diff --git a/include/boost/unordered/detail/foa/core.hpp b/include/boost/unordered/detail/foa/core.hpp
index fab35866..38d50e68 100644
--- a/include/boost/unordered/detail/foa/core.hpp
+++ b/include/boost/unordered/detail/foa/core.hpp
@@ -923,6 +923,8 @@ inline unsigned int unchecked_countr_zero(int x)
   unsigned long r;
   _BitScanForward(&r,(unsigned long)x);
   return (unsigned int)r;
+#elif defined(BOOST_GCC)||defined(BOOST_CLANG)
+  return (unsigned int)__builtin_ctz((unsigned int)x);
 #else
   BOOST_UNORDERED_ASSUME(x!=0);
   return (unsigned int)boost::core::countr_zero((unsigned int)x);