diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 25014e66ed..a67304fef2 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1210,15 +1210,15 @@ psa_status_t psa_wipe_key_slot(psa_key_slot_t *slot)
         case PSA_SLOT_PENDING_DELETION:
             /* In this state psa_wipe_key_slot() must only be called if the
              * caller is the last reader. */
-            if (slot->registered_readers != 1) {
-                MBEDTLS_TEST_HOOK_TEST_ASSERT(slot->registered_readers == 1);
+            if (slot->var.occupied.registered_readers != 1) {
+                MBEDTLS_TEST_HOOK_TEST_ASSERT(slot->var.occupied.registered_readers == 1);
                 status = PSA_ERROR_CORRUPTION_DETECTED;
             }
             break;
         case PSA_SLOT_FILLING:
             /* In this state registered_readers must be 0. */
-            if (slot->registered_readers != 0) {
-                MBEDTLS_TEST_HOOK_TEST_ASSERT(slot->registered_readers == 0);
+            if (slot->var.occupied.registered_readers != 0) {
+                MBEDTLS_TEST_HOOK_TEST_ASSERT(slot->var.occupied.registered_readers == 0);
                 status = PSA_ERROR_CORRUPTION_DETECTED;
             }
             break;
diff --git a/library/psa_crypto_core.h b/library/psa_crypto_core.h
index e7ff151a8b..a893c149ab 100644
--- a/library/psa_crypto_core.h
+++ b/library/psa_crypto_core.h
@@ -59,6 +59,8 @@ typedef enum {
  * and metadata for one key.
  */
 typedef struct {
+    /* This field is accessed in a lot of places. Putting it first
+     * reduces the code size. */
     psa_key_attributes_t attr;
 
     /*
@@ -81,32 +83,38 @@ typedef struct {
      * PSA_SLOT_FULL. */
     psa_key_slot_state_t state;
 
-    /*
-     * Number of functions registered as reading the material in the key slot.
-     *
-     * Library functions must not write directly to registered_readers
-     *
-     * A function must call psa_register_read(slot) before reading the current
-     * contents of the slot for an operation.
-     * They then must call psa_unregister_read(slot) once they have finished
-     * reading the current contents of the slot. If the key slot mutex is not
-     * held (when mutexes are enabled), this call must be done via a call to
-     * psa_unregister_read_under_mutex(slot).
-     * A function must call psa_key_slot_has_readers(slot) to check if
-     * the slot is in use for reading.
-     *
-     * This counter is used to prevent resetting the key slot while the library
-     * may access it. For example, such control is needed in the following
-     * scenarios:
-     * . In case of key slot starvation, all key slots contain the description
-     *   of a key, and the library asks for the description of a persistent
-     *   key not present in the key slots, the key slots currently accessed by
-     *   the library cannot be reclaimed to free a key slot to load the
-     *   persistent key.
-     * . In case of a multi-threaded application where one thread asks to close
-     *   or purge or destroy a key while it is in use by the library through
-     *   another thread. */
-    size_t registered_readers;
+    union {
+        struct {
+            /*
+             * Number of functions registered as reading the material in the key slot.
+             *
+             * Library functions must not write directly to registered_readers
+             *
+             * A function must call psa_register_read(slot) before reading
+             * the current contents of the slot for an operation.
+             * They then must call psa_unregister_read(slot) once they have
+             * finished reading the current contents of the slot. If the key
+             * slot mutex is not held (when mutexes are enabled), this call
+             * must be done via a call to
+             * psa_unregister_read_under_mutex(slot).
+             * A function must call psa_key_slot_has_readers(slot) to check if
+             * the slot is in use for reading.
+             *
+             * This counter is used to prevent resetting the key slot while
+             * the library may access it. For example, such control is needed
+             * in the following scenarios:
+             * . In case of key slot starvation, all key slots contain the
+             *   description of a key, and the library asks for the
+             *   description of a persistent key not present in the
+             *   key slots, the key slots currently accessed by the
+             *   library cannot be reclaimed to free a key slot to load
+             *   the persistent key.
+             * . In case of a multi-threaded application where one thread
+             *   asks to close or purge or destroy a key while it is in use
+             *   by the library through another thread. */
+            size_t registered_readers;
+        } occupied;
+    } var;
 
     /* Dynamically allocated key data buffer.
      * Format as specified in psa_export_key(). */
@@ -169,7 +177,7 @@ typedef struct {
  */
 static inline int psa_key_slot_has_readers(const psa_key_slot_t *slot)
 {
-    return slot->registered_readers > 0;
+    return slot->var.occupied.registered_readers > 0;
 }
 
 #if defined(MBEDTLS_PSA_CRYPTO_SE_C)
diff --git a/library/psa_crypto_slot_management.c b/library/psa_crypto_slot_management.c
index 7e0a1c12c1..327be0a4f3 100644
--- a/library/psa_crypto_slot_management.c
+++ b/library/psa_crypto_slot_management.c
@@ -268,7 +268,7 @@ void psa_wipe_all_key_slots(void)
     for (size_t slice_idx = 0; slice_idx < KEY_SLICE_COUNT; slice_idx++) {
         for (size_t slot_idx = 0; slot_idx < key_slice_length(slice_idx); slot_idx++) {
             psa_key_slot_t *slot = get_key_slot(slice_idx, slot_idx);
-            slot->registered_readers = 1;
+            slot->var.occupied.registered_readers = 1;
             slot->state = PSA_SLOT_PENDING_DELETION;
             (void) psa_wipe_key_slot(slot);
         }
@@ -568,12 +568,12 @@ psa_status_t psa_unregister_read(psa_key_slot_t *slot)
     /* If we are the last reader and the slot is marked for deletion,
      * we must wipe the slot here. */
     if ((slot->state == PSA_SLOT_PENDING_DELETION) &&
-        (slot->registered_readers == 1)) {
+        (slot->var.occupied.registered_readers == 1)) {
         return psa_wipe_key_slot(slot);
     }
 
     if (psa_key_slot_has_readers(slot)) {
-        slot->registered_readers--;
+        slot->var.occupied.registered_readers--;
         return PSA_SUCCESS;
     }
 
@@ -707,7 +707,7 @@ psa_status_t psa_close_key(psa_key_handle_t handle)
         return status;
     }
 
-    if (slot->registered_readers == 1) {
+    if (slot->var.occupied.registered_readers == 1) {
         status = psa_wipe_key_slot(slot);
     } else {
         status = psa_unregister_read(slot);
@@ -742,7 +742,7 @@ psa_status_t psa_purge_key(mbedtls_svc_key_id_t key)
     }
 
     if ((!PSA_KEY_LIFETIME_IS_VOLATILE(slot->attr.lifetime)) &&
-        (slot->registered_readers == 1)) {
+        (slot->var.occupied.registered_readers == 1)) {
         status = psa_wipe_key_slot(slot);
     } else {
         status = psa_unregister_read(slot);
diff --git a/library/psa_crypto_slot_management.h b/library/psa_crypto_slot_management.h
index 88b7c837cc..cccf8f2f19 100644
--- a/library/psa_crypto_slot_management.h
+++ b/library/psa_crypto_slot_management.h
@@ -174,10 +174,10 @@ static inline psa_status_t psa_key_slot_state_transition(
 static inline psa_status_t psa_register_read(psa_key_slot_t *slot)
 {
     if ((slot->state != PSA_SLOT_FULL) ||
-        (slot->registered_readers >= SIZE_MAX)) {
+        (slot->var.occupied.registered_readers >= SIZE_MAX)) {
         return PSA_ERROR_CORRUPTION_DETECTED;
     }
-    slot->registered_readers++;
+    slot->var.occupied.registered_readers++;
 
     return PSA_SUCCESS;
 }