diff --git a/h_malloc.c b/h_malloc.c
index 6221d0b..4f6ed19 100644
--- a/h_malloc.c
+++ b/h_malloc.c
@@ -20,6 +20,13 @@
 #include "random.h"
 #include "util.h"
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wundef"
+#if __x86_64__
+#include "immintrin.h"
+#endif
+#pragma GCC diagnostic pop
+
 #ifdef USE_PKEY
 #include <sys/mman.h>
 #endif
@@ -405,8 +412,14 @@ static size_t get_free_slot(struct random_state *rng, size_t slots, const struct
         // randomize start location for linear search (uniform random choice is too slow)
         size_t random_index = get_random_u16_uniform(rng, slots);
         size_t first_bitmap = random_index / U64_WIDTH;
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wundef"
+#if __x86_64__ && ((__GNU__ && __BMI2__ ) || (__clang__ && __BMI2INTRIN_H_))
+#pragma GCC diagnostic pop
+        u64 random_split = ~(~0UL << _pext_u64(random_index, 8));
+#else
         u64 random_split = ~(~0UL << (random_index - first_bitmap * U64_WIDTH));
-
+#endif
         size_t i = first_bitmap;
         u64 masked = metadata->bitmap[i];
         masked |= random_split;