sync/atomic: make unaligned 64-bit atomics crash on 386

R=golang-dev, bradfitz, dvyukov
CC=golang-dev
https://golang.org/cl/7702043

src/pkg/sync/atomic/asm_386.s

@@ -28,6 +28,9 @@ TEXT ·CompareAndSwapInt64(SB),7,$0
 
 TEXT ·CompareAndSwapUint64(SB),7,$0
 	MOVL	addr+0(FP), BP
+	TESTL	$7, BP
+	JZ	2(PC)
+	MOVL	0, AX // crash with nil ptr deref
 	MOVL	old+4(FP), AX
 	MOVL	old+8(FP), DX
 	MOVL	new+12(FP), BX
@@ -61,6 +64,9 @@ TEXT ·AddInt64(SB),7,$0
 TEXT ·AddUint64(SB),7,$0
 	// no XADDQ so use CMPXCHG8B loop
 	MOVL	addr+0(FP), BP
+	TESTL	$7, BP
+	JZ	2(PC)
+	MOVL	0, AX // crash with nil ptr deref
 	// DI:SI = delta
 	MOVL	delta+4(FP), SI
 	MOVL	delta+8(FP), DI
@@ -105,6 +111,9 @@ TEXT ·LoadInt64(SB),7,$0
 
 TEXT ·LoadUint64(SB),7,$0
 	MOVL	addr+0(FP), AX
+	TESTL	$7, AX
+	JZ	2(PC)
+	MOVL	0, AX // crash with nil ptr deref
 	// MOVQ and EMMS were introduced on the Pentium MMX.
 	// MOVQ (%EAX), %MM0
 	BYTE $0x0f; BYTE $0x6f; BYTE $0x00
@@ -133,6 +142,9 @@ TEXT ·StoreInt64(SB),7,$0
 
 TEXT ·StoreUint64(SB),7,$0
 	MOVL	addr+0(FP), AX
+	TESTL	$7, AX
+	JZ	2(PC)
+	MOVL	0, AX // crash with nil ptr deref
 	// MOVQ and EMMS were introduced on the Pentium MMX.
 	// MOVQ 0x8(%ESP), %MM0
 	BYTE $0x0f; BYTE $0x6f; BYTE $0x44; BYTE $0x24; BYTE $0x08

src/pkg/sync/atomic/atomic_test.go

@@ -1177,3 +1177,29 @@ func TestStoreLoadRelAcq64(t *testing.T) {
 	<-c
 	<-c
 }
+
+func shouldPanic(t *testing.T, name string, f func()) {
+	defer func() {
+		if recover() == nil {
+			t.Errorf("%s did not panic", name)
+		}
+	}()
+	f()
+}
+
+func TestUnaligned64(t *testing.T) {
+	// Unaligned 64-bit atomics on 32-bit systems are
+	// a continual source of pain. Test that on 386 they crash
+	// instead of failing silently.
+	if runtime.GOARCH != "386" {
+		t.Skip("test only runs on 386")
+	}
+
+	x := make([]uint32, 4)
+	p := (*uint64)(unsafe.Pointer(&x[1])) // misaligned
+
+	shouldPanic(t, "LoadUint64", func() { LoadUint64(p) })
+	shouldPanic(t, "StoreUint64", func() { StoreUint64(p, 1) })
+	shouldPanic(t, "CompareAndSwapUint64", func() { CompareAndSwapUint64(p, 1, 2) })
+	shouldPanic(t, "AddUint64", func() { AddUint64(p, 3) })
+}