diff --git a/MagneticField/MagFieldElements/CMakeLists.txt b/MagneticField/MagFieldElements/CMakeLists.txt
index 9095bd3a90a39879278cf19111c78ecfa4eb77a3..93d930c8287e6c82614afeefcee668c3baac996c 100644
--- a/MagneticField/MagFieldElements/CMakeLists.txt
+++ b/MagneticField/MagFieldElements/CMakeLists.txt
@@ -28,6 +28,5 @@ if ( "${CMAKE_BUILD_TYPE}" STREQUAL "Debug" )
   set_source_files_properties(
      ${CMAKE_CURRENT_SOURCE_DIR}/src/AtlasFieldCache.cxx
      PROPERTIES
-     COMPILE_FLAGS "${CMAKE_CXX_FLAGS_RELWITHDEBINFO}"
-     COMPILE_DEFINITIONS "FLATTEN" )
+     COMPILE_FLAGS "${CMAKE_CXX_FLAGS_RELWITHDEBINFO}")
 endif()
diff --git a/MagneticField/MagFieldElements/src/AtlasFieldCache.cxx b/MagneticField/MagFieldElements/src/AtlasFieldCache.cxx
index 0e8179dfcc06168de354a650181c4b9819aa9a64..8e722a5f058cd8041a9ac7e634479450c8cb691b 100644
--- a/MagneticField/MagFieldElements/src/AtlasFieldCache.cxx
+++ b/MagneticField/MagFieldElements/src/AtlasFieldCache.cxx
@@ -26,13 +26,13 @@ namespace {
 constexpr double defaultB = 0.1 * Gaudi::Units::gauss;
 }
 
-#if defined(FLATTEN) && defined(__GNUC__)
+#if defined(__GNUC__)
 // We compile this package with optimization, even in debug builds; otherwise,
 // the heavy use of Eigen makes it too slow.  However, from here we may call
 // to out-of-line Eigen code that is linked from other DSOs; in that case,
 // it would not be optimized.  Avoid this by forcing all Eigen code
 // to be inlined here if possible.
-__attribute__((flatten))
+[[gnu::flatten]]
 #endif
 void
 MagField::AtlasFieldCache::getField(const double* ATH_RESTRICT xyz,