cmd/compile: CSE copied tuple selectors

In CSE if a tuple generator is CSE'd to a different block, its
selectors are copied to the same block. In this case, also CES
the copied selectors.

Test copied from Keith's CL 27202.

Fixes #16741.

Change-Id: I2fc8b9513d430f10d6104275cfff5fb75d3ef3d9
Reviewed-on: https://go-review.googlesource.com/27236
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -166,7 +166,10 @@ func cse(f *Func) {
 	// if we rewrite a tuple generator to a new one in a different block,
 	// copy its selectors to the new generator's block, so tuple generator
 	// and selectors stay together.
+	// be careful not to copy same selectors more than once (issue 16741).
+	copiedSelects := make(map[ID][]*Value)
 	for _, b := range f.Blocks {
+	out:
 		for _, v := range b.Values {
 			if rewrite[v.ID] != nil {
 				continue
@@ -180,8 +183,16 @@ func cse(f *Func) {
 			t := rewrite[v.Args[0].ID]
 			if t != nil && t.Block != b {
 				// v.Args[0] is tuple generator, CSE'd into a different block as t, v is left behind
+				for _, c := range copiedSelects[t.ID] {
+					if v.Op == c.Op {
+						// an equivalent selector is already copied
+						rewrite[v.ID] = c
+						continue out
+					}
+				}
 				c := v.copyInto(t.Block)
 				rewrite[v.ID] = c
+				copiedSelects[t.ID] = append(copiedSelects[t.ID], c)
 			}
 		}
 	}

test/fixedbugs/issue16741.go

+// compile
+
+// Copyright 2016 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.
+
+// Make sure CSE of multi-output opcodes works correctly
+// with select0/1 operations.
+
+package main
+
+func div(d, r int64) int64 {
+	if m := d % r; m > 0 {
+		return d/r + 1
+	}
+	return d / r
+}