编译器未定义行为与程序正确性

#include <stdio.h>
#include <stdint.h>

void checkAdd(int a)
{
    if (a + 100 > a)
    {
        printf("%d > %d\n", a + 100, a);
    }
    else
    {
        printf("%d > %d\n", a + 100, a);
    }
}

int main(int argc, char *argv[])
{
    checkAdd(100);
    checkAdd(INT32_MAX);
    return 0;
}

[yanyg@x1{192.168.0.106} ~/test ]
$ gcc -Wall overflow.c -ubsan -fsanitize=undefined
[yanyg@x1{192.168.0.106} ~/test ]
$ ./a.out
200 > 100
overflow.c:6:17: runtime error: signed integer overflow: 2147483647 + 99 cannot be represented in type 'int'
overflow.c:12:9: runtime error: signed integer overflow: 2147483647 + 100 cannot be represented in type 'int'
-2147483549 > 2147483647

#include <stdio.h>
#include <string.h>

typedef struct Bar
{
    char name[16];
    int mode;
    int zero;
} Bar;

#define MIN(a, b)       ((a) < (b) ? (a) : (b))

int main(int argc, char *argv[])
{
    Bar bar;
    memset(&bar, 0xff, sizeof(bar));
    bar.zero = 0;
    size_t len = strlen(bar.name);
    size_t min = MIN(16, len);
    printf("len=%zu, min=%zu\n", len, min);

    bar.name[14] = '\0';
    len = strlen(bar.name);
    min = MIN(16, len);
    printf("len=%zu, min=%zu\n", len, min);
    return 0;
}

[yanyg@x1{192.168.0.106} ~/test ]
$ gcc -Wall array-overflow.c -O2
[yanyg@x1{192.168.0.106} ~/test ]
$ ./a.out
len=20, min=20
len=14, min=14

// -O0
movl    $16, %eax
cmpq    $16, -8(%rbp)
cmovbe  -8(%rbp), %rax
movq    %rax, -16(%rbp)

// -O2
sbbq    $3, %rsi
xorl    %eax, %eax
subq    %rbx, %rsi
movq    %rsi, %rdx // min = MIN(16, len);