++i 是原子操作吗？

#include <iostream>
#include <atomic>
#include <thread>

using namespace std;

static atomic<int> atomicvalue(0);
static int value = 0;

constexpr int count = 10000;

void increase() {
    for (auto i = 0; i < count; i ++) {
        ++atomicvalue;
        ++value;
    }
}

void decrease() {
    for (auto i = 0; i < count; i ++) {
        --atomicvalue;
        --value;
    }
}

int main() {
    thread incr(increase);
    thread decr(decrease);

    incr.join();
    decr.join();

    // when thread all done
    // atomic value will be zero
    // non-atomic value may not be zero
    cout << "atomic value:" << atomicvalue << endl;
    cout << "non-atomic value:" << value << endl;

    return 0;
}

atomic value:0
non-atomic value:269

atomic value:0
non-atomic value:-2027

int main() {
    int i = 1;
    ++i;
    return 0;
}

	.file	"mm.cpp"
	.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
	movl	$1, -4(%rbp)
	addl	$1, -4(%rbp)
	movl	$0, %eax
	popq	%rbp
	.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

movl	$1, -4(%rbp)
addl	$1, -4(%rbp)
movl	$0, %eax