matmul example in doc is out of date

[hahaxxz]

In doc/rvv-intrinsic-examples.adoc, the matmul example is out of date. The pointer ptr_a and ptr_b are not updated in the loop.

void matmul_rvv(double *a, double *b, double *c, int n, int m, int p) {
  size_t vlmax = __riscv_vsetvlmax_e64m1();
  for (int i = 0; i < n; ++i)
    for (int j = 0; j < m; ++j) {
      double *ptr_a = &a[i * p];
      double *ptr_b = &b[j];
      int k = p;
      // Set accumulator to  zero.
      vfloat64m1_t vec_s = __riscv_vfmv_v_f_f64m1(0.0, vlmax);
      vfloat64m1_t vec_zero = __riscv_vfmv_v_f_f64m1(0.0, vlmax);
      for (size_t vl; k > 0; k -= vl) {
        vl = __riscv_vsetvl_e64m1(k);

        // Load row a[i][k..k+vl)
        vfloat64m1_t vec_a = __riscv_vle64_v_f64m1(ptr_a, vl);
        // Load column b[k..k+vl)[j]
        vfloat64m1_t vec_b =
          __riscv_vlse64_v_f64m1(ptr_b, sizeof(double) * m, vl);

        // Accumulate dot product of row and column. If vl < vlmax we need to
        // preserve the existing values of vec_s, hence the tu policy.
        vec_s = __riscv_vfmacc_vv_f64m1_tu(vec_s, vec_a, vec_b, vl);
      }

      // Final accumulation.
      vfloat64m1_t vec_sum =
        __riscv_vfredusum_vs_f64m1_f64m1(vec_s, vec_zero, vlmax);
      double sum = __riscv_vfmv_f_s_f64m1_f64(vec_sum);
      c[i * m + j] = sum;
    }
}

The correct example is presented in examples/rvv_matmul.c. I think we should either use the code in examples/rvv_matmul.c directly or fix it in the following way:

- for (size_t vl; k > 0; k -= vl) {
+ for (size_t vl; k > 0; k -= vl, ptr_a += vl, ptr_b += vl * m) {
...
}

[kito-cheng]

Could you create a PR to fix that? :)

[hahaxxz]

Sure.

#360