diff --git a/m1/perf/sched.c b/m1/perf/sched.c
index 9e3fb76..d00aa3f 100644
--- a/m1/perf/sched.c
+++ b/m1/perf/sched.c
@@ -22,6 +22,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
+#include <assert.h>
 
 #include <fpvm/is.h>
 #include <fpvm/fpvm.h>
@@ -135,6 +136,21 @@ static void get_registers(struct fpvm_fragment *fragment,
  * Use the naming conventions of include/linux/list.h
  */
 
+
+#ifdef DEBUG
+
+static void list_poison(struct list *list)
+{
+	list->next = list->prev = NULL;
+}
+
+#else /* DEBUG */
+
+#define list_poison(list)
+
+#endif /* !DEBUG */
+
+
 static void list_init(struct list *list)
 {
 	list->next = list->prev = list;
@@ -143,8 +159,10 @@ static void list_init(struct list *list)
 
 static void list_del(struct list *item)
 {
+	assert(item->next != item);
 	item->prev->next = item->next;
 	item->next->prev = item->prev;
+	list_poison(item);
 }
 
 
@@ -186,7 +204,7 @@ static void list_concat(struct list *a, struct list *b)
 		b->prev->next = a;
 		a->prev = b->prev;
 	}
-	list_init(b);
+	list_poison(b);
 }
 
 
@@ -249,12 +267,14 @@ static void put_reg(int vm_reg)
 		return;
 
 	vm_idx = reg2idx(vm_reg);
+	assert(sc->regs[vm_idx].refs);
 	if (--sc->regs[vm_idx].refs)
 		return;
 
 	Dprintf("  free reg %d\n", regs[vm_idx].pfpu_reg);
 
 #ifdef REG_STATS
+	assert(sc->curr_regs);
 	sc->curr_regs--;
 #endif
 
@@ -335,8 +355,6 @@ static struct vm_reg *add_data_ref(struct insn *insn, struct data_ref *ref,
 		Dprintf("insn %lu: reg %d setter %lu unresolved %d\n",
 		    insn-sc->insns, reg_num, reg->setter-sc->insns,
 		    insn->unresolved);
-	} else {
-		list_init(&ref->more);
 	}
 	return reg;
 }
@@ -428,6 +446,7 @@ static void issue(struct insn *insn, int cycle, unsigned *code)
 	foreach (ref, &insn->dependants) {
 		if (ref->insn->earliest <= end)
 			ref->insn->earliest = end+1;
+		assert(ref->insn->unresolved);
 		if (!--ref->insn->unresolved) {
 			Dprintf("  unlocked %lu -> %u\n", ref->insn-insns,
 			    ref->insn->earliest);
@@ -509,7 +528,7 @@ static int schedule(unsigned int *code)
 		end = PFPU_PROGSIZE;
 	while (i != end) {
 		if (FIELD(code[i]).dest)
-			last = i+1; /* @@@ ? */
+			last = i+1;
 		i++;
 	}
 	return last;