add two avx instructions example & simple clear screen

avx/dotproduct.c

@@ -0,0 +1,61 @@
+/* Compute the dot product of two (properly aligned) vectors. */
+#include <stdio.h>
+#include <immintrin.h>
+#include <math.h>
+
+const int N = 83;
+
+double slow_dot_product(const double *a, const double *b) {
+    double answer = 0.0;
+    for(int ii = 0; ii < N; ++ii)
+        answer += a[ii]*b[ii];
+    return answer;
+}
+
+/* Horizontal add works within 128-bit lanes. Use scalar ops to add
+ * across the boundary. */
+double reduce_vector1(__m256d input) {
+    __m256d temp = _mm256_hadd_pd(input, input);
+    return ((double*)&temp)[0] + ((double*)&temp)[2];
+}
+
+/* Another way to get around the 128-bit boundary: grab the first 128
+ * bits, grab the lower 128 bits and then add them together with a 128
+ * bit add instruction. */
+double reduce_vector2(__m256d input) {
+    __m256d temp = _mm256_hadd_pd(input, input);
+    __m128d sum_high = _mm256_extractf128_pd(temp, 1);
+    __m128d result = _mm_add_pd(sum_high, _mm256_castpd256_pd128(temp));
+    return ((double*)&result)[0];
+}
+
+double dot_product(const double *a, const double *b) {
+    __m256d sum_vec = _mm256_set_pd(0.0, 0.0, 0.0, 0.0);
+
+    /* Add up partial dot-products in blocks of 256 bits */
+    for(int ii = 0; ii < N/4; ++ii) {
+        __m256d x = _mm256_load_pd(a+4*ii);
+        __m256d y = _mm256_load_pd(b+4*ii);
+        __m256d z = _mm256_mul_pd(x,y);
+        sum_vec = _mm256_add_pd(sum_vec, z);
+    }
+
+    /* Find the partial dot-product for the remaining elements after
+    * dealing with all 256-bit blocks. */
+    double final = 0.0;
+    for(int ii = N-N%4; ii < N; ++ii)
+        final += a[ii] * b[ii];
+
+    return reduce_vector2(sum_vec) + final;
+}
+
+int main() {
+    __attribute__ ((aligned (32))) double a[N], b[N];
+
+    for(int ii = 0; ii < N; ++ii)
+        a[ii] = b[ii] = ii/sqrt(N);
+
+    double answer = dot_product(a, b);
+    printf("%f\n", answer);
+    printf("%f\n", slow_dot_product(a,b));
+}