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From: "rafal at bursig dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/99693] -O2 not move 'if' checks on const data outside the loops
Date: Mon, 22 Mar 2021 09:43:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99693

--- Comment #5 from rafal at bursig dot org ---
Additional when I use c++ variant of this code and throw exception in else =
then
in  -O2 level the 'if' is removed outside loop:

typedef struct Update {
    int m_update;
    //...
} Update;

extern void antagonizer( Update * );

void antagonize(Update *data, unsigned int n)
{
    int _update =3D data->m_update;
    while(--n)
    {

        if (_update)
        {
            antagonizer( data );
        } else throw  std::runtime_error("error");

    }
}

The asm looks like:


.LC0:
        .string "error"
antagonize(Update*, unsigned int):
        mov     eax, DWORD PTR [rdi]
        sub     esi, 1
        je      .L12
        push    r12
        push    rbp
        mov     rbp, rdi
        push    rbx
        mov     ebx, esi
        test    eax, eax
        je      .L3
.L4:
        mov     rdi, rbp
        call    antagonizer(Update*)
        sub     ebx, 1
        jne     .L4
        pop     rbx
        pop     rbp
        pop     r12
        ret
.L12:
        ret
antagonize(Update*, unsigned int) [clone .cold]:
.L3:
        mov     edi, 16
        call    __cxa_allocate_exception
        mov     esi, OFFSET FLAT:.LC0
        mov     rdi, rax
        mov     rbp, rax
        call    std::runtime_error::runtime_error(char const*) [complete ob=
ject
constructor]
        mov     edx, OFFSET FLAT:_ZNSt13runtime_errorD1Ev
        mov     esi, OFFSET FLAT:_ZTISt13runtime_error
        mov     rdi, rbp
        call    __cxa_throw
        mov     r12, rax
        mov     rdi, rbp
        call    __cxa_free_exception
        mov     rdi, r12
        call    _Unwind_Resume


when I comment the "else throw ..." then the if check return int loop=