cmd/compile: fix half multiply issue

In walkdiv, an OMUL node was created and passed to typecheck,
before the op was changed back to OHMUL.  In some instances,
the node that came back was an evaluated literal constant that
occurred with a full multiply.  The end result was a literal node
with a non-shifted value and an OHMUL op. This change causes code
to be generated for the OHMUL.

Fixes #11358
Fixes #11369

Change-Id: If42a98c6830d07fe065d5ca57717704fb8cfbd33
Reviewed-on: https://go-review.googlesource.com/11400Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -554,6 +554,7 @@ func typeinit() {
 	okfor[OLE] = okforcmp[:]
 	okfor[OLT] = okforcmp[:]
 	okfor[OMOD] = okforand[:]
+	okfor[OHMUL] = okforarith[:]
 	okfor[OMUL] = okforarith[:]
 	okfor[ONE] = okforeq[:]
 	okfor[OOR] = okforand[:]

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

@@ -524,6 +524,7 @@ OpSwitch:
 		OEQ,
 		OGE,
 		OGT,
+		OHMUL,
 		OLE,
 		OLT,
 		OLSH,

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

@@ -3599,9 +3599,8 @@ func walkdiv(np **Node, init **NodeList) {
 			nc := Nod(OXXX, nil, nil)
 
 			Nodconst(nc, nl.Type, int64(m.Um))
-			n1 := Nod(OMUL, nl, nc)
+			n1 := Nod(OHMUL, nl, nc)
 			typecheck(&n1, Erv)
-			n1.Op = OHMUL
 			if m.Ua != 0 {
 				// Select a Go type with (at least) twice the width.
 				var twide *Type
@@ -3644,9 +3643,8 @@ func walkdiv(np **Node, init **NodeList) {
 			nc := Nod(OXXX, nil, nil)
 
 			Nodconst(nc, nl.Type, m.Sm)
-			n1 := Nod(OMUL, nl, nc)
+			n1 := Nod(OHMUL, nl, nc)
 			typecheck(&n1, Erv)
-			n1.Op = OHMUL
 			if m.Sm < 0 {
 				// add the numerator.
 				n1 = Nod(OADD, n1, nl)

test/fixedbugs/issue11369.go

+// run
+
+// Copyright 2015 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.
+
+// Test that the half multiply resulting from a division
+// by a constant generates correct code.
+
+package main
+
+func main() {
+	var _ = 7 / "0"[0] // test case from #11369
+	var _ = 1 / "."[0] // test case from #11358
+	var x = 0 / "0"[0]
+	var y = 48 / "0"[0]
+	var z = 5 * 48 / "0"[0]
+	if x != 0 {
+		panic("expected 0")
+	}
+	if y != 1 {
+		panic("expected 1")
+	}
+	if z != 5 {
+		panic("expected 5")
+	}
+}