cmd/compile: fix write barrier coalescing

We can't coalesce a non-WB store with a subsequent Move, as the
result of the store might be the source of the move.

There's a simple codegen test. Not sure how we might do a real test,
as all the repro's I've come up with are very expensive and unreliable.

Fixes #71228

Change-Id: If18bf181a266b9b90964e2591cd2e61a7168371c
Reviewed-on: https://go-review.googlesource.com/c/go/+/642197
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -252,6 +252,7 @@ func writebarrier(f *Func) {
 		var start, end int
 		var nonPtrStores int
 		values := b.Values
+		hasMove := false
 	FindSeq:
 		for i := len(values) - 1; i >= 0; i-- {
 			w := values[i]
@@ -263,6 +264,9 @@ func writebarrier(f *Func) {
 					end = i + 1
 				}
 				nonPtrStores = 0
+				if w.Op == OpMoveWB {
+					hasMove = true
+				}
 			case OpVarDef, OpVarLive:
 				continue
 			case OpStore:
@@ -273,6 +277,17 @@ func writebarrier(f *Func) {
 				if nonPtrStores > 2 {
 					break FindSeq
 				}
+				if hasMove {
+					// We need to ensure that this store happens
+					// before we issue a wbMove, as the wbMove might
+					// use the result of this store as its source.
+					// Even though this store is not write-barrier
+					// eligible, it might nevertheless be the store
+					// of a pointer to the stack, which is then the
+					// source of the move.
+					// See issue 71228.
+					break FindSeq
+				}
 			default:
 				if last == nil {
 					continue