Share some Node4/16 and Node48/256 implementations

This cuts down on the number of instructions (confirmed with
cachegrind). Also avoid initializing some memory unnecessarily.

ConflictSet.cpp

@@ -21,6 +21,7 @@ limitations under the License.
 #include <bit>
 #include <cassert>
 #include <compare>
+#include <cstddef>
 #include <cstdint>
 #include <cstring>
 #include <inttypes.h>
@@ -89,45 +90,6 @@ private:
   void *freeList = nullptr;
 };
 
-enum class Type : int8_t {
-  Node4,
-  Node16,
-  Node48,
-  Node256,
-  Invalid,
-};
-struct Node {
-  /* begin section that's copied to the next node */
-  Node *parent = nullptr;
-  // The max write version over all keys that start with the search path up to
-  // this point
-  int64_t maxVersion;
-  Entry entry;
-  int16_t numChildren = 0;
-  bool entryPresent = false;
-  uint8_t parentsIndex = 0;
-  constexpr static auto kPartialKeyMaxLen = 26;
-  uint8_t partialKey[kPartialKeyMaxLen];
-  int8_t partialKeyLen = 0;
-  /* end section that's copied to the next node */
-
-  Type type = Type::Invalid;
-};
-
-struct Node4 : Node {
-  // Sorted
-  uint8_t index[4] = {};
-  Node *children[4] = {};
-  Node4() { this->type = Type::Node4; }
-};
-
-struct Node16 : Node {
-  // Sorted
-  uint8_t index[16] = {};
-  Node *children[16] = {};
-  Node16() { this->type = Type::Node16; }
-};
-
 struct BitSet {
   bool test(int i) const {
     assert(0 <= i);
@@ -185,9 +147,49 @@ private:
   __uint128_t hi = 0;
 };
 
+enum class Type : int8_t {
+  Node4,
+  Node16,
+  Node48,
+  Node256,
+  Invalid,
+};
+struct Node {
+  /* begin section that's copied to the next node */
+  Node *parent = nullptr;
+  // The max write version over all keys that start with the search path up to
+  // this point
+  int64_t maxVersion;
+  Entry entry;
+  int16_t numChildren = 0;
+  bool entryPresent = false;
+  uint8_t parentsIndex = 0;
+  constexpr static auto kPartialKeyMaxLen = 26;
+  uint8_t partialKey[kPartialKeyMaxLen];
+  int8_t partialKeyLen = 0;
+  /* end section that's copied to the next node */
+
+  Type type = Type::Invalid;
+};
+
+struct Node4 : Node {
+  // Sorted
+  uint8_t index[16]; // 16 so that we can use the same simd index search
+                     // implementation for Node4 as Node16
+  Node *children[4];
+  Node4() { this->type = Type::Node4; }
+};
+
+struct Node16 : Node {
+  // Sorted
+  uint8_t index[16];
+  Node *children[16];
+  Node16() { this->type = Type::Node16; }
+};
+
 struct Node48 : Node {
   BitSet bitSet;
-  Node *children[48] = {};
+  Node *children[48];
   int8_t nextFree = 0;
   int8_t index[256];
   Node48() {
@@ -209,15 +211,6 @@ struct NodeAllocators {
   BoundedFreeListAllocator<Node256> node256;
 };
 
-int getNodeIndex(Node4 *self, uint8_t index) {
-  for (int i = 0; i < self->numChildren; ++i) {
-    if (self->index[i] == index) {
-      return i;
-    }
-  }
-  return -1;
-}
-
 int getNodeIndex(Node16 *self, uint8_t index) {
 #ifdef HAS_AVX
   // Based on https://www.the-paper-trail.org/post/art-paper-notes/
@@ -278,10 +271,7 @@ int getNodeIndex(Node16 *self, uint8_t index) {
 
 // Precondition - an entry for index must exist in the node
 Node *&getChildExists(Node *self, uint8_t index) {
-  if (self->type == Type::Node4) {
-    auto *self4 = static_cast<Node4 *>(self);
-    return self4->children[getNodeIndex(self4, index)];
-  } else if (self->type == Type::Node16) {
+  if (self->type <= Type::Node16) {
     auto *self16 = static_cast<Node16 *>(self);
     return self16->children[getNodeIndex(self16, index)];
   } else if (self->type == Type::Node48) {
@@ -299,17 +289,7 @@ int getChildGeq(Node *self, int child) {
   if (child > 255) {
     return -1;
   }
-  if (self->type == Type::Node4) {
-    auto *self4 = static_cast<Node4 *>(self);
-    for (int i = 0; i < self->numChildren; ++i) {
-      if (i > 0) {
-        assert(self4->index[i - 1] < self4->index[i]);
-      }
-      if (self4->index[i] >= child) {
-        return self4->index[i];
-      }
-    }
-  } else if (self->type == Type::Node16) {
+  if (self->type <= Type::Node16) {
     auto *self16 = static_cast<Node16 *>(self);
 #ifdef HAS_AVX
     __m128i key_vec = _mm_set1_epi8(child);
@@ -363,12 +343,10 @@ int getChildGeq(Node *self, int child) {
       }
     }
 #endif
-  } else if (self->type == Type::Node48) {
+  } else {
+    static_assert(offsetof(Node48, bitSet) == offsetof(Node256, bitSet));
     auto *self48 = static_cast<Node48 *>(self);
     return self48->bitSet.firstSetGeq(child);
-  } else {
-    auto *self256 = static_cast<Node256 *>(self);
-    return self256->bitSet.firstSetGeq(child);
   }
   return -1;
 }
@@ -386,7 +364,7 @@ Node *&getOrCreateChild(Node *&self, uint8_t index,
   if (self->type == Type::Node4) {
     auto *self4 = static_cast<Node4 *>(self);
     {
-      int i = getNodeIndex(self4, index);
+      int i = getNodeIndex((Node16 *)self4, index);
       if (i >= 0) {
         return self4->children[i];
       }
@@ -517,15 +495,7 @@ void eraseChild(Node *self, uint8_t index, NodeAllocators *allocators) {
     __builtin_unreachable(); // GCOVR_EXCL_LINE
   }
 
-  if (self->type == Type::Node4) {
-    auto *self4 = static_cast<Node4 *>(self);
-    int nodeIndex = getNodeIndex(self4, index);
-    memmove(self4->index + nodeIndex, self4->index + nodeIndex + 1,
-            sizeof(self4->index[0]) * (self->numChildren - (nodeIndex + 1)));
-    memmove(self4->children + nodeIndex, self4->children + nodeIndex + 1,
-            sizeof(self4->children[0]) * // NOLINT
-                (self->numChildren - (nodeIndex + 1)));
-  } else if (self->type == Type::Node16) {
+  if (self->type <= Type::Node16) {
     auto *self16 = static_cast<Node16 *>(self);
     int nodeIndex = getNodeIndex(self16, index);
     memmove(self16->index + nodeIndex, self16->index + nodeIndex + 1,
@@ -735,15 +705,8 @@ int64_t maxBetweenExclusive(Node *n, int begin, int end) {
     }
   }
   switch (n->type) {
-  case Type::Node4: {
-    auto *self = static_cast<Node4 *>(n);
-    for (int i = 0; i < self->numChildren && self->index[i] < end; ++i) {
-      if (begin < self->index[i]) {
-        result = std::max(result, self->children[i]->maxVersion);
-      }
-    }
-    break;
-  }
+  case Type::Node4:
+    [[fallthrough]];
   case Type::Node16: {
     auto *self = static_cast<Node16 *>(n);
     for (int i = 0; i < self->numChildren && self->index[i] < end; ++i) {