问题描述
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。