restrict

void cong(int *a, int *b, int *c) {
    *a += *c;
    *b += *c;
}

void cong_r(int *restrict a, int *restrict b, int *restrict c) {
    *a += *c;
    *b += *c;
}

void chep(int *d, const int *s, size_t n) {
    for (size_t i = 0; i < n; ++i) d[i] = s[i];
}

void chep_r(int *restrict d, const int *restrict s, size_t n) {
    for (size_t i = 0; i < n; ++i) d[i] = s[i];
}

/* Nhan ma tran: vong lap trong doc B va ghi C.
   Khong co restrict, trinh bien dich phai gia dinh
   C co the trung voi A hoac B, nen khong giu duoc
   tong trong thanh ghi. */

void nhan(double *restrict C,
          const double *restrict A,
          const double *restrict B,
          size_t n)
{
    for (size_t i = 0; i < n; ++i)
        for (size_t j = 0; j < n; ++j) {
            double t = 0.0;

            for (size_t k = 0; k < n; ++k)
                t += A[i * n + k] * B[k * n + j];

            C[i * n + j] = t;
        }
}

/* Bien cuc bo t da giai quyet phan lon van de o vong trong.
   Nhung restrict van giup o vong ngoai: khong co no,
   trinh bien dich phai gia dinh viec ghi C[i*n+j] co the
   sua A hoac B, nen phai nap lai chung o vong sau. */

#include <stdio.h>

void cong_r(int *restrict a, int *restrict b, int *restrict c) {
    *a += *c;
    *b += *c;
}

int main(void) {
    int x = 1, y = 10;

    cong_r(&x, &y, &x);      /* a va c TRUNG NHAU: pha loi hua */

    printf("x = %d, y = %d\n", x, y);

    return 0;
}

/* GCC va Clang o -O2 mac dinh bat -fstrict-aliasing.

   Quy tac: hai con tro KHAC KIEU thi trinh bien dich duoc phep
   gia dinh chung khong trung nhau. */

void f(int *a, float *b) {
    *a = 1;
    *b = 2.0f;     /* trinh bien dich gia dinh khong dung *a */
}

/* Nghia la doi khi ban duoc toi uu MIEN PHI, khong can restrict,
   chi vi kieu khac nhau.

   Nhung dieu do cung nghia la ma nhu sau la SAI: */

float f_bit(int x) {
    return *(float *)&x;      /* vi pham quy tac trung bi danh */
}

float f_dung(int x) {
    float y;

    memcpy(&y, &x, sizeof y);  /* dung: memcpy luon hop le */

    return y;
}

/* Hoac union, trong C11 thi doc thanh vien khac cua union
   la hop le va co dinh nghia: */

float f_union(int x) {
    union { int i; float f; } u = { .i = x };

    return u.f;
}

/* Phan 13 se noi ky ve quy tac nay va ve char *, kieu duy nhat
   duoc phep tro vao bat cu thu gi. */