[Bug tree-optimization/68274] __builtin_unreachable pessimizes code

[matt at godbolt dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68274

--- Comment #2 from Matt Godbolt <matt at godbolt dot org> ---
Thanks for updating the bug. As a corollary, moving the unreachability above
the returns yields the same code as the non-unreachable: https://goo.gl/MdULOs

--
int test_with_unreach_First(Side side, const Foo &foo) {
  if (side != Ask && side != Bid) __builtin_unreachable();
  if (side == Bid) return foo.a;
  return foo.b;
}
--
test_with_unreach_First(Side, Foo const&):
        mov     eax, DWORD PTR [rsi+4]
        test    edi, edi
        cmove   eax, DWORD PTR [rsi]
        ret
--

For what it's worth I've been unable to coax either clang or icc (13.0.1
anyway) into the code I'd ideally like (the rsi+eax*4 case).