cmd/compile: don't eliminate all registers when restricting to desired ones

We shouldn't mask to desired registers if we haven't masked out all the
forbidden registers yet.  In this path we haven't masked out the nospill
registers yet. If the resulting mask contains only nospill registers, then
allocReg fails.

This can only happen on resultNotInArgs-marked instructions, which exist
only on the ARM64, MIPS, MIPS64, and PPC64 ports.

Maybe there's a better way to handle resultNotInArgs instructions.
But for 1.13, this is a low-risk fix.

Fixes #33355

Change-Id: I1082f78f798d1371bde65c58cc265540480e4fa4
Reviewed-on: https://go-review.googlesource.com/c/go/+/188178
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -1455,7 +1455,7 @@ func (s *regAllocState) regalloc(f *Func) {
 						}
 					}
 					// Avoid registers we're saving for other values.
-					if mask&^desired.avoid != 0 {
+					if mask&^desired.avoid&^s.nospill != 0 {
 						mask &^= desired.avoid
 					}
 					r := s.allocReg(mask, v)

test/fixedbugs/issue33355.go

+// compile
+
+// Copyright 2019 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.
+
+// This code failed on arm64 in the register allocator.
+// See issue 33355.
+
+package server
+
+import (
+	"bytes"
+	"sync"
+)
+
+type client struct {
+	junk [4]int
+	mu   sync.Mutex
+	srv  *Server
+	gw   *gateway
+	msgb [100]byte
+}
+
+type gateway struct {
+	cfg    *gatewayCfg
+	outsim *sync.Map
+}
+
+type gatewayCfg struct {
+	replyPfx []byte
+}
+
+type Account struct {
+	Name string
+}
+
+type Server struct {
+	gateway *srvGateway
+}
+
+type srvGateway struct {
+	outo     []*client
+}
+
+type subscription struct {
+	queue   []byte
+	client  *client
+}
+
+type outsie struct {
+	ni    map[string]struct{}
+	sl    *Sublist
+	qsubs int
+}
+
+type Sublist struct {
+}
+
+type SublistResult struct {
+	psubs []*subscription
+	qsubs [][]*subscription
+}
+
+var subPool = &sync.Pool{}
+
+func (c *client) sendMsgToGateways(acc *Account, msg, subject, reply []byte, qgroups [][]byte) {
+	var gws []*client
+	gw := c.srv.gateway
+	for i := 0; i < len(gw.outo); i++ {
+		gws = append(gws, gw.outo[i])
+	}
+	var (
+		subj       = string(subject)
+		queuesa    = [512]byte{}
+		queues     = queuesa[:0]
+		mreply     []byte
+		dstPfx     []byte
+		checkReply = len(reply) > 0
+	)
+
+	sub := subPool.Get().(*subscription)
+
+	if subjectStartsWithGatewayReplyPrefix(subject) {
+		dstPfx = subject[:8]
+	}
+	for i := 0; i < len(gws); i++ {
+		gwc := gws[i]
+		if dstPfx != nil {
+			gwc.mu.Lock()
+			ok := bytes.Equal(dstPfx, gwc.gw.cfg.replyPfx)
+			gwc.mu.Unlock()
+			if !ok {
+				continue
+			}
+		} else {
+			qr := gwc.gatewayInterest(acc.Name, subj)
+			queues = queuesa[:0]
+			for i := 0; i < len(qr.qsubs); i++ {
+				qsubs := qr.qsubs[i]
+				queue := qsubs[0].queue
+				add := true
+				for _, qn := range qgroups {
+					if bytes.Equal(queue, qn) {
+						add = false
+						break
+					}
+				}
+				if add {
+					qgroups = append(qgroups, queue)
+				}
+			}
+			if len(queues) == 0 {
+				continue
+			}
+		}
+		if checkReply {
+			checkReply = false
+			mreply = reply
+		}
+		mh := c.msgb[:10]
+		mh = append(mh, subject...)
+		if len(queues) > 0 {
+			mh = append(mh, mreply...)
+			mh = append(mh, queues...)
+		}
+		sub.client = gwc
+	}
+	subPool.Put(sub)
+}
+
+func subjectStartsWithGatewayReplyPrefix(subj []byte) bool {
+	return len(subj) > 8 && string(subj[:4]) == "foob"
+}
+
+func (c *client) gatewayInterest(acc, subj string) *SublistResult {
+	ei, _ := c.gw.outsim.Load(acc)
+	var r *SublistResult
+	e := ei.(*outsie)
+	r = e.sl.Match(subj)
+	return r
+}
+
+func (s *Sublist) Match(subject string) *SublistResult {
+	return nil
+}
+