Add AVX512 support to the dgemv_n_microk_haswell-4.c kernel

Now that the kernel is written in C-with-intrinsics, adding
AVX512 support to this kernel is trivial and yields a pretty significant
performance increase

kernel/x86_64/dgemv_n_microk_haswell-4.c

@@ -25,7 +25,12 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
 USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *****************************************************************************/
 
+/* Ensure that the compiler knows how to generate AVX2 instructions if it doesn't already*/
+#ifndef __AVX512CD_
+#pragma GCC target("avx2,fma")
+#endif
 
+#ifdef __AVX2__
 
 #define HAVE_KERNEL_4x4 1
 
@@ -46,8 +51,39 @@ static void dgemv_kernel_4x4( BLASLONG n, FLOAT **ap, FLOAT *x, FLOAT *y, FLOAT
 
 	__alpha = _mm256_broadcastsd_pd(_mm_load_sd(alpha));
 
+#ifdef __AVX512CD__
+	int n5;
+	__m512d x05, x15, x25, x35;
+	__m512d __alpha5;
+	n5 = n & ~7;
+
+	x05 = _mm512_broadcastsd_pd(_mm_load_sd(&x[0]));
+	x15 = _mm512_broadcastsd_pd(_mm_load_sd(&x[1]));
+	x25 = _mm512_broadcastsd_pd(_mm_load_sd(&x[2]));
+	x35 = _mm512_broadcastsd_pd(_mm_load_sd(&x[3]));
+
+	__alpha5 = _mm512_broadcastsd_pd(_mm_load_sd(alpha));
+
+	for (; i < n5; i+= 8) {
+		__m512d tempY;
+		__m512d sum;
+
+		sum = _mm512_add_pd(
+				_mm512_add_pd(
+					_mm512_mul_pd(_mm512_loadu_pd(&ap[0][i]), x05),
+					_mm512_mul_pd(_mm512_loadu_pd(&ap[1][i]), x15)),
+				_mm512_add_pd(
+					_mm512_mul_pd(_mm512_loadu_pd(&ap[2][i]), x25),
+					_mm512_mul_pd(_mm512_loadu_pd(&ap[3][i]), x35))
+			);
 
-	for (i = 0; i < n; i+= 4) {
+		tempY = _mm512_loadu_pd(&y[i]);
+		tempY = _mm512_add_pd(tempY, _mm512_mul_pd(sum, __alpha5));
+		_mm512_storeu_pd(&y[i], tempY);
+	}
+#endif
+
+	for (; i < n; i+= 4) {
 		__m256d tempY;
 		__m256d sum;
 
@@ -99,3 +135,5 @@ static void dgemv_kernel_4x2( BLASLONG n, FLOAT **ap, FLOAT *x, FLOAT *y, FLOAT
 	}
 
 }
+
+#endif /* AVX2 */