runtime: use a distinct pattern to mark free blocks in need of zeroing

R=golang-dev, dvyukov, khr, cshapiro
CC=golang-dev
https://golang.org/cl/8392043

src/pkg/runtime/malloc.goc

@@ -160,7 +160,7 @@ runtime·free(void *v)
 	if(sizeclass == 0) {
 		// Large object.
 		size = s->npages<<PageShift;
-		*(uintptr*)(s->start<<PageShift) = 1;	// mark as "needs to be zeroed"
+		*(uintptr*)(s->start<<PageShift) = (uintptr)0xfeedfeedfeedfeedll;	// mark as "needs to be zeroed"
 		// Must mark v freed before calling unmarkspan and MHeap_Free:
 		// they might coalesce v into other spans and change the bitmap further.
 		runtime·markfreed(v, size);
@@ -170,7 +170,7 @@ runtime·free(void *v)
 		// Small object.
 		size = runtime·class_to_size[sizeclass];
 		if(size > sizeof(uintptr))
-			((uintptr*)v)[1] = 1;	// mark as "needs to be zeroed"
+			((uintptr*)v)[1] = (uintptr)0xfeedfeedfeedfeedll;	// mark as "needs to be zeroed"
 		// Must mark v freed before calling MCache_Free:
 		// it might coalesce v and other blocks into a bigger span
 		// and change the bitmap further.

src/pkg/runtime/mgc0.c

@@ -1607,7 +1607,7 @@ sweepspan(ParFor *desc, uint32 idx)
 		if(cl == 0) {
 			// Free large span.
 			runtime·unmarkspan(p, 1<<PageShift);
-			*(uintptr*)p = 1;	// needs zeroing
+			*(uintptr*)p = (uintptr)0xdeaddeaddeaddeadll;	// needs zeroing
 			runtime·MHeap_Free(runtime·mheap, s, 1);
 			c->local_alloc -= size;
 			c->local_nfree++;
@@ -1622,7 +1622,7 @@ sweepspan(ParFor *desc, uint32 idx)
 				break;
 			}
 			if(size > sizeof(uintptr))
-				((uintptr*)p)[1] = 1;	// mark as "needs to be zeroed"
+				((uintptr*)p)[1] = (uintptr)0xdeaddeaddeaddeadll;	// mark as "needs to be zeroed"
 			
 			end->next = (MLink*)p;
 			end = (MLink*)p;