cmd/compile: handle DOT STRUCTLIT for zero-valued struct in SSA

CL 35261 makes SSA handle zero-valued STRUCTLIT, but DOT operation
was not handled.

Fixes #18994.

Change-Id: Ic7976036acca1523b0b14afac4d170797e8aee20
Reviewed-on: https://go-review.googlesource.com/36565
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1956,6 +1956,15 @@ func (s *state) expr(n *Node) *ssa.Value {
 			v := s.expr(n.Left)
 			return s.newValue1I(ssa.OpStructSelect, n.Type, int64(fieldIdx(n)), v)
 		}
+		if n.Left.Op == OSTRUCTLIT {
+			// 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.Left) {
+				Fatalf("literal with nonzero value in SSA: %v", n.Left)
+			}
+			return s.zeroVal(n.Type)
+		}
 		p, _ := s.addr(n, false)
 		return s.newValue2(ssa.OpLoad, n.Type, p, s.mem())
 

test/fixedbugs/issue18994.go

+// run
+
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 18994: SSA didn't handle DOT STRUCTLIT for zero-valued
+// STRUCTLIT.
+
+package main
+
+// large struct - not SSA-able
+type T struct {
+	a, b, c, d, e, f, g, h int
+}
+
+func main() {
+	x := T{}.a
+	if x != 0 {
+		panic("FAIL")
+	}
+}