Vì sao UB nguy hiểm hơn chương trình sập

#include <limits.h>

int kiem_tra_tran(int x) {
    if (x + 1 < x) return 1;
    return 0;
}

int kiem_tra_dung(int x) {
    if (x > INT_MAX - 1) return 1;
    return 0;
}

int kiem_tra_builtin(int x) {
    int r;
    if (__builtin_add_overflow(x, 1, &r)) return 1;
    return 0;
}

#include <stddef.h>

struct S { int truong; };

void ghi_log(int x);

int f(struct S *p) {
    int x = p->truong;          /* giai tham chieu TRUOC */

    if (p == NULL) return -1;   /* kiem tra SAU */

    ghi_log(x);
    return x;
}

int g(struct S *p) {
    if (p == NULL) return -1;   /* kiem tra TRUOC */

    int x = p->truong;

    ghi_log(x);
    return x;
}

__attribute__((noinline)) int thieu_return(int x) {
    if (x > 0) return 1;
}                                   /* khong co return cho x <= 0 */

__attribute__((noinline)) int *tra_ve_cuc_bo(void) {
    int x = 42;
    return &x;                      /* tra ve dia chi bien cuc bo */
}

/* Neu n == INT_MAX thi i tran o vong cuoi, tuc UB. */

for (int i = 0; i <= n; ++i) { ... }

/* Trinh bien dich duoc phep gia dinh khong tran, nen no biet
   i < n + 1 luon dung, nen dieu kien dung khong bao gio thoa.
   Ket qua co the la mot vong lap vo tan.

   Va do la CAI GIA cua viec dinh nghia tran so co dau: neu chuan
   bat buoc quan vong, trinh bien dich phai cat i ve 32 bit moi vong
   thay vi de no trong thanh ghi 64 bit. Voi vong lap dai thi do la
   mot lenh thua moi vong.

   Cach viet an toan: */

for (int i = 0; i < n; ++i)      /* dung < thay vi <=, neu duoc */

for (size_t i = 0; i <= n; ++i)  /* so khong dau: quan vong la hop le,
                                    nhung can than vong vo tan neu
                                    n == SIZE_MAX */

int f(int x) {
    if (x == 0) {
        ghi_log("truong hop dac biet");
        return 1 / x;                    /* chia cho 0: UB */
    }

    return 100 / x;
}

/* Clang co the:
     - thay nhanh x == 0 chac chan dan toi UB
     - suy ra x == 0 khong bao gio xay ra
     - xoa ca nhanh, KE CA loi goi ghi_log
     - va xoa luon cau if, vi dieu kien luon sai

   Ket qua: ban mat dong nhat ky ma ban dat vao de go loi. */

# Ban go loi: bat het, dung ngay khi gap loi
CFLAGS_DEBUG = -std=c11 -g3 -O1 \
               -Wall -Wextra -Wpedantic \
               -Wshadow -Wconversion -Wcast-qual \
               -Wstrict-prototypes -Wmissing-prototypes \
               -Wnull-dereference -Wdouble-promotion \
               -fsanitize=address,undefined \
               -fno-omit-frame-pointer \
               -fno-sanitize-recover=all

# Ban phat hanh: toi uu, van giu canh bao
CFLAGS_RELEASE = -std=c11 -O2 -DNDEBUG \
                 -Wall -Wextra -Wpedantic

# Trong tich hop lien tuc: canh bao la loi
CFLAGS_CI = $(CFLAGS_DEBUG) -Werror