It seems gcc-4.4 uses external calls. --- source/common/putilimp.h (revision 40154) +++ source/common/putilimp.h (working copy) @@ -191,7 +191,7 @@ /* Use the predefined value. */ #elif U_PLATFORM == U_PF_MINGW #define U_HAVE_GCC_ATOMICS 0 -#elif U_GCC_MAJOR_MINOR >= 404 || defined(__clang__) +#elif U_GCC_MAJOR_MINOR >= 405 || defined(__clang__) /* TODO: Intel icc and IBM xlc on AIX also support gcc atomics. (Intel originated them.) * Add them for these compilers. * Note: Clang sets __GNUC__ defines for version 4.2, so misses the 4.4 test here.