diff --git a/ext/json/ext/parser/extconf.rb b/ext/json/ext/parser/extconf.rb
index a9d740c7..bd537c9c 100644
--- a/ext/json/ext/parser/extconf.rb
+++ b/ext/json/ext/parser/extconf.rb
@@ -8,6 +8,31 @@
 have_func("rb_hash_bulk_insert", "ruby.h") # Missing on TruffleRuby
 have_func("ruby_xfree_sized", "ruby.h") # RUBY_VERSION >= 4.1
 
+def have_builtin_func(name, check_expr, opt = "", &b)
+  checking_for checking_message(name.funcall_style, nil, opt) do
+    if try_compile(<<SRC, opt, &b)
+int foo;
+int main() { #{check_expr}; return 0; }
+SRC
+      $defs.push(format("-DHAVE_BUILTIN_%s", name.tr_cpp))
+      true
+    else
+      false
+    end
+  end
+end
+
+have_builtin_func("__builtin_clzll", "__builtin_clzll(0)")
+
+if have_header("x86intrin.h")
+  have_func("_lzcnt_u64", "x86intrin.h")
+end
+
+if have_header("intrin.h")
+  have_func("__lzcnt64", "intrin.h")
+  have_func("_BitScanReverse64", "intrin.h")
+end
+
 if RUBY_ENGINE == "ruby"
   have_const("RUBY_TYPED_EMBEDDABLE", "ruby.h") # RUBY_VERSION >= 3.3
 end
diff --git a/ext/json/ext/vendor/fast_float_parser.h b/ext/json/ext/vendor/fast_float_parser.h
index 66e1b132..67bc6900 100644
--- a/ext/json/ext/vendor/fast_float_parser.h
+++ b/ext/json/ext/vendor/fast_float_parser.h
@@ -707,6 +707,12 @@ static inline double ffp_bits2double(uint64_t bits) {
     return d;
 }
 
+#if defined(__x86_64__) && defined(HAVE_X86INTRIN_H)
+# include <x86intrin.h>
+#elif defined(_MSC_VER) && defined(HAVE_INTRIN_H)
+# include <intrin.h>
+#endif
+
 static inline unsigned int
 ffc_nlz_int64(uint64_t x)
 {