diff --git a/h_malloc.c b/h_malloc.c
index 865c73d..1e303d9 100644
--- a/h_malloc.c
+++ b/h_malloc.c
@@ -374,7 +374,7 @@ static size_t get_free_slot(struct random_state *rng, size_t slots, const struct
             }
 
             if (masked != ~0UL) {
-                return ffzl(masked) - 1 + i * U64_WIDTH;
+                return ffz64(masked) - 1 + i * U64_WIDTH;
             }
 
             i = i == (slots - 1) / U64_WIDTH ? 0 : i + 1;
@@ -388,7 +388,7 @@ static size_t get_free_slot(struct random_state *rng, size_t slots, const struct
             }
 
             if (masked != ~0UL) {
-                return ffzl(masked) - 1 + i * U64_WIDTH;
+                return ffz64(masked) - 1 + i * U64_WIDTH;
             }
         }
     }
@@ -703,7 +703,7 @@ static inline void deallocate_small(void *p, const size_t *expected_size) {
 
     set_quarantine_slot(metadata, slot);
 
-    size_t quarantine_shift = __builtin_clzl(size) - (63 - MAX_SLAB_SIZE_CLASS_SHIFT);
+    size_t quarantine_shift = clz64(size) - (63 - MAX_SLAB_SIZE_CLASS_SHIFT);
 
 #if SLAB_QUARANTINE_RANDOM_LENGTH > 0
     size_t slab_quarantine_random_length = SLAB_QUARANTINE_RANDOM_LENGTH << quarantine_shift;
@@ -1200,7 +1200,7 @@ static size_t get_large_size_class(size_t size) {
         // 1 MiB [5 MiB, 6 MiB, 7 MiB, 8 MiB]
         // etc.
         size = max(size, (size_t)PAGE_SIZE);
-        size_t spacing_shift = U64_WIDTH - __builtin_clzl(size - 1) - 3;
+        size_t spacing_shift = U64_WIDTH - clz64(size - 1) - 3;
         size_t spacing_class = 1ULL << spacing_shift;
         return align(size, spacing_class);
     }
@@ -1766,7 +1766,7 @@ EXPORT int h_malloc_trim(UNUSED size_t pad) {
 
 #if SLAB_QUARANTINE && CONFIG_EXTENDED_SIZE_CLASSES
             if (size >= min_extended_size_class) {
-                size_t quarantine_shift = __builtin_clzl(size) - (63 - MAX_SLAB_SIZE_CLASS_SHIFT);
+                size_t quarantine_shift = clz64(size) - (63 - MAX_SLAB_SIZE_CLASS_SHIFT);
 
 #if SLAB_QUARANTINE_RANDOM_LENGTH > 0
                 size_t slab_quarantine_random_length = SLAB_QUARANTINE_RANDOM_LENGTH << quarantine_shift;
diff --git a/util.h b/util.h
index f0f2058..7ec6bf3 100644
--- a/util.h
+++ b/util.h
@@ -29,8 +29,18 @@
 #define STRINGIFY(s) #s
 #define ALIAS(f) __attribute__((alias(STRINGIFY(f))))
 
-static inline int ffzl(unsigned long x) {
-    return __builtin_ffsl(~x);
+typedef uint8_t u8;
+typedef uint16_t u16;
+typedef uint32_t u32;
+typedef uint64_t u64;
+typedef unsigned __int128 u128;
+
+static inline int clz64(u64 x) {
+    return __builtin_clzll(x);
+}
+
+static inline int ffz64(u64 x) {
+    return __builtin_ffsll(~x);
 }
 
 static inline size_t align(size_t size, size_t align) {
@@ -40,12 +50,6 @@ static inline size_t align(size_t size, size_t align) {
 
 COLD noreturn void fatal_error(const char *s);
 
-typedef uint8_t u8;
-typedef uint16_t u16;
-typedef uint32_t u32;
-typedef uint64_t u64;
-typedef unsigned __int128 u128;
-
 #define U64_WIDTH 64
 
 #if CONFIG_SEAL_METADATA