runtime: minor refactoring in preparation for parallel GC

factor sweepspan() out of sweep(), no logical changes

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5991047

src/pkg/runtime/mgc0.c

@@ -719,22 +719,17 @@ handlespecial(byte *p, uintptr size)
 	return true;
 }
 
+static void sweepspan(MSpan *s);
+
 // Sweep frees or collects finalizers for blocks not marked in the mark phase.
 // It clears the mark bits in preparation for the next GC round.
 static void
 sweep(void)
 {
 	MSpan *s;
-	int32 cl, n, npages;
-	uintptr size;
-	byte *p;
-	MCache *c;
-	byte *arena_start;
 	int64 now;
 
-	arena_start = runtime·mheap.arena_start;
 	now = runtime·nanotime();
-
 	for(;;) {
 		s = work.spans;
 		if(s == nil)
@@ -750,6 +745,20 @@ sweep(void)
 		if(s->state != MSpanInUse)
 			continue;
 
+		sweepspan(s);
+	}
+}
+
+static void
+sweepspan(MSpan *s)
+{
+	int32 cl, n, npages;
+	uintptr size;
+	byte *p;
+	MCache *c;
+	byte *arena_start;
+
+	arena_start = runtime·mheap.arena_start;
 	p = (byte*)(s->start << PageShift);
 	cl = s->sizeclass;
 	if(cl == 0) {
@@ -813,7 +822,6 @@ sweep(void)
 		c->local_alloc -= size;
 		c->local_nfree++;
 	}
-	}
 }
 
 void