[Bug target/79671] [7 Regression] mapnik miscompilation on armv7hl since r235622

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671

--- Comment #57 from Jonathan Wakely <redi at gcc dot gnu.org> ---
OK, so this appears to work (assuming may_alias on function_buffer):

  function_buffer& aliasing_ref = f.functor;
  this->functor = aliasing_ref;

It changes the GIMPLE from:

  f1_2(D)->D.2297.functor = MEM[(const struct function
*)f2_3(D)].D.2297.functor;

to:

  f1_2(D)->D.2297.functor = MEM[(const union function_buffer &
{ref-all})f2_3(D)];

Can I rely on that continuing to work? Is it likely to perform better than just
doing memcpy?

  __builtin_memcpy(&this->functor, &f.functor, sizeof(f.functor));