cmd/compile: add special case for testing first field of struct variable

The change to the write barrier in https://golang.org/cl/16899 means
that the compiler now emits tests of the first field of a struct.  That
was using a register that was not used before.  This change fixes that
for amd64 by adding a special case for the first field of a struct.

Update #12416.

Change-Id: Ia57baa62cd741592fbeb9be82f1e846be73d6edd
Reviewed-on: https://go-review.googlesource.com/16933
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/compile/internal/amd64/gsubr.go

@@ -108,7 +108,15 @@ func ginscmp(op gc.Op, t *gc.Type, n1, n2 *gc.Node, likely int) *obj.Prog {
 	}
 	// General case.
 	var r1, r2, g1, g2 gc.Node
-	if n1.Op == gc.ONAME && n1.Class&gc.PHEAP == 0 || n1.Op == gc.OINDREG {
+
+	// A special case to make write barriers more efficient.
+	// Comparing the first field of a named struct can be done directly.
+	base := n1
+	if n1.Op == gc.ODOT && n1.Left.Type.Etype == gc.TSTRUCT && n1.Left.Type.Type.Sym == n1.Right.Sym {
+		base = n1.Left
+	}
+
+	if base.Op == gc.ONAME && base.Class&gc.PHEAP == 0 || n1.Op == gc.OINDREG {
 		r1 = *n1
 	} else {
 		gc.Regalloc(&r1, t, n1)

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

@@ -404,6 +404,17 @@ func Naddr(a *obj.Addr, n *Node) {
 
 		a.Sym = Linksym(s)
 
+	case ODOT:
+		// A special case to make write barriers more efficient.
+		// Taking the address of the first field of a named struct
+		// is the same as taking the address of the struct.
+		if n.Left.Type.Etype != TSTRUCT || n.Left.Type.Type.Sym != n.Right.Sym {
+			Debug['h'] = 1
+			Dump("naddr", n)
+			Fatalf("naddr: bad %v %v", Oconv(int(n.Op), 0), Ctxt.Dconv(a))
+		}
+		Naddr(a, n.Left)
+
 	case OLITERAL:
 		if Thearch.Thechar == '8' {
 			a.Width = 0