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From: "grantrwittmann at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/112870] New: incorrect jmp when using goto on a function,
 causing infinite loop
Date: Tue, 05 Dec 2023 18:41:30 +0000
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X-Bugzilla-Version: 13.2.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112870

            Bug ID: 112870
           Summary: incorrect jmp when using goto on a function, causing
                    infinite loop
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: grantrwittmann at gmail dot com
  Target Milestone: ---

When first setting a variable to a label, then jumping after that label to a
function (not calling the function), GCC generates incorrect assembly causi=
ng
the goto statement to jump to the previous label and not the function.

The code:

void function() {
    return;
}

int main() {
    void* label_var =3D &&label;
    label:
        goto *&function;
}

compiles to this assembly:

function:
        push    rbp
        mov     rbp, rsp
        nop
        pop     rbp
        ret
main:
        push    rbp
        mov     rbp, rsp
        mov     QWORD PTR [rbp-8], OFFSET FLAT:.L4
.L4:
        nop
        jmp     .L4

The last jmp goes back to .L4 (label) even though it should be going to
function. The assembly is compiled correctly when the `label_var` declarati=
on
is removed.

This same code compiles correctly on other compilers like clang (17.0.1):

function:                               # @function
        push    rbp
        mov     rbp, rsp
        pop     rbp
        ret
main:                                   # @main
        push    rbp
        mov     rbp, rsp
        mov     dword ptr [rbp - 4], 0
        lea     rax, [rip + .Ltmp2]
        mov     qword ptr [rbp - 16], rax
.Ltmp2:                                 # Block address taken
.LBB1_1:                                # =3D>This Inner Loop Header: Depth=
=3D1
        lea     rax, [rip + function]
        mov     qword ptr [rbp - 24], rax       # 8-byte Spill
        jmp     .LBB1_2
.LBB1_2:                                #   in Loop: Header=3DBB1_1 Depth=
=3D1
        mov     rax, qword ptr [rbp - 24]       # 8-byte Reload
        jmp     rax


All examples from this issue were compiled and ran through the godbolt.org
online compiler explorer.=