cmd/compile: fix runtime.KeepAlive

KeepAlive needs to introduce a use of the spill of the
value it is keeping alive.  Without that, we don't guarantee
that the spill dominates the KeepAlive.

This bug was probably introduced with the code to move spills
down to the dominator of the restores, instead of always spilling
just after the value itself (CL 34822).

Fixes #22458.

Change-Id: I94955a21960448ffdacc4df775fe1213967b1d4c
Reviewed-on: https://go-review.googlesource.com/74210
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -1133,12 +1133,15 @@ func (s *regAllocState) regalloc(f *Func) {
 			if v.Op == OpKeepAlive {
 				// Make sure the argument to v is still live here.
 				s.advanceUses(v)
-				vi := &s.values[v.Args[0].ID]
-				if vi.spill != nil {
+				a := v.Args[0]
+				vi := &s.values[a.ID]
+				if vi.regs == 0 && !vi.rematerializeable {
 					// Use the spill location.
-					v.SetArg(0, vi.spill)
+					// This forces later liveness analysis to make the
+					// value live at this point.
+					v.SetArg(0, s.makeSpill(a, b))
 				} else {
-					// No need to keep unspilled values live.
+					// In-register and rematerializeable values are already live.
 					// These are typically rematerializeable constants like nil,
 					// or values of a variable that were modified since the last call.
 					v.Op = OpCopy

test/fixedbugs/issue22458.go

@@ -0,0 +1,26 @@
+// compile
+
+// 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.
+
+// Make sure KeepAlive introduces a use of the spilled variable.
+
+package main
+
+import "runtime"
+
+type node struct {
+        next *node
+}
+
+var x bool
+
+func main() {
+        var head *node
+        for x {
+                head = &node{head}
+        }
+
+        runtime.KeepAlive(head)
+}