错误：'%' 的操作数*UND* 和 *UND* 部分无效

如何解决错误：'%' 的操作数*UND* 和 *UND* 部分无效

这个 cppreference example 在我的 Archlinux 机器上的 g++ 11.1.0 上以 Error: invalid operands (*UND* and *UND* sections) for %'` 失败，但在 coliru 上没有。为什么？

文件名是_1h6.cpp：

$ g++ _1h6.cpp -S && as _1h6.s # same error as g++ _1h6.cpp
_1h6.s: Assembler messages:
_1h6.s:11: Error: invalid operands (*UND* and *UND* sections) for `%'
_1h6.cpp:29: Error: invalid operands (*UND* and *UND* sections) for `%'
_1h6.cpp:30: Error: invalid operands (*UND* and *UND* sections) for `%'

这里是 C++ 代码：

#include <iostream>
extern "C" int func();
// the definition of func is written in assembly language
// raw string literal could be very useful
asm(R"(
.globl func
    .type func,@function
    func:
    .cfi_startproc
    movl $7, %eax
    ret
    .cfi_endproc
)");
int main()
{
    int n = func();
    // extended inline assembly
    asm ("leal (%0,%0,4),%0"
         : "=r" (n)
         : "0" (n));
    std::cout << "7*5 = " << n << std::endl; // flush is intentional
    // standard inline assembly
    asm ("movq $60, %rax\n\t" // the exit syscall number on Linux
         "movq $2, %rdi\n\t" // this program returns 2
         "syscall");
}

UND 指的是一个临时 elf 部分（未定义），用于没有目标的引用。我的 g++ 能否在内部链接与 coliru 不同的库？我使用与 coliru 相同的命令：

g++ -std=c++20 -pthread  -O2 -Wall -Wextra -pedantic -pthread -pedantic-errors _1h6.cpp -lm  -latomic -lstdc++fs  && ./a.out

当我使用 -S 创建程序集时，错误发生在 as _1h6.s。 这里是 -S 输出：

    .file   "_1h6.cpp"
    .text
    .local  _ZStL8__ioinit
    .comm   _ZStL8__ioinit,1,1
#APP
    
.globl func
    .type func, %eax
    ret
    .cfi_endproc

    .section    .rodata
.LC0:
    .string "7*5 = "
#NO_APP
    .text
    .globl  main
    .type   main,@function
main:
.LFB1729:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6,-16
    movq    %rsp,%rbp
    .cfi_def_cfa_register 6
    subq    $16,%rsp
    call    func@PLT
    movl    %eax,-4(%rbp)
    movl    -4(%rbp),%eax
#APP
# 27 "_1h6.cpp" 1
    leal (%eax,%eax,%eax
# 0 "" 2
#NO_APP
    movl    %eax,-4(%rbp)
    leaq    .LC0(%rip),%rax
    movq    %rax,%rsi
    leaq    _ZSt4cout(%rip),%rdi
    call    _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc@PLT
    movq    %rax,%rdx
    movl    -4(%rbp),%eax
    movl    %eax,%esi
    movq    %rdx,%rdi
    call    _ZNSolsEi@PLT
    movq    _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_@GOTPCREL(%rip),%rdx
    movq    %rdx,%rsi
    movq    %rax,%rdi
    call    _ZNSolsEPFRSoS_E@PLT
#APP
# 32 "_1h6.cpp" 1
    movq $60, %rax
    movq $2, %rdi
    syscall
# 0 "" 2
#NO_APP
    movl    $0,%eax
    leave
    .cfi_def_cfa 7,8
    ret
    .cfi_endproc
.LFE1729:
    .size   main,.-main
    .type   _Z41__static_initialization_and_destruction_0ii,@function
_Z41__static_initialization_and_destruction_0ii:
.LFB2232:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6,%rsp
    movl    %edi,-4(%rbp)
    movl    %esi,-8(%rbp)
    cmpl    $1,-4(%rbp)
    jne .L5
    cmpl    $65535,-8(%rbp)
    jne .L5
    leaq    _ZStL8__ioinit(%rip),%rdi
    call    _ZNSt8ios_base4InitC1Ev@PLT
    leaq    __dso_handle(%rip),%rdx
    leaq    _ZStL8__ioinit(%rip),%rsi
    movq    _ZNSt8ios_base4InitD1Ev@GOTPCREL(%rip),%rdi
    call    __cxa_atexit@PLT
.L5:
    nop
    leave
    .cfi_def_cfa 7,8
    ret
    .cfi_endproc
.LFE2232:
    .size   _Z41__static_initialization_and_destruction_0ii,.-_Z41__static_initialization_and_destruction_0ii
    .type   _GLOBAL__sub_I_main,@function
_GLOBAL__sub_I_main:
.LFB2233:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6,%rbp
    .cfi_def_cfa_register 6
    movl    $65535,%esi
    movl    $1,%edi
    call    _Z41__static_initialization_and_destruction_0ii
    popq    %rbp
    .cfi_def_cfa 7,8
    ret
    .cfi_endproc
.LFE2233:
    .size   _GLOBAL__sub_I_main,.-_GLOBAL__sub_I_main
    .section    .init_array,"aw"
    .align 8
    .quad   _GLOBAL__sub_I_main
    .hidden __dso_handle
    .ident  "GCC: (GNU) 11.1.0"
    .section    .note.GNU-stack,"",@progbits

由于错误来自 as，因此是 binutils 问题。 as --version：

GNU assembler (GNU Binutils) 2.36.1
Copyright (C) 2021 Free Software Foundation,Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or later.
This program has absolutely no warranty.
This assembler was configured for a target of `x86_64-pc-linux-gnu'.

类似的问题没有帮助：

• Error using GCC Intel Assembly: invalid operands (.text and UND sections) for +
• error: invalid operands (*UND* and *UND* sections) for `*'

解决方法

导致错误的代码是自动从HTML中提取的，不可见空格不是空格。在编辑器中看起来像空格的东西可能不是空格。此处 \xc2\xa0 位于 % 之前，这使 as 产生错误。