背景

写的代码在x64上单元测试一点问题没有，交叉编译出aarch64架构程序，跑在测试那里一开就崩，查了半天竟然是没有加return引发的血案…

调试过程

函数调用funca，调用完funca后本应该回到调用函数，但gdb查core却看到奇怪的堆栈信息funcb在funca之上，且崩在funcb里，这里简单写个小demo测试了一下（当然这个demo不会崩，因为没有访问成员变量）

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#include <iostream>

using namespace std;

int funca()
{
    cout << "this is func A" << endl;
    //return 0;
}

int funcb()
{
    cout << "this is func B" << endl;
    return 0;
}

int main()
{
    funca();
    return 0;
}

编译指令为 （注意需要加上 -O2 优化才会复现现象）

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/opt/aarch64-linux/bin/aarch64-linux-gnu-g++ -O2 main.cpp -o main

按道理来讲应该只会输出 “this is func A” 然后退出程序，结果程序在输出 “this is func A” 后，开始死循环输出 “this is func B”

根据openai的回答

在C++中，如果声明了返回值类型但没有显式地使用return语句，编译器通常会根据函数的返回类型自动插入一个隐式的return语句。对于非void类型的函数，如果没有显式地提供返回值，编译器通常会插入一个默认的return语句，例如return 0;，以确保函数能够正常返回一个值
。

然而，在ARM架构上，这种行为可能会有所不同。ARM架构下的编译器和链接器可能会有不同的处理方式，特别是在处理函数返回时。例如，某些编译器可能不会自动插入return语句，而是依赖于程序员显式地使用return语句来确保函数正确返回

也就是说没有加return的funca，执行完之后下一条指令不再是返回调用函数，而是指向了代码顺序里的下一条命令

汇编

使用

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/opt/aarch64-linux/bin/aarch64-linux-gnu-g++ -O2 main.cpp -S

分别输出不加return和加return的汇编代码

  • 不加return
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	.arch armv8-a+crc
	.file	"main.cpp"
	.text
	.section	.text._ZNKSt5ctypeIcE8do_widenEc,"axG",@progbits,_ZNKSt5ctypeIcE8do_widenEc,comdat
	.align	2
	.p2align 4,,15
	.weak	_ZNKSt5ctypeIcE8do_widenEc
	.type	_ZNKSt5ctypeIcE8do_widenEc, %function
_ZNKSt5ctypeIcE8do_widenEc:
.LFB1296:
	.cfi_startproc
	mov	w0, w1
	ret
	.cfi_endproc
.LFE1296:
	.size	_ZNKSt5ctypeIcE8do_widenEc, .-_ZNKSt5ctypeIcE8do_widenEc
	.text
	.align	2
	.p2align 4,,15
	.global	_Z5funcav
	.type	_Z5funcav, %function
_Z5funcav:
.LFB1539:
	.cfi_startproc
	stp	x29, x30, [sp, -32]!
	.cfi_def_cfa_offset 32
	.cfi_offset 29, -32
	.cfi_offset 30, -24
	adrp	x1, .LC0
	add	x1, x1, :lo12:.LC0
	mov	x29, sp
	mov	x2, 14
	str	x19, [sp, 16]
	.cfi_offset 19, -16
	adrp	x19, _ZSt4cout
	add	x19, x19, :lo12:_ZSt4cout
	mov	x0, x19
	bl	_ZSt16__ostream_insertIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_PKS3_l
	mov	x0, x19
	bl	_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_
	.cfi_endproc
.LFE1539:
	.size	_Z5funcav, .-_Z5funcav
	.align	2
	.p2align 4,,15
	.global	_Z5funcbv
	.type	_Z5funcbv, %function
_Z5funcbv:
.LFB1540:
	.cfi_startproc
	stp	x29, x30, [sp, -32]!
	.cfi_def_cfa_offset 32
	.cfi_offset 29, -32
	.cfi_offset 30, -24
	adrp	x1, .LC1
	mov	x2, 14
	mov	x29, sp
	stp	x19, x20, [sp, 16]
	.cfi_offset 19, -16
	.cfi_offset 20, -8
	adrp	x19, _ZSt4cout
	add	x20, x19, :lo12:_ZSt4cout
	mov	x0, x20
	add	x1, x1, :lo12:.LC1
	bl	_ZSt16__ostream_insertIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_PKS3_l
	ldr	x0, [x19, #:lo12:_ZSt4cout]
	ldr	x0, [x0, -24]
	add	x0, x20, x0
	ldr	x19, [x0, 240]
	cbz	x19, .L11
	ldrb	w0, [x19, 56]
	cbz	w0, .L7
	ldrb	w1, [x19, 67]
.L8:
	mov	x0, x20
	bl	_ZNSo3putEc
	bl	_ZNSo5flushEv
	mov	w0, 0
	ldp	x19, x20, [sp, 16]
	ldp	x29, x30, [sp], 32
	.cfi_remember_state
	.cfi_restore 30
	.cfi_restore 29
	.cfi_restore 19
	.cfi_restore 20
	.cfi_def_cfa_offset 0
	ret
	.p2align 2
.L7:
	.cfi_restore_state
	mov	x0, x19
	bl	_ZNKSt5ctypeIcE13_M_widen_initEv
	ldr	x2, [x19]
	adrp	x0, _ZNKSt5ctypeIcE8do_widenEc
	add	x0, x0, :lo12:_ZNKSt5ctypeIcE8do_widenEc
	mov	w1, 10
	ldr	x2, [x2, 48]
	cmp	x2, x0
	beq	.L8
	mov	x0, x19
	blr	x2
	and	w1, w0, 255
	b	.L8
.L11:
	bl	_ZSt16__throw_bad_castv
	.cfi_endproc
.LFE1540:
	.size	_Z5funcbv, .-_Z5funcbv
	.section	.text.startup,"ax",@progbits
	.align	2
	.p2align 4,,15
	.global	main
	.type	main, %function
main:
.LFB1541:
	.cfi_startproc
	stp	x29, x30, [sp, -16]!
	.cfi_def_cfa_offset 16
	.cfi_offset 29, -16
	.cfi_offset 30, -8
	mov	x29, sp
	bl	_Z5funcav
	.cfi_endproc
.LFE1541:
	.size	main, .-main
	.align	2
	.p2align 4,,15
	.type	_GLOBAL__sub_I__Z5funcav, %function
_GLOBAL__sub_I__Z5funcav:
.LFB2030:
	.cfi_startproc
	stp	x29, x30, [sp, -32]!
	.cfi_def_cfa_offset 32
	.cfi_offset 29, -32
	.cfi_offset 30, -24
	mov	x29, sp
	str	x19, [sp, 16]
	.cfi_offset 19, -16
	adrp	x19, .LANCHOR0
	add	x19, x19, :lo12:.LANCHOR0
	mov	x0, x19
	bl	_ZNSt8ios_base4InitC1Ev
	mov	x1, x19
	adrp	x2, __dso_handle
	ldr	x19, [sp, 16]
	adrp	x0, _ZNSt8ios_base4InitD1Ev
	ldp	x29, x30, [sp], 32
	.cfi_restore 30
	.cfi_restore 29
	.cfi_restore 19
	.cfi_def_cfa_offset 0
	add	x2, x2, :lo12:__dso_handle
	add	x0, x0, :lo12:_ZNSt8ios_base4InitD1Ev
	b	__cxa_atexit
	.cfi_endproc
.LFE2030:
	.size	_GLOBAL__sub_I__Z5funcav, .-_GLOBAL__sub_I__Z5funcav
	.section	.init_array,"aw",%init_array
	.align	3
	.xword	_GLOBAL__sub_I__Z5funcav
	.bss
	.align	3
	.set	.LANCHOR0,. + 0
	.type	_ZStL8__ioinit, %object
	.size	_ZStL8__ioinit, 1
_ZStL8__ioinit:
	.zero	1
	.section	.rodata.str1.8,"aMS",@progbits,1
	.align	3
.LC0:
	.string	"this is func A"
	.zero	1
.LC1:
	.string	"this is func B"
	.hidden	__dso_handle
	.ident	"GCC: (Kedacom VC 2019-12-09) 8.3.0"
	.section	.note.GNU-stack,"",@progbits
  • 加return
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	.arch armv8-a+crc
	.file	"main.cpp"
	.text
	.section	.text._ZNKSt5ctypeIcE8do_widenEc,"axG",@progbits,_ZNKSt5ctypeIcE8do_widenEc,comdat
	.align	2
	.p2align 4,,15
	.weak	_ZNKSt5ctypeIcE8do_widenEc
	.type	_ZNKSt5ctypeIcE8do_widenEc, %function
_ZNKSt5ctypeIcE8do_widenEc:
.LFB1296:
	.cfi_startproc
	mov	w0, w1
	ret
	.cfi_endproc
.LFE1296:
	.size	_ZNKSt5ctypeIcE8do_widenEc, .-_ZNKSt5ctypeIcE8do_widenEc
	.text
	.align	2
	.p2align 4,,15
	.global	_Z5funcav
	.type	_Z5funcav, %function
_Z5funcav:
.LFB1539:
	.cfi_startproc
	stp	x29, x30, [sp, -32]!
	.cfi_def_cfa_offset 32
	.cfi_offset 29, -32
	.cfi_offset 30, -24
	adrp	x1, .LC0
	mov	x2, 14
	mov	x29, sp
	stp	x19, x20, [sp, 16]
	.cfi_offset 19, -16
	.cfi_offset 20, -8
	adrp	x19, _ZSt4cout
	add	x20, x19, :lo12:_ZSt4cout
	mov	x0, x20
	add	x1, x1, :lo12:.LC0
	bl	_ZSt16__ostream_insertIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_PKS3_l
	ldr	x0, [x19, #:lo12:_ZSt4cout]
	ldr	x0, [x0, -24]
	add	x0, x20, x0
	ldr	x19, [x0, 240]
	cbz	x19, .L9
	ldrb	w0, [x19, 56]
	cbz	w0, .L5
	ldrb	w1, [x19, 67]
.L6:
	mov	x0, x20
	bl	_ZNSo3putEc
	bl	_ZNSo5flushEv
	mov	w0, 0
	ldp	x19, x20, [sp, 16]
	ldp	x29, x30, [sp], 32
	.cfi_remember_state
	.cfi_restore 30
	.cfi_restore 29
	.cfi_restore 19
	.cfi_restore 20
	.cfi_def_cfa_offset 0
	ret
	.p2align 2
.L5:
	.cfi_restore_state
	mov	x0, x19
	bl	_ZNKSt5ctypeIcE13_M_widen_initEv
	ldr	x2, [x19]
	adrp	x0, _ZNKSt5ctypeIcE8do_widenEc
	add	x0, x0, :lo12:_ZNKSt5ctypeIcE8do_widenEc
	mov	w1, 10
	ldr	x2, [x2, 48]
	cmp	x2, x0
	beq	.L6
	mov	x0, x19
	blr	x2
	and	w1, w0, 255
	b	.L6
.L9:
	bl	_ZSt16__throw_bad_castv
	.cfi_endproc
.LFE1539:
	.size	_Z5funcav, .-_Z5funcav
	.align	2
	.p2align 4,,15
	.global	_Z5funcbv
	.type	_Z5funcbv, %function
_Z5funcbv:
.LFB1540:
	.cfi_startproc
	stp	x29, x30, [sp, -32]!
	.cfi_def_cfa_offset 32
	.cfi_offset 29, -32
	.cfi_offset 30, -24
	adrp	x1, .LC1
	mov	x2, 14
	mov	x29, sp
	stp	x19, x20, [sp, 16]
	.cfi_offset 19, -16
	.cfi_offset 20, -8
	adrp	x19, _ZSt4cout
	add	x20, x19, :lo12:_ZSt4cout
	mov	x0, x20
	add	x1, x1, :lo12:.LC1
	bl	_ZSt16__ostream_insertIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_PKS3_l
	ldr	x0, [x19, #:lo12:_ZSt4cout]
	ldr	x0, [x0, -24]
	add	x0, x20, x0
	ldr	x19, [x0, 240]
	cbz	x19, .L16
	ldrb	w0, [x19, 56]
	cbz	w0, .L12
	ldrb	w1, [x19, 67]
.L13:
	mov	x0, x20
	bl	_ZNSo3putEc
	bl	_ZNSo5flushEv
	mov	w0, 0
	ldp	x19, x20, [sp, 16]
	ldp	x29, x30, [sp], 32
	.cfi_remember_state
	.cfi_restore 30
	.cfi_restore 29
	.cfi_restore 19
	.cfi_restore 20
	.cfi_def_cfa_offset 0
	ret
	.p2align 2
.L12:
	.cfi_restore_state
	mov	x0, x19
	bl	_ZNKSt5ctypeIcE13_M_widen_initEv
	ldr	x2, [x19]
	adrp	x0, _ZNKSt5ctypeIcE8do_widenEc
	add	x0, x0, :lo12:_ZNKSt5ctypeIcE8do_widenEc
	mov	w1, 10
	ldr	x2, [x2, 48]
	cmp	x2, x0
	beq	.L13
	mov	x0, x19
	blr	x2
	and	w1, w0, 255
	b	.L13
.L16:
	bl	_ZSt16__throw_bad_castv
	.cfi_endproc
.LFE1540:
	.size	_Z5funcbv, .-_Z5funcbv
	.section	.text.startup,"ax",@progbits
	.align	2
	.p2align 4,,15
	.global	main
	.type	main, %function
main:
.LFB1541:
	.cfi_startproc
	stp	x29, x30, [sp, -16]!
	.cfi_def_cfa_offset 16
	.cfi_offset 29, -16
	.cfi_offset 30, -8
	mov	x29, sp
	bl	_Z5funcav
	mov	w0, 0
	ldp	x29, x30, [sp], 16
	.cfi_restore 30
	.cfi_restore 29
	.cfi_def_cfa_offset 0
	ret
	.cfi_endproc
.LFE1541:
	.size	main, .-main
	.align	2
	.p2align 4,,15
	.type	_GLOBAL__sub_I__Z5funcav, %function
_GLOBAL__sub_I__Z5funcav:
.LFB2030:
	.cfi_startproc
	stp	x29, x30, [sp, -32]!
	.cfi_def_cfa_offset 32
	.cfi_offset 29, -32
	.cfi_offset 30, -24
	mov	x29, sp
	str	x19, [sp, 16]
	.cfi_offset 19, -16
	adrp	x19, .LANCHOR0
	add	x19, x19, :lo12:.LANCHOR0
	mov	x0, x19
	bl	_ZNSt8ios_base4InitC1Ev
	mov	x1, x19
	adrp	x2, __dso_handle
	ldr	x19, [sp, 16]
	adrp	x0, _ZNSt8ios_base4InitD1Ev
	ldp	x29, x30, [sp], 32
	.cfi_restore 30
	.cfi_restore 29
	.cfi_restore 19
	.cfi_def_cfa_offset 0
	add	x2, x2, :lo12:__dso_handle
	add	x0, x0, :lo12:_ZNSt8ios_base4InitD1Ev
	b	__cxa_atexit
	.cfi_endproc
.LFE2030:
	.size	_GLOBAL__sub_I__Z5funcav, .-_GLOBAL__sub_I__Z5funcav
	.section	.init_array,"aw",%init_array
	.align	3
	.xword	_GLOBAL__sub_I__Z5funcav
	.bss
	.align	3
	.set	.LANCHOR0,. + 0
	.type	_ZStL8__ioinit, %object
	.size	_ZStL8__ioinit, 1
_ZStL8__ioinit:
	.zero	1
	.section	.rodata.str1.8,"aMS",@progbits,1
	.align	3
.LC0:
	.string	"this is func A"
	.zero	1
.LC1:
	.string	"this is func B"
	.hidden	__dso_handle
	.ident	"GCC: (Kedacom VC 2019-12-09) 8.3.0"
	.section	.note.GNU-stack,"",@progbits

确实是不一致的，当然目前我还看不懂aarch64的汇编，挖个坑，有空要学一下

总结

不要过度信任所有编译器都会处理好不加return的情况，还是要规范写代码呀！定义了返回类型就要加return。