cmd/cc: generate error if #pragma pack off does anything

We can't translate misaligned things to Go, so start rejecting them in C.

The only one in any build appears to be EpollEvent on linux/amd64.
Fix that.

LGTM=r
R=golang-codereviews, r, dvyukov
CC=golang-codereviews, iant
https://golang.org/cl/137020043

src/cmd/5c/swt.c

@@ -374,10 +374,11 @@ align(int32 i, Type *t, int op, int32 *maxalign)
 {
 	int32 o;
 	Type *v;
-	int w;
+	int w, packw;
 
 	o = i;
 	w = 1;
+	packw = 0;
 	switch(op) {
 	default:
 		diag(Z, "unknown align opcode %d", op);
@@ -388,7 +389,7 @@ align(int32 i, Type *t, int op, int32 *maxalign)
 		if(w < 1)
 			w = 1;
 		if(packflg)
-			w = packflg;
+			packw = packflg;
 		break;
 
 	case Ael1:	/* initial align of struct element */
@@ -404,7 +405,7 @@ align(int32 i, Type *t, int op, int32 *maxalign)
 		if(w < 1 || w > SZ_LONG)
 			fatal(Z, "align");
 		if(packflg) 
-			w = packflg;
+			packw = packflg;
 		break;
 
 	case Ael2:	/* width of a struct element */
@@ -440,6 +441,8 @@ align(int32 i, Type *t, int op, int32 *maxalign)
 		w = SZ_LONG;	/* because of a pun in cc/dcl.c:contig() */
 		break;
 	}
+	if(packw != 0 && xround(o, w) != xround(o, packw))
+		diag(Z, "#pragma pack changes offset of %T", t);
 	o = xround(o, w);
 	if(maxalign != nil && *maxalign < w)
 		*maxalign = w;

src/cmd/6c/swt.c

@@ -250,10 +250,11 @@ align(int32 i, Type *t, int op, int32 *maxalign)
 {
 	int32 o;
 	Type *v;
-	int w;
+	int w, packw;
 
 	o = i;
 	w = 1;
+	packw = 0;
 	switch(op) {
 	default:
 		diag(Z, "unknown align opcode %d", op);
@@ -264,7 +265,7 @@ align(int32 i, Type *t, int op, int32 *maxalign)
 		if(w < 1)
 			w = 1;
 		if(packflg)
-			w = packflg;
+			packw = packflg;
 		break;
 
 	case Ael1:	/* initial align of struct element */
@@ -277,7 +278,7 @@ align(int32 i, Type *t, int op, int32 *maxalign)
 		if(w < 1 || w > SZ_VLONG)
 			fatal(Z, "align");
 		if(packflg) 
-			w = packflg;
+			packw = packflg;
 		break;
 
 	case Ael2:	/* width of a struct element */
@@ -331,6 +332,8 @@ align(int32 i, Type *t, int op, int32 *maxalign)
 		o = align(o, t, Ael2, nil);
 		break;
 	}
+	if(packw != 0 && xround(o, w) != xround(o, packw))
+		diag(Z, "#pragma pack changes offset of %T", t);
 	o = xround(o, w);
 	if(maxalign && *maxalign < w)
 		*maxalign = w;

src/cmd/8c/swt.c

@@ -255,10 +255,11 @@ align(int32 i, Type *t, int op, int32 *maxalign)
 {
 	int32 o;
 	Type *v;
-	int w;
+	int w, packw;
 
 	o = i;
 	w = 1;
+	packw = 0;
 	switch(op) {
 	default:
 		diag(Z, "unknown align opcode %d", op);
@@ -269,7 +270,7 @@ align(int32 i, Type *t, int op, int32 *maxalign)
 		if(w < 1)
 			w = 1;
 		if(packflg)
-			w = packflg;
+			packw = packflg;
 		break;
 
 	case Ael1:	/* initial align of struct element */
@@ -285,7 +286,7 @@ align(int32 i, Type *t, int op, int32 *maxalign)
 		if(w < 1 || w > SZ_LONG)
 			fatal(Z, "align");
 		if(packflg) 
-			w = packflg;
+			packw = packflg;
 		break;
 
 	case Ael2:	/* width of a struct element */
@@ -320,6 +321,8 @@ align(int32 i, Type *t, int op, int32 *maxalign)
 		o = align(o, t, Ael2, nil);
 		break;
 	}
+	if(packw != 0 && xround(o, w) != xround(o, packw))
+		diag(Z, "#pragma pack changes offset of %T", t);
 	o = xround(o, w);
 	if(maxalign && *maxalign < w)
 		*maxalign = w;

src/pkg/runtime/defs_linux_amd64.h

@@ -122,7 +122,7 @@ struct Itimerval {
 };
 struct EpollEvent {
 	uint32	events;
-	uint64	data;
+	byte	data[8]; // unaligned uintptr
 };
 
 

src/pkg/runtime/netpoll_epoll.c

@@ -37,7 +37,7 @@ runtime·netpollopen(uintptr fd, PollDesc *pd)
 	int32 res;
 
 	ev.events = EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET;
-	ev.data = (uint64)pd;
+	*(uintptr*)ev.data = (uintptr)pd;
 	res = runtime·epollctl(epfd, EPOLL_CTL_ADD, (int32)fd, &ev);
 	return -res;
 }