1 files changed, 6 insertions, 2 deletions
diff --git a/crypto/openssh/libcrux_mlkem768_sha3.h b/crypto/openssh/libcrux_mlkem768_sha3.h
index b8ac1436f218..885e82bafa06 100644
--- a/crypto/openssh/libcrux_mlkem768_sha3.h
+++ b/crypto/openssh/libcrux_mlkem768_sha3.h
@@ -177,10 +177,14 @@ static inline uint32_t core_num__u32_8__from_le_bytes(uint8_t buf[4]) {
 }
 
 static inline uint32_t core_num__u8_6__count_ones(uint8_t x0) {
-#ifdef _MSC_VER
+#if defined(_MSC_VER)
   return __popcnt(x0);
-#else
+#elif !defined(MISSING_BUILTIN_POPCOUNT)
   return __builtin_popcount(x0);
+#else
+  const uint8_t v[16] = { 0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4 };
+  return v[x0 & 0xf] + v[(x0 >> 4) & 0xf];
+
 #endif
 }