https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108552
--- Comment #33 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
It is the default unless -pthread is specified:
%{fprofile-arcs|fprofile-generate*|coverage:\
%{!fprofile-update=single:\
%{pthread:-fprofile-update=prefer-atomic}}}
So, when one uses -fprofile-arcs, -fprofile-generate* or -converage together
with -pthread and doesn't use -fprofile-uupdate=single, then
-fprofile-update=prefer-atomic is added (which is -fprofile-update=atomic if
the architecture supports it).
