会溢出吗?

我们写个程序测试一下。

test.c

#include <inttypes.h>
#include <stdio.h>

int main() {
  int64_t a = INT64_MIN;
  printf("a = %" PRId64 "\n", a);
  return 0;
}

先编译到 64 位平台以便对照:

gcc -S test.c

结果

	.file	"test.c"
	.text
	.section	.rodata
.LC0:
	.string	"a = %ld\n"
	.text
	.globl	main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register 6
	subq	$16, %rsp
	movabsq	$-9223372036854775808, %rax
	movq	%rax, -8(%rbp)
	movq	-8(%rbp), %rax
	movq	%rax, %rsi
	leaq	.LC0(%rip), %rdi
	movl	$0, %eax
	call	printf@PLT
	movl	$0, %eax
	leave
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.ident	"GCC: (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0"
	.section	.note.GNU-stack,"",@progbits

并且运行它能够得到输出:

a = -9223372036854775808

这是 int64 的最小值,没问题。

下面编译到 32 位平台。首先安装依赖:

sudo apt-get install gcc-multilib
sudo apt install lib32gcc-7-dev

编译:

gcc -S -m32 test.c

结果:

编译也没有报错。但是汇编变成:

	.file	"test.c"
	.text
	.section	.rodata
.LC0:
	.string	"a = %lld\n"
	.text
	.globl	main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	leal	4(%esp), %ecx
	.cfi_def_cfa 1, 0
	andl	$-16, %esp
	pushl	-4(%ecx)
	pushl	%ebp
	.cfi_escape 0x10,0x5,0x2,0x75,0
	movl	%esp, %ebp
	pushl	%ebx
	pushl	%ecx
	.cfi_escape 0xf,0x3,0x75,0x78,0x6
	.cfi_escape 0x10,0x3,0x2,0x75,0x7c
	subl	$16, %esp
	call	__x86.get_pc_thunk.ax
	addl	$_GLOBAL_OFFSET_TABLE_, %eax
	movl	$0, -16(%ebp)
	movl	$-2147483648, -12(%ebp)
	subl	$4, %esp
	pushl	-12(%ebp)
	pushl	-16(%ebp)
	leal	.LC0@GOTOFF(%eax), %edx
	pushl	%edx
	movl	%eax, %ebx
	call	printf@PLT
	addl	$16, %esp
	movl	$0, %eax
	leal	-8(%ebp), %esp
	popl	%ecx
	.cfi_restore 1
	.cfi_def_cfa 1, 0
	popl	%ebx
	.cfi_restore 3
	popl	%ebp
	.cfi_restore 5
	leal	-4(%ecx), %esp
	.cfi_def_cfa 4, 4
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.section	.text.__x86.get_pc_thunk.ax,"axG",@progbits,__x86.get_pc_thunk.ax,comdat
	.globl	__x86.get_pc_thunk.ax
	.hidden	__x86.get_pc_thunk.ax
	.type	__x86.get_pc_thunk.ax, @function
__x86.get_pc_thunk.ax:
.LFB1:
	.cfi_startproc
	movl	(%esp), %eax
	ret
	.cfi_endproc
.LFE1:
	.ident	"GCC: (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0"
	.section	.note.GNU-stack,"",@progbits

未完待续……