runtime: keep objects in free lists marked as allocated.

Restore https://golang.org/cl/41040043 after GC rewrite.
Original description:
On the plus side, we don't need to change the bits on malloc and free.
On the downside, we need to mark objects in the free lists during GC.
But the free lists are small at GC time, so it should be a net win.

benchmark             old ns/op     new ns/op     delta
BenchmarkMalloc8      21.9          20.4          -6.85%
BenchmarkMalloc16     31.1          29.6          -4.82%

LGTM=khr
R=khr
CC=golang-codereviews, rlh, rsc
https://golang.org/cl/122280043

1 files changed, 15 insertions, 7 deletions

diff --git a/src/pkg/runtime/heapdump.c b/src/pkg/runtime/heapdump.c
index b002feb1c2..e5032783a8 100644
--- a/src/pkg/runtime/heapdump.c
+++ b/src/pkg/runtime/heapdump.c
@@ -525,11 +525,17 @@ dumproots(void)
 	runtime·iterate_finq(finq_callback);
 }
 
+// Bit vector of free marks.	
+// Needs to be as big as the largest number of objects per span.	
+#pragma dataflag NOPTR
+static byte free[PageSize/8];	
+
 static void
 dumpobjs(void)
 {
-	uintptr i, j, size, n, off, shift, *bitp, bits;
+	uintptr i, j, size, n;
 	MSpan *s;
+	MLink *l;
 	byte *p;
 
 	for(i = 0; i < runtime·mheap.nspan; i++) {
@@ -539,13 +545,15 @@ dumpobjs(void)
 		p = (byte*)(s->start << PageShift);
 		size = s->elemsize;
 		n = (s->npages << PageShift) / size;
+		if(n > nelem(free))	
+			runtime·throw("free array doesn't have enough entries");	
+		for(l = s->freelist; l != nil; l = l->next)
+			free[((byte*)l - p) / size] = true;	
 		for(j = 0; j < n; j++, p += size) {
-			off = (uintptr*)p - (uintptr*)runtime·mheap.arena_start;
-			bitp = (uintptr*)runtime·mheap.arena_start - off/wordsPerBitmapWord - 1;
-			shift = (off % wordsPerBitmapWord) * gcBits;
-			bits = (*bitp >> shift) & bitMask;
-			if(bits != bitAllocated)
-				continue;
+			if(free[j]) {	
+				free[j] = false;	
+				continue;	
+			}
 			dumpobj(p, size, makeheapobjbv(p, size));
 		}
 	}