PR target/80640
	* doc/md.texi (mem_thread_fence): Remove mention of mode.  Rewrite.
	* optabs.c (expand_mem_thread_fence): Emit a compiler barrier when
	using targetm.gen_mem_thread_fence.
testsuite/
	* gcc.dg/atomic/pr80640.c: New testcase.

From-SVN: r251377