diff --git a/test/unittest/unittest.h b/test/unittest/unittest.h
index 800fbab..87bb083 100644
--- a/test/unittest/unittest.h
+++ b/test/unittest/unittest.h
@@ -40,6 +40,14 @@
 #pragma GCC diagnostic ignored "-Weffc++"
 #endif
 
+// __SSE2__ and __SSE4_2__ are recognized by gcc, clang, and the Intel compiler.
+// We use -march=native with gmake to enable -msse2 and -msse4.2, if supported.
+#if defined(__SSE4_2__)
+#  define RAPIDJSON_SSE42
+#elif defined(__SSE2__)
+#  define RAPIDJSON_SSE2
+#endif
+
 #include "gtest/gtest.h"
 #include <stdexcept>