1 files changed, 16 insertions, 13 deletions

diff --git a/libgo/go/runtime/runtime1.go b/libgo/go/runtime/runtime1.go
index 99c0f11930f..dd3f7b277a6 100644
--- a/libgo/go/runtime/runtime1.go
+++ b/libgo/go/runtime/runtime1.go
@@ -112,10 +112,10 @@ var test_z64, test_x64 uint64
 func testAtomic64() {
 	test_z64 = 42
 	test_x64 = 0
-	// prefetcht0(uintptr(unsafe.Pointer(&test_z64)))
-	// prefetcht1(uintptr(unsafe.Pointer(&test_z64)))
-	// prefetcht2(uintptr(unsafe.Pointer(&test_z64)))
-	// prefetchnta(uintptr(unsafe.Pointer(&test_z64)))
+	prefetcht0(uintptr(unsafe.Pointer(&test_z64)))
+	prefetcht1(uintptr(unsafe.Pointer(&test_z64)))
+	prefetcht2(uintptr(unsafe.Pointer(&test_z64)))
+	prefetchnta(uintptr(unsafe.Pointer(&test_z64)))
 	if atomic.Cas64(&test_z64, test_x64, 1) {
 		throw("cas64 failed")
 	}
@@ -151,14 +151,6 @@ func testAtomic64() {
 }
 
 func check() {
-
-	// This doesn't currently work for gccgo.  Because escape
-	// analysis is not turned on by default, the code below that
-	// takes the address of local variables causes memory
-	// allocation, but this function is called before the memory
-	// allocator has been initialized.
-	return
-
 	var (
 		a     int8
 		b     uint8
@@ -390,7 +382,18 @@ var dbgvars = []dbgVar{
 func parsedebugvars() {
 	// defaults
 	debug.cgocheck = 1
-	debug.invalidptr = 1
+
+	// Unfortunately, because gccgo uses conservative stack scanning,
+	// we can not enable invalid pointer checking. It is possible for
+	// memory block M1 to point to M2, and for both to be dead.
+	// We release M2, causing the entire span to be released.
+	// Before we release M1, a stack pointer appears that point into it.
+	// This stack pointer is presumably dead, but causes M1 to be marked.
+	// We scan M1 and see the pointer to M2 on a released span.
+	// At that point, if debug.invalidptr is set, we crash.
+	// This is not a problem, assuming that M1 really is dead and
+	// the pointer we discovered to it will not be used.
+	// debug.invalidptr = 1
 
 	for p := gogetenv("GODEBUG"); p != ""; {
 		field := ""