[Bug ipa/102528] Unused out-of-line functions emitted for trivial coroutines

["iains at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102528

--- Comment #6 from Iain Sandoe <iains at gcc dot gnu.org> ---
#include <coroutine>

struct simple {
    struct promise_type {
        void return_void() {}
        std::suspend_never initial_suspend() { return {}; }
        std::suspend_never final_suspend() noexcept { return {}; }
        void unhandled_exception() { /*throw;*/ }
        simple get_return_object() { return {}; }
    };

    std::true_type await_ready() {return {}; }
    void await_suspend(std::coroutine_handle<>) {}
    void await_resume() {}
};

inline simple
test1() {
    co_return;
}

inline simple
test2() {
    co_await test1();
    co_return;
}

void test() {
    test1();
}

is enough to reproduce at -O2
-std=c++20 -O2 -S -fno-reorder-blocks-and-partition -fno-exceptions
-fno-unwind-tables -fno-asynchronous-unwind-tables 
^^ makes the output a little less complicated.

        .text
        .p2align 4
__Z5test1PZ5test1vE15_Z5test1v.Frame.actor:
<SNIP>
        jmp     __ZdlPv
L3:
        ud2
        .p2align 4
__Z5test1PZ5test1vE15_Z5test1v.Frame.destroy:
        movzwl  32(%rdi), %eax
<SNIP>
        ret
        .p2align 4,,10
        .p2align 3
L19:
        jmp     __ZdlPv
L16:
        ud2
        .p2align 4
        .globl __Z4testv
__Z4testv:
        ret
L21:
        nop
        .ident  "GCC: (GNU) 12.0.0 20211003 (experimental)"

NOTES:
1/ All traces of test2 are correctly elided (so that it seems we need to have
had a use at some earlier point).

2/ at the start of DSE2 we still hold references to the actor and destroyer
functions (the ramp has been inlined, and the ramp function elided).

3/ DSE2 elides the coroutine frame and the references to the two functions, but
it seems that is too late in the pipeline for them to be removed?