https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66509
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> --- This is due to HAVE_AS_IX86_FILDS not being defined. So basically they made the clang assembly not backwards compatible with the GNU one. You can add a check to configure.ac if you want which does the same the check for the gnu assembly: gcc_GAS_CHECK_FEATURE([filds and fists mnemonics], gcc_cv_as_ix86_filds,,, [filds mem; fists mem],, [AC_DEFINE(HAVE_AS_IX86_FILDS, 1, [Define if your assembler uses filds and fists mnemonics.])]) Actually on second thought, this should be happening. Can you attach config.log form gcc directory.