test: update tinyfin test

* use new(int32) to be pedantic about documented SetFinalizer rules:
  "The argument x must be a pointer to an object allocated by calling
  new or by taking the address of a composite literal"

* remove the amd64-only restriction. The GC is fully precise everywhere
  now, even on 32-bit. (keep the gccgo restriction, though)

* remove a data race (perhaps the actual bug) and use atomic.LoadInt32
  for the final check. The race detector is now happy, too.

Updates #13100

Change-Id: I8d05c0ac4f046af9ba05701ad709c57984b34893
Reviewed-on: https://go-review.googlesource.com/16535
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/tinyfin.go

@@ -15,12 +15,8 @@ import (
 )
 
 func main() {
-	// Does not work on 32-bits due to partially conservative GC.
+	// Does not work on gccgo due to partially conservative GC.
 	// Try to enable when we have fully precise GC.
-	if runtime.GOARCH != "amd64" {
-		return
-	}
-	// Likewise for gccgo.
 	if runtime.Compiler == "gccgo" {
 		return
 	}
@@ -28,9 +24,10 @@ func main() {
 	count := N
 	done := make([]bool, N)
 	for i := int32(0); i < N; i++ {
-		x := i // subject to tiny alloc
+		x := new(int32) // subject to tiny alloc
+		*x = i
 		// the closure must be big enough to be combined
-		runtime.SetFinalizer(&x, func(p *int32) {
+		runtime.SetFinalizer(x, func(p *int32) {
 			// Check that p points to the correct subobject of the tiny allocation.
 			// It's a bit tricky, because we can't capture another variable
 			// with the expected value (it would be combined as well).
@@ -54,9 +51,8 @@ func main() {
 	// if the outermost allocations are combined with something persistent.
 	// Currently 4 int32's are combined into a 16-byte block,
 	// ensure that most of them are finalized.
-	if count >= N/4 {
+	if atomic.LoadInt32(&count) >= N/4 {
 		println(count, "out of", N, "finalizer are not called")
 		panic("not all finalizers are called")
 	}
 }
-