Changed pivot code back to previous logic for performance reasons

Author: sterrettm2

src/avx512-common-qsort.h

@@ -688,7 +688,7 @@ static inline int64_t partition_avx512(type_t1 *keys,
 }
 
 template <typename vtype, typename type_t>
-X86_SIMD_SORT_INLINE type_t get_pivot(type_t *arr,
+X86_SIMD_SORT_INLINE type_t get_pivot_scalar(type_t *arr,
                                       const int64_t left,
                                       const int64_t right)
 {
@@ -703,9 +703,132 @@ X86_SIMD_SORT_INLINE type_t get_pivot(type_t *arr,
 
     auto vec = vtype::loadu(samples);
     vec = vtype::sort_vec(vec);
-    vtype::storeu(samples, vec);
+    return ((type_t *)&vec)[numSamples / 2];
+}
+
+template <typename vtype, typename reg_t>
+X86_SIMD_SORT_INLINE reg_t sort_zmm_16bit(reg_t zmm);
+
+template <typename vtype, typename reg_t>
+X86_SIMD_SORT_INLINE reg_t sort_zmm_32bit(reg_t zmm);
+
+template <typename vtype, typename reg_t>
+X86_SIMD_SORT_INLINE reg_t sort_zmm_64bit(reg_t zmm);
+
+template <typename vtype, typename type_t>
+X86_SIMD_SORT_INLINE type_t get_pivot_16bit(type_t *arr,
+                                            const int64_t left,
+                                            const int64_t right)
+{
+    // median of 32
+    int64_t size = (right - left) / 32;
+    type_t vec_arr[32] = {arr[left],
+                          arr[left + size],
+                          arr[left + 2 * size],
+                          arr[left + 3 * size],
+                          arr[left + 4 * size],
+                          arr[left + 5 * size],
+                          arr[left + 6 * size],
+                          arr[left + 7 * size],
+                          arr[left + 8 * size],
+                          arr[left + 9 * size],
+                          arr[left + 10 * size],
+                          arr[left + 11 * size],
+                          arr[left + 12 * size],
+                          arr[left + 13 * size],
+                          arr[left + 14 * size],
+                          arr[left + 15 * size],
+                          arr[left + 16 * size],
+                          arr[left + 17 * size],
+                          arr[left + 18 * size],
+                          arr[left + 19 * size],
+                          arr[left + 20 * size],
+                          arr[left + 21 * size],
+                          arr[left + 22 * size],
+                          arr[left + 23 * size],
+                          arr[left + 24 * size],
+                          arr[left + 25 * size],
+                          arr[left + 26 * size],
+                          arr[left + 27 * size],
+                          arr[left + 28 * size],
+                          arr[left + 29 * size],
+                          arr[left + 30 * size],
+                          arr[left + 31 * size]};
+    typename vtype::reg_t rand_vec = vtype::loadu(vec_arr);
+    typename vtype::reg_t sort = sort_zmm_16bit<vtype>(rand_vec);
+    return ((type_t *)&sort)[16];
+}
+
+template <typename vtype, typename type_t>
+X86_SIMD_SORT_INLINE type_t get_pivot_32bit(type_t *arr,
+                                            const int64_t left,
+                                            const int64_t right)
+{
+    // median of 16
+    int64_t size = (right - left) / 16;
+    using zmm_t = typename vtype::reg_t;
+    using ymm_t = typename vtype::halfreg_t;
+    __m512i rand_index1 = _mm512_set_epi64(left + size,
+                                           left + 2 * size,
+                                           left + 3 * size,
+                                           left + 4 * size,
+                                           left + 5 * size,
+                                           left + 6 * size,
+                                           left + 7 * size,
+                                           left + 8 * size);
+    __m512i rand_index2 = _mm512_set_epi64(left + 9 * size,
+                                           left + 10 * size,
+                                           left + 11 * size,
+                                           left + 12 * size,
+                                           left + 13 * size,
+                                           left + 14 * size,
+                                           left + 15 * size,
+                                           left + 16 * size);
+    ymm_t rand_vec1
+            = vtype::template i64gather<sizeof(type_t)>(rand_index1, arr);
+    ymm_t rand_vec2
+            = vtype::template i64gather<sizeof(type_t)>(rand_index2, arr);
+    zmm_t rand_vec = vtype::merge(rand_vec1, rand_vec2);
+    zmm_t sort = sort_zmm_32bit<vtype>(rand_vec);
+    // pivot will never be a nan, since there are no nan's!
+    return ((type_t *)&sort)[8];
+}
+
+template <typename vtype, typename type_t>
+X86_SIMD_SORT_INLINE type_t get_pivot_64bit(type_t *arr,
+                                            const int64_t left,
+                                            const int64_t right)
+{
+    // median of 8
+    int64_t size = (right - left) / 8;
+    using zmm_t = typename vtype::reg_t;
+    __m512i rand_index = _mm512_set_epi64(left + size,
+                                          left + 2 * size,
+                                          left + 3 * size,
+                                          left + 4 * size,
+                                          left + 5 * size,
+                                          left + 6 * size,
+                                          left + 7 * size,
+                                          left + 8 * size);
+    zmm_t rand_vec = vtype::template i64gather<sizeof(type_t)>(rand_index, arr);
+    // pivot will never be a nan, since there are no nan's!
+    zmm_t sort = sort_zmm_64bit<vtype>(rand_vec);
+    return ((type_t *)&sort)[4];
+}
 
-    return samples[numSamples / 2];
+template <typename vtype, typename type_t>
+X86_SIMD_SORT_INLINE type_t get_pivot(type_t *arr,
+                                      const int64_t left,
+                                      const int64_t right)
+{
+    if constexpr (vtype::numlanes == 8)
+        return get_pivot_64bit<vtype>(arr, left, right);
+    else if constexpr (vtype::numlanes == 16)
+        return get_pivot_32bit<vtype>(arr, left, right);
+    else if constexpr (vtype::numlanes == 32)
+        return get_pivot_16bit<vtype>(arr, left, right);
+    else
+        return get_pivot_scalar<vtype>(arr, left, right);
 }
 
 template <typename vtype, int64_t maxN>