Make assert have no side effects

This makes sure symbols referenced by assert uses may not be generated
if unused. The recently introduced __unused macro is used to suppress
compiler warnings resulting from this change.

24 files changed, 88 insertions, 69 deletions

diff --git a/arch/x86/machine/cga.c b/arch/x86/machine/cga.c
index b519ba1f..d086326d 100644
--- a/arch/x86/machine/cga.c
+++ b/arch/x86/machine/cga.c
@@ -216,7 +216,7 @@ static void
 cga_bbuf_redraw(struct cga_bbuf *bbuf)
 {
     size_t size;
-    int error;
+    __unused int error;
 
     size = CGA_MEMORY_SIZE;
     error = cbuf_read(&bbuf->cbuf, bbuf->view, cga_memory, &size);
@@ -298,7 +298,7 @@ cga_bbuf_newline(struct cga_bbuf *bbuf)
 {
     uint16_t cursor = 0, spaces[CGA_COLUMNS];
     size_t i, nr_spaces, offset, size;
-    int error;
+    __unused int error;
 
     cga_bbuf_reset_view(bbuf);
 
diff --git a/arch/x86/machine/cpu.c b/arch/x86/machine/cpu.c
index 98d3680e..e971d99c 100644
--- a/arch/x86/machine/cpu.c
+++ b/arch/x86/machine/cpu.c
@@ -466,7 +466,9 @@ cpu_init(struct cpu *cpu)
     cpu->phys_addr_width = 0;
     cpu->virt_addr_width = 0;
 
-    assert(max_basic >= 1);
+    if (max_basic == 0) {
+        panic("cpu: unsupported maximum input value for basic information");
+    }
 
     eax = 1;
     cpu_cpuid(&eax, &ebx, &ecx, &edx);
diff --git a/arch/x86/machine/ioapic.c b/arch/x86/machine/ioapic.c
index 82fbd6d5..d70ec92b 100644
--- a/arch/x86/machine/ioapic.c
+++ b/arch/x86/machine/ioapic.c
@@ -209,7 +209,7 @@ ioapic_create(unsigned int apic_id, uintptr_t addr, unsigned int gsi_base)
     return ioapic;
 }
 
-static bool
+__unused static bool
 ioapic_has_gsi(const struct ioapic *ioapic, unsigned int gsi)
 {
     return ((gsi >= ioapic->first_gsi) && (gsi <= ioapic->last_gsi));
diff --git a/include/assert.h b/include/assert.h
index 8fdc2fe2..9bf3bdf0 100644
--- a/include/assert.h
+++ b/include/assert.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2010 Richard Braun.
+ * Copyright (c) 2010-2017 Richard Braun.
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -21,7 +21,7 @@
 #define static_assert _Static_assert
 
 #ifdef NDEBUG
-#define assert(expression) ((void)(expression))
+#define assert(expression)
 #else /* NDEBUG */
 
 #include <kern/macros.h>
diff --git a/kern/cbuf.c b/kern/cbuf.c
index 2093f428..eca0c33c 100644
--- a/kern/cbuf.c
+++ b/kern/cbuf.c
@@ -72,7 +72,7 @@ cbuf_push(struct cbuf *cbuf, const void *buf, size_t size, bool erase)
 int
 cbuf_pop(struct cbuf *cbuf, void *buf, size_t *sizep)
 {
-    int error;
+    __unused int error;
 
     if (cbuf_size(cbuf) == 0) {
         return ERROR_AGAIN;
diff --git a/kern/condition.c b/kern/condition.c
index e6d65951..b12068fa 100644
--- a/kern/condition.c
+++ b/kern/condition.c
@@ -88,7 +88,7 @@ condition_wait_common(struct condition *condition, struct mutex *mutex,
 void
 condition_wait(struct condition *condition, struct mutex *mutex)
 {
-    int error;
+    __unused int error;
 
     error = condition_wait_common(condition, mutex, false, 0);
     assert(!error);
diff --git a/kern/kmem.c b/kern/kmem.c
index d98be0d6..5423c0a0 100644
--- a/kern/kmem.c
+++ b/kern/kmem.c
@@ -646,7 +646,7 @@ kmem_cache_register(struct kmem_cache *cache, struct kmem_slab *slab)
     uintptr_t va, end;
     phys_addr_t pa;
     bool virtual;
-    int error;
+    __unused int error;
 
     assert(kmem_cache_registration_required(cache));
     assert(slab->nr_refs == 0);
diff --git a/kern/log.c b/kern/log.c
index a48afc5a..10131d12 100644
--- a/kern/log.c
+++ b/kern/log.c
@@ -459,7 +459,7 @@ INIT_OP_DEFINE(log_start,
 static void
 log_write(const void *s, size_t size)
 {
-    int error;
+    __unused int error;
 
     error = cbuf_push(&log_cbuf, s, size, true);
     assert(!error);
diff --git a/kern/mutex/mutex_adaptive.c b/kern/mutex/mutex_adaptive.c
index 68b006ad..8da55270 100644
--- a/kern/mutex/mutex_adaptive.c
+++ b/kern/mutex/mutex_adaptive.c
@@ -24,6 +24,7 @@
 #include <kern/clock.h>
 #include <kern/error.h>
 #include <kern/init.h>
+#include <kern/macros.h>
 #include <kern/mutex.h>
 #include <kern/mutex_types.h>
 #include <kern/sleepq.h>
@@ -235,7 +236,7 @@ out:
 void
 mutex_adaptive_lock_slow(struct mutex *mutex)
 {
-    int error;
+    __unused int error;
 
     error = mutex_adaptive_lock_slow_common(mutex, false, 0);
     assert(!error);
diff --git a/kern/mutex/mutex_plain.c b/kern/mutex/mutex_plain.c
index 2c655940..abfa3396 100644
--- a/kern/mutex/mutex_plain.c
+++ b/kern/mutex/mutex_plain.c
@@ -22,6 +22,7 @@
 
 #include <kern/atomic.h>
 #include <kern/init.h>
+#include <kern/macros.h>
 #include <kern/mutex.h>
 #include <kern/mutex_types.h>
 #include <kern/sleepq.h>
@@ -133,7 +134,7 @@ out:
 void
 mutex_plain_lock_slow(struct mutex *mutex)
 {
-    int error;
+    __unused int error;
 
     error = mutex_plain_lock_slow_common(mutex, false, 0);
     assert(!error);
diff --git a/kern/rtmutex.c b/kern/rtmutex.c
index c07bbfef..09637514 100644
--- a/kern/rtmutex.c
+++ b/kern/rtmutex.c
@@ -183,7 +183,7 @@ out:
 void
 rtmutex_lock_slow(struct rtmutex *rtmutex)
 {
-    int error;
+    __unused int error;
 
     error = rtmutex_lock_slow_common(rtmutex, false, 0);
     assert(!error);
diff --git a/kern/semaphore.c b/kern/semaphore.c
index 72e843a9..003f053c 100644
--- a/kern/semaphore.c
+++ b/kern/semaphore.c
@@ -20,6 +20,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include <kern/macros.h>
 #include <kern/semaphore.h>
 #include <kern/semaphore_i.h>
 #include <kern/sleepq.h>
@@ -63,7 +64,7 @@ semaphore_wait_slow_common(struct semaphore *semaphore,
 void
 semaphore_wait_slow(struct semaphore *semaphore)
 {
-    int error;
+    __unused int error;
 
     error = semaphore_wait_slow_common(semaphore, false, 0);
     assert(!error);
diff --git a/kern/sleepq.c b/kern/sleepq.c
index 170b1a9f..0c20ae42 100644
--- a/kern/sleepq.c
+++ b/kern/sleepq.c
@@ -110,14 +110,14 @@ sleepq_waiter_wakeup(struct sleepq_waiter *waiter)
     thread_wakeup(waiter->thread);
 }
 
-static void
-sleepq_assert_init_state(const struct sleepq *sleepq)
+__unused static bool
+sleepq_state_initialized(const struct sleepq *sleepq)
 {
-    assert(sleepq->bucket == NULL);
-    assert(sleepq->sync_obj == NULL);
-    assert(list_empty(&sleepq->waiters));
-    assert(sleepq->oldest_waiter == NULL);
-    assert(sleepq->next_free == NULL);
+    return ((sleepq->bucket == NULL)
+            && (sleepq->sync_obj == NULL)
+            && (list_empty(&sleepq->waiters))
+            && (sleepq->oldest_waiter == NULL)
+            && (sleepq->next_free == NULL));
 }
 
 static void
@@ -134,7 +134,7 @@ sleepq_unuse(struct sleepq *sleepq)
     sleepq->sync_obj = NULL;
 }
 
-static bool
+__unused static bool
 sleepq_in_use(const struct sleepq *sleepq)
 {
     return sleepq->sync_obj != NULL;
@@ -186,7 +186,8 @@ sleepq_bucket_add(struct sleepq_bucket *bucket, struct sleepq *sleepq)
 }
 
 static void
-sleepq_bucket_remove(struct sleepq_bucket *bucket, struct sleepq *sleepq)
+sleepq_bucket_remove(__unused struct sleepq_bucket *bucket,
+                     struct sleepq *sleepq)
 {
     assert(sleepq->bucket == bucket);
     sleepq->bucket = NULL;
@@ -253,14 +254,14 @@ sleepq_create(void)
         return NULL;
     }
 
-    sleepq_assert_init_state(sleepq);
+    assert(sleepq_state_initialized(sleepq));
     return sleepq;
 }
 
 void
 sleepq_destroy(struct sleepq *sleepq)
 {
-    sleepq_assert_init_state(sleepq);
+    assert(sleepq_state_initialized(sleepq));
     kmem_cache_free(&sleepq_cache, sleepq);
 }
 
@@ -352,7 +353,7 @@ sleepq_lend(const void *sync_obj, bool condition, unsigned long *flags)
     assert(sync_obj != NULL);
 
     sleepq = thread_sleepq_lend();
-    sleepq_assert_init_state(sleepq);
+    assert(sleepq_state_initialized(sleepq));
 
     bucket = sleepq_bucket_get(sync_obj, condition);
 
@@ -390,7 +391,7 @@ sleepq_return(struct sleepq *sleepq, unsigned long flags)
 
     spinlock_unlock_intr_restore(&bucket->lock, flags);
 
-    sleepq_assert_init_state(free_sleepq);
+    assert(sleepq_state_initialized(free_sleepq));
     thread_sleepq_return(free_sleepq);
 }
 
@@ -468,7 +469,7 @@ sleepq_wait_common(struct sleepq *sleepq, const char *wchan,
 void
 sleepq_wait(struct sleepq *sleepq, const char *wchan)
 {
-    int error;
+    __unused int error;
 
     error = sleepq_wait_common(sleepq, wchan, false, 0);
     assert(!error);
diff --git a/kern/spinlock.h b/kern/spinlock.h
index d4105da0..50ac5401 100644
--- a/kern/spinlock.h
+++ b/kern/spinlock.h
@@ -26,6 +26,8 @@
 #ifndef _KERN_SPINLOCK_H
 #define _KERN_SPINLOCK_H
 
+#include <stdbool.h>
+
 #include <kern/init.h>
 #include <kern/macros.h>
 #include <kern/spinlock_i.h>
@@ -35,13 +37,17 @@
 
 struct spinlock;
 
-#define spinlock_assert_locked(lock) assert((lock)->value != SPINLOCK_UNLOCKED)
-
 /*
  * Initialize a spin lock.
  */
 void spinlock_init(struct spinlock *lock);
 
+static inline bool
+spinlock_locked(const struct spinlock *lock)
+{
+    return lock->value != SPINLOCK_UNLOCKED;
+}
+
 /*
  * Attempt to lock the given spin lock.
  *
diff --git a/kern/sref.c b/kern/sref.c
index 3603e781..95daeb0f 100644
--- a/kern/sref.c
+++ b/kern/sref.c
@@ -242,7 +242,7 @@ sref_queue_concat(struct sref_queue *queue1, struct sref_queue *queue2)
     queue1->size += queue2->size;
 }
 
-static inline bool
+__unused static inline bool
 sref_counter_aligned(const struct sref_counter *counter)
 {
     return (((uintptr_t)counter & (~SREF_WEAKREF_MASK)) == 0);
diff --git a/kern/task.h b/kern/task.h
index 1711fbee..149ff49d 100644
--- a/kern/task.h
+++ b/kern/task.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012 Richard Braun.
+ * Copyright (c) 2012-2017 Richard Braun.
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -21,6 +21,7 @@
 #include <kern/atomic.h>
 #include <kern/init.h>
 #include <kern/list.h>
+#include <kern/macros.h>
 #include <kern/spinlock.h>
 #include <kern/thread.h>
 #include <vm/vm_map.h>
@@ -50,7 +51,7 @@ extern struct task *kernel_task;
 static inline void
 task_ref(struct task *task)
 {
-    unsigned long nr_refs;
+    __unused unsigned long nr_refs;
 
     nr_refs = atomic_fetch_add(&task->nr_refs, 1, ATOMIC_RELAXED);
     assert(nr_refs != (unsigned long)-1);
diff --git a/kern/thread.c b/kern/thread.c
index 4a9cb2a1..5d8ac11d 100644
--- a/kern/thread.c
+++ b/kern/thread.c
@@ -475,7 +475,7 @@ thread_runq_add(struct thread_runq *runq, struct thread *thread)
     const struct thread_sched_ops *ops;
 
     assert(!cpu_intr_enabled());
-    spinlock_assert_locked(&runq->lock);
+    assert(spinlock_locked(&runq->lock));
     assert(!thread->in_runq);
 
     ops = thread_get_real_sched_ops(thread);
@@ -502,7 +502,7 @@ thread_runq_remove(struct thread_runq *runq, struct thread *thread)
     const struct thread_sched_ops *ops;
 
     assert(!cpu_intr_enabled());
-    spinlock_assert_locked(&runq->lock);
+    assert(spinlock_locked(&runq->lock));
     assert(thread->in_runq);
 
     runq->nr_threads--;
@@ -523,7 +523,7 @@ thread_runq_put_prev(struct thread_runq *runq, struct thread *thread)
     const struct thread_sched_ops *ops;
 
     assert(!cpu_intr_enabled());
-    spinlock_assert_locked(&runq->lock);
+    assert(spinlock_locked(&runq->lock));
 
     ops = thread_get_real_sched_ops(thread);
 
@@ -539,7 +539,7 @@ thread_runq_get_next(struct thread_runq *runq)
     unsigned int i;
 
     assert(!cpu_intr_enabled());
-    spinlock_assert_locked(&runq->lock);
+    assert(spinlock_locked(&runq->lock));
 
     for (i = 0; i < ARRAY_SIZE(thread_sched_ops); i++) {
         thread = thread_sched_ops[i].get_next(runq);
@@ -572,7 +572,7 @@ static void
 thread_runq_wakeup(struct thread_runq *runq, struct thread *thread)
 {
     assert(!cpu_intr_enabled());
-    spinlock_assert_locked(&runq->lock);
+    assert(spinlock_locked(&runq->lock));
     assert(thread->state == THREAD_RUNNING);
 
     thread_runq_add(runq, thread);
@@ -612,7 +612,7 @@ thread_runq_schedule(struct thread_runq *runq)
            && (__builtin_frame_address(0) < (prev->stack + TCB_STACK_SIZE)));
     assert(prev->preempt == THREAD_SUSPEND_PREEMPT_LEVEL);
     assert(!cpu_intr_enabled());
-    spinlock_assert_locked(&runq->lock);
+    assert(spinlock_locked(&runq->lock));
 
     llsync_report_context_switch();
 
@@ -657,7 +657,7 @@ thread_runq_schedule(struct thread_runq *runq)
 
     assert(prev->preempt == THREAD_SUSPEND_PREEMPT_LEVEL);
     assert(!cpu_intr_enabled());
-    spinlock_assert_locked(&runq->lock);
+    assert(spinlock_locked(&runq->lock));
     return runq;
 }
 
@@ -762,7 +762,8 @@ thread_sched_rt_get_next(struct thread_runq *runq)
 }
 
 static void
-thread_sched_rt_reset_priority(struct thread *thread, unsigned short priority)
+thread_sched_rt_reset_priority(struct thread *thread,
+                               __unused unsigned short priority)
 {
     assert(priority <= THREAD_SCHED_RT_PRIO_MAX);
     thread->rt_data.time_slice = THREAD_DEFAULT_RR_TIME_SLICE;
@@ -1888,12 +1889,12 @@ thread_unlock_runq(struct thread_runq *runq, unsigned long flags)
 static void *
 thread_alloc_stack(void)
 {
-    struct vm_page *first_page, *last_page;
+    __unused struct vm_page *first_page, *last_page;
     phys_addr_t first_pa, last_pa;
     size_t stack_size;
     uintptr_t va;
     void *mem;
-    int error;
+    __unused int error;
 
     stack_size = vm_page_round(TCB_STACK_SIZE);
     mem = vm_kmem_alloc((PAGE_SIZE * 2) + stack_size);
@@ -2546,7 +2547,7 @@ void
 thread_sleep(struct spinlock *interlock, const void *wchan_addr,
              const char *wchan_desc)
 {
-    int error;
+    __unused int error;
 
     error = thread_sleep_common(interlock, wchan_addr, wchan_desc, false, 0);
     assert(!error);
@@ -2776,14 +2777,14 @@ thread_pi_setscheduler(struct thread *thread, unsigned char policy,
                        unsigned short priority)
 {
     const struct thread_sched_ops *ops;
+    __unused struct turnstile_td *td;
     struct thread_runq *runq;
-    struct turnstile_td *td;
     unsigned int global_priority;
     unsigned long flags;
     bool requeue, current;
 
     td = thread_turnstile_td(thread);
-    turnstile_td_assert_lock(td);
+    assert(turnstile_td_locked(td));
 
     ops = thread_get_sched_ops(thread_policy_to_class(policy));
     global_priority = ops->get_global_priority(priority);
diff --git a/kern/thread.h b/kern/thread.h
index a3f2670d..1a811685 100644
--- a/kern/thread.h
+++ b/kern/thread.h
@@ -43,6 +43,7 @@
 #include <kern/init.h>
 #include <kern/condition.h>
 #include <kern/cpumap.h>
+#include <kern/macros.h>
 #include <kern/spinlock_types.h>
 #include <kern/turnstile_types.h>
 #include <machine/cpu.h>
@@ -281,7 +282,7 @@ void thread_pi_setscheduler(struct thread *thread, unsigned char policy,
 static inline void
 thread_ref(struct thread *thread)
 {
-    unsigned long nr_refs;
+    __unused unsigned long nr_refs;
 
     nr_refs = atomic_fetch_add(&thread->nr_refs, 1, ATOMIC_RELAXED);
     assert(nr_refs != (unsigned long)-1);
diff --git a/kern/timer.c b/kern/timer.c
index 77a6bb36..57f1765f 100644
--- a/kern/timer.c
+++ b/kern/timer.c
@@ -135,7 +135,7 @@ timer_unlock_cpu_data(struct timer_cpu_data *cpu_data, unsigned long flags)
  * Timer state functions.
  */
 
-static bool
+__unused static bool
 timer_ready(const struct timer *timer)
 {
     return timer->state == TIMER_TS_READY;
diff --git a/kern/turnstile.c b/kern/turnstile.c
index e59a7f3f..a9b994d5 100644
--- a/kern/turnstile.c
+++ b/kern/turnstile.c
@@ -401,14 +401,14 @@ turnstile_td_propagate_priority(struct turnstile_td *td)
     turnstile_td_propagate_priority_loop(td);
 }
 
-static void
-turnstile_assert_init_state(const struct turnstile *turnstile)
+__unused static bool
+turnstile_state_initialized(const struct turnstile *turnstile)
 {
-    assert(turnstile->bucket == NULL);
-    assert(turnstile->sync_obj == NULL);
-    assert(plist_empty(&turnstile->waiters));
-    assert(turnstile->next_free == NULL);
-    assert(turnstile->top_waiter == NULL);
+    return ((turnstile->bucket == NULL)
+            && (turnstile->sync_obj == NULL)
+            && (plist_empty(&turnstile->waiters))
+            && (turnstile->next_free == NULL)
+            && (turnstile->top_waiter == NULL));
 }
 
 static void
@@ -425,7 +425,7 @@ turnstile_unuse(struct turnstile *turnstile)
     turnstile->sync_obj = NULL;
 }
 
-static bool
+__unused static bool
 turnstile_in_use(const struct turnstile *turnstile)
 {
     return turnstile->sync_obj != NULL;
@@ -464,7 +464,7 @@ turnstile_bucket_add(struct turnstile_bucket *bucket,
 }
 
 static void
-turnstile_bucket_remove(struct turnstile_bucket *bucket,
+turnstile_bucket_remove(__unused struct turnstile_bucket *bucket,
                         struct turnstile *turnstile)
 {
     assert(turnstile->bucket == bucket);
@@ -529,14 +529,14 @@ turnstile_create(void)
         return NULL;
     }
 
-    turnstile_assert_init_state(turnstile);
+    assert(turnstile_state_initialized(turnstile));
     return turnstile;
 }
 
 void
 turnstile_destroy(struct turnstile *turnstile)
 {
-    turnstile_assert_init_state(turnstile);
+    assert(turnstile_state_initialized(turnstile));
     kmem_cache_free(&turnstile_cache, turnstile);
 }
 
@@ -603,7 +603,7 @@ turnstile_lend(const void *sync_obj)
     assert(sync_obj != NULL);
 
     turnstile = thread_turnstile_lend();
-    turnstile_assert_init_state(turnstile);
+    assert(turnstile_state_initialized(turnstile));
 
     td = thread_turnstile_td(thread_self());
     bucket = turnstile_bucket_get(sync_obj);
@@ -653,7 +653,7 @@ turnstile_return(struct turnstile *turnstile)
 
     spinlock_unlock(&bucket->lock);
 
-    turnstile_assert_init_state(free_turnstile);
+    assert(turnstile_state_initialized(free_turnstile));
     thread_turnstile_return(free_turnstile);
 }
 
@@ -774,7 +774,7 @@ void
 turnstile_wait(struct turnstile *turnstile, const char *wchan,
                struct thread *owner)
 {
-    int error;
+    __unused int error;
 
     error = turnstile_wait_common(turnstile, wchan, owner, false, 0);
     assert(!error);
@@ -806,7 +806,7 @@ turnstile_own(struct turnstile *turnstile)
 {
     struct turnstile_td *td;
     struct thread *owner;
-    unsigned int top_priority;
+    __unused unsigned int top_priority;
 
     assert(turnstile->owner == NULL);
 
diff --git a/kern/turnstile.h b/kern/turnstile.h
index e7b4a5e3..f7ba0847 100644
--- a/kern/turnstile.h
+++ b/kern/turnstile.h
@@ -43,8 +43,6 @@ struct turnstile;
  */
 struct turnstile_td;
 
-#define turnstile_td_assert_lock(td) spinlock_assert_locked(&(td)->lock)
-
 /*
  * Initialize turnstile thread data.
  */
@@ -62,6 +60,12 @@ turnstile_td_init(struct turnstile_td *td)
  * Turnstile thread data locking functions.
  */
 
+static inline bool
+turnstile_td_locked(struct turnstile_td *td)
+{
+    return spinlock_locked(&(td)->lock);
+}
+
 static inline void
 turnstile_td_lock(struct turnstile_td *td)
 {
diff --git a/vm/vm_kmem.c b/vm/vm_kmem.c
index 101c9815..d526804c 100644
--- a/vm/vm_kmem.c
+++ b/vm/vm_kmem.c
@@ -64,7 +64,7 @@ INIT_OP_DEFINE(vm_kmem_setup,
                INIT_OP_DEP(vm_object_setup, true),
                INIT_OP_DEP(vm_page_setup, true));
 
-static int
+__unused static int
 vm_kmem_alloc_check(size_t size)
 {
     if (!vm_page_aligned(size)
@@ -75,7 +75,7 @@ vm_kmem_alloc_check(size_t size)
     return 0;
 }
 
-static int
+__unused static int
 vm_kmem_free_check(uintptr_t va, size_t size)
 {
     if (!vm_page_aligned(va)) {
diff --git a/vm/vm_page.c b/vm/vm_page.c
index 49c2c11e..19f6f4be 100644
--- a/vm/vm_page.c
+++ b/vm/vm_page.c
@@ -790,7 +790,7 @@ vm_page_lookup(phys_addr_t pa)
     return NULL;
 }
 
-static bool
+__unused static bool
 vm_page_block_referenced(const struct vm_page *page, unsigned int order)
 {
     unsigned int i, nr_pages;
diff --git a/vm/vm_page.h b/vm/vm_page.h
index 70a0091a..e9ca321c 100644
--- a/vm/vm_page.h
+++ b/vm/vm_page.h
@@ -240,7 +240,7 @@ vm_page_referenced(const struct vm_page *page)
 static inline void
 vm_page_ref(struct vm_page *page)
 {
-    unsigned int nr_refs;
+    __unused unsigned int nr_refs;
 
     nr_refs = atomic_fetch_add(&page->nr_refs, 1, ATOMIC_RELAXED);
     assert(nr_refs != (unsigned int)-1);