The GCC middle-end has started emitting "control reaches end of
non-void function" warnings. This are not too useful for Go, which
implements its own error for this in the frontend.  Avoid the
middle-end warnings for Go by 1) marking the builtin function panic
and the compiler-generated function __go_runtime_error as not
returning and 2) adding a default case to the switch used for select
statements that simply calls __builtin_unreachable.  This fixes
https://golang.org/issue/22767.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline.

Ian


2017-12-01  Ian Lance Taylor  <iant@golang.org>

* go-gcc.cc (Gcc_backend::Gcc_backend): Define
__builtin_unreachable.
(Gcc_backend::function): Add does_not_return parameter.