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

[redbeard0531 at gmail dot com via Gcc-bugs]

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

--- Comment #1 from Mathias Stearn <redbeard0531 at gmail dot com> ---
Sorry, there was a typo in the initial code. I forgot the trivial
implementation of await_resume(). (D'oh!)

Now I can see that test() is just a ret instruction, but there is still a lot
of dead code emitted for the coroutine functions. While it isn't too much for
the trivial functions, for a real use case it would be. And it seems like a bug
anyway  that unused code makes it to the obj file in -O3:
https://godbolt.org/z/bTncofrzd

#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() {
    test2();
}

test1(test1()::_Z5test1v.frame*) [clone .actor]:
        movzx   eax, WORD PTR [rdi+18]
        test    al, 1
        je      .L2
        cmp     ax, 5
        ja      .L3
        mov     edx, 42
        bt      rdx, rax
        jnc     .L3
.L4:
        cmp     BYTE PTR [rdi+17], 0
        jne     .L14
.L1:
        ret
.L2:
        cmp     ax, 2
        je      .L5
        cmp     ax, 4
        je      .L4
        test    ax, ax
        jne     .L3
        mov     QWORD PTR [rdi+24], rdi
.L5:
        cmp     BYTE PTR [rdi+17], 0
        mov     BYTE PTR [rdi+32], 1
        mov     QWORD PTR [rdi], 0
        je      .L1
.L14:
        jmp     operator delete(void*)
test1(test1()::_Z5test1v.frame*) [clone .actor] [clone .cold]:
.L3:
        ud2
test2(test2()::_Z5test2v.frame*) [clone .actor]:
        movzx   eax, WORD PTR [rdi+18]
        test    al, 1
        je      .L16
        cmp     ax, 7
        ja      .L17
        mov     edx, 170
        bt      rdx, rax
        jnc     .L17
.L18:
        cmp     BYTE PTR [rdi+17], 0
        jne     .L33
.L15:
        ret
.L16:
        cmp     ax, 4
        je      .L19
        ja      .L20
        test    ax, ax
        jne     .L34
        mov     QWORD PTR [rdi+24], rdi
.L22:
        mov     BYTE PTR [rdi+32], 1
.L19:
        cmp     BYTE PTR [rdi+17], 0
        mov     QWORD PTR [rdi], 0
        je      .L15
.L33:
        jmp     operator delete(void*)
.L34:
        cmp     ax, 2
        je      .L22
        jmp     .L17
.L20:
        cmp     ax, 6
        je      .L18
        jmp     .L17
test2(test2()::_Z5test2v.frame*) [clone .actor] [clone .cold]:
.L17:
        ud2
test1(test1()::_Z5test1v.frame*) [clone .destroy]:
        movzx   eax, WORD PTR [rdi+18]
        or      eax, 1
        mov     WORD PTR [rdi+18], ax
        cmp     ax, 5
        ja      .L36
        mov     edx, 42
        bt      rdx, rax
        jnc     .L36
        cmp     BYTE PTR [rdi+17], 0
        jne     .L39
        ret
.L39:
        jmp     operator delete(void*)
test1(test1()::_Z5test1v.frame*) [clone .destroy] [clone .cold]:
.L36:
        ud2
test2(test2()::_Z5test2v.frame*) [clone .destroy]:
        movzx   eax, WORD PTR [rdi+18]
        or      eax, 1
        mov     WORD PTR [rdi+18], ax
        cmp     ax, 7
        ja      .L41
        mov     edx, 170
        bt      rdx, rax
        jnc     .L41
        cmp     BYTE PTR [rdi+17], 0
        jne     .L44
        ret
.L44:
        jmp     operator delete(void*)
test2(test2()::_Z5test2v.frame*) [clone .destroy] [clone .cold]:
.L41:
        ud2
test():
        ret