__cxa_atexit@plt is used when -fno-plt is supplied

[pinskia]

(forwarded from GCC's bugzilla: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115170 as GCC is doing the right thing, only clang is not).
Take:

struct s
{
        ~s();
};
s f;
void g();
void h()
{
        g();
}

With -fno-plt -fPIE, clang will call __cxa_atexit@plt rather than doing an indirect call to __cxa_atexit via a load from the GOT.

        .cfi_def_cfa_register %rbp
        movq    _ZN1sD1Ev@GOTPCREL(%rip), %rdi
        leaq    f(%rip), %rsi
        leaq    __dso_handle(%rip), %rdx
        callq   __cxa_atexit@PLT
        popq    %rbp

vs what is done for the call to g:

        .cfi_def_cfa_register %rbp
        callq   *_Z1gv@GOTPCREL(%rip)
        popq    %rbp
        .cfi_def_cfa %rsp, 8
        retq

GCC will do an indirect call to the __cxa_atexit@GOTPCREL:

        movq    %rax, %rdi
        call    *__cxa_atexit@GOTPCREL(%rip)
        nop
        popq    %rbp

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Andrew Pinski (pinskia)

(forwarded from GCC's bugzilla: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115170 as GCC is doing the right thing, only clang is not). Take: ``` struct s { ~s(); }; s f; void g(); void h() { g(); } ``` With `-fno-plt -fPIE`, clang will call __cxa_atexit@plt rather than doing an indirect call to __cxa_atexit via a load from the GOT. ```

    .cfi_def_cfa_register %rbp
    movq    _ZN1sD1Ev@<!-- -->GOTPCREL(%rip), %rdi
    leaq    f(%rip), %rsi
    leaq    __dso_handle(%rip), %rdx
    callq   __cxa_atexit@<!-- -->PLT
    popq    %rbp

vs what is done for the call to g:

    .cfi_def_cfa_register %rbp
    callq   *_Z1gv@<!-- -->GOTPCREL(%rip)
    popq    %rbp
    .cfi_def_cfa %rsp, 8
    retq

GCC will do an indirect call to the `__cxa_atexit@<!-- -->GOTPCREL`:

    movq    %rax, %rdi
    call    *__cxa_atexit@<!-- -->GOTPCREL(%rip)
    nop
    popq    %rbp

</details>

[MaskRay]

The issue is -O0 specific.

-fno-plt sets RtLibUseGOT. -O1 and above infer the function attribute nonlazybind from RtLibUseGOT. Instruction selectors handle nonlazybind calls.

-O0 does not infer nonlazybind. We can add a simplified InferFunctionAttrsPass for -O0.