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# Restricted pointer

From cppreference.com
< c‎ | keyword

| This section is incomplete Reason: needs content

### [edit] Keywords

`restrict`

### [edit] Example

| This section is incomplete Reason: replace by a concise example

To verify the 15% savings in this matrix-vector product, remove "restrict" from function Axb.

Run this code

    #include <stdio.h>
    #include <time.h>

    /*
    #define restrict
    */

    // Ax = b
    // Arrays A, x, and b could be aliased, so restrict pointers.
    void Axb (float * restrict A, float * restrict x, float * restrict b, int n)
             __attribute__ ((noinline));

    void Axb (float * restrict A, float * restrict x, float * restrict b, int n)
    {
      int i;
      for (i=0; i<n/5; i++)
      {
        b[0] =  A[0]*x[0] +  A[1]*x[1] +  A[2]*x[2] +  A[3]*x[3] +  A[4]*x[4];
        b[1] =  A[5]*x[0] +  A[6]*x[1] +  A[7]*x[2] +  A[8]*x[3] +  A[9]*x[4];
        b[2] = A[10]*x[0] + A[11]*x[1] + A[12]*x[2] + A[13]*x[3] + A[14]*x[4];
        b[3] = A[15]*x[0] + A[16]*x[1] + A[17]*x[2] + A[18]*x[3] + A[19]*x[4];
        b[4] = A[20]*x[0] + A[21]*x[1] + A[22]*x[2] + A[23]*x[3] + A[24]*x[4];
        A += 25;
        b += 5;
      }
    }

    /*    A    *  x   ==>    b    */
    /* 50000x5 * 5x1  ==> 50000x1 */
    #define NROW 50000
    #define NCOL 5
    float A[NROW*NCOL], x[NCOL], b[NROW];

    int main(void)
    {
        // Initialize matrix A and vector x.
        int i;
        for (i=0; i<NCOL; ++i) x[i] = 1;
        for (i=0; i<NROW*NCOL; ++i) A[i] = i;

        clock_t t = clock();   // start clock
        for (i=0; i<10000; ++i)
            Axb(A,x,b,NROW);
        t = clock()-t;         // stop clock
        printf("Time used: %f seconds\n", ((float)t)/CLOCKS_PER_SEC);

        return 0;
    }

Possible output:

    Time used: 1.880000 seconds

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