = r
 
 	case OARRAYLIT, OSLICELIT, OMAPLIT, OSTRUCTLIT, OPTRLIT:
+		if n.Op == OSTRUCTLIT && iszero(n) && !instrumenting { // TODO: SSA doesn't yet handle ARRAYLIT with length > 1
+			break
+		}
 		if isStaticCompositeLiteral(n) {
 			// n can be directly represented in the read-only data section.
 			// Make direct reference to the static data. See issue 12841.
@@ -3143,6 +3146,16 @@ func walkcompare(n *Node, init *Nodes) *Node {
 
 	// Chose not to inline. Call equality function directly.
 	if !inline {
+		if isvaluelit(cmpl) {
+			var_ := temp(cmpl.Type)
+			anylit(cmpl, var_, init)
+			cmpl = var_
+		}
+		if isvaluelit(cmpr) {
+			var_ := temp(cmpr.Type)
+			anylit(cmpr, var_, init)
+			cmpr = var_
+		}
 		if !islvalue(cmpl) || !islvalue(cmpr) {
 			Fatalf("arguments of comparison must be lvalues - %v %v", cmpl, cmpr)
 		}