[GitHub] [arrow] pitrou commented on a change in pull request #7963: ARROW-9699: [C++][Compute] Optimize mode kernel for small integer types

[GitBox <gi...@apache.org>]

pitrou commented on a change in pull request #7963:
URL: https://github.com/apache/arrow/pull/7963#discussion_r474146349



##########
File path: cpp/src/arrow/compute/kernels/aggregate_mode.cc
##########
@@ -69,28 +72,83 @@ struct ModeState {
   std::unordered_map<T, int64_t> value_counts{};
 };
 
+// Use array to count small integers(bool, int8, uint8), improves performance 2x ~ 6x
+template <typename ArrowType>
+struct ModeState<ArrowType, enable_if_t<(sizeof(typename ArrowType::c_type) == 1)>> {
+  using ThisType = ModeState<ArrowType>;
+  using T = typename ArrowType::c_type;
+  using Limits = std::numeric_limits<T>;
+
+  ModeState() : value_counts(Limits::max() - Limits::min() + 1) {}
+
+  void MergeFrom(const ThisType& state) {
+    std::transform(this->value_counts.cbegin(), this->value_counts.cend(),
+                   state.value_counts.cbegin(), this->value_counts.begin(),
+                   std::plus<int64_t>{});
+  }
+
+  void MergeOne(T value) { ++this->value_counts[value - Limits::min()]; }
+
+  std::pair<T, int64_t> Finalize() {
+    T mode = Limits::min();
+    int64_t count = 0;
+
+    for (int i = 0; i <= Limits::max() - Limits::min(); ++i) {
+      T this_value = static_cast<T>(i + Limits::min());
+      int64_t this_count = this->value_counts[i];
+      if (this_count > count || (this_count == count && this_value < mode)) {
+        count = this_count;
+        mode = this_value;
+      }
+    }
+    return std::make_pair(mode, count);
+  }
+
+  std::vector<int64_t> value_counts;
+};
+
+// read raw_values[] directly improves performance ~15%

Review comment:
       This is probably heavily compiler-dependent. In any case, such micro-optimizations mostly complicate the code for no real benefit.

##########
File path: cpp/src/arrow/compute/kernels/aggregate_test.cc
##########
@@ -685,5 +687,10 @@ TYPED_TEST(TestFloatingModeKernel, Floats) {
   this->AssertModeIsNull("[]");
 }
 
+TEST_F(TestInt8ModeKernelValueRange, Basics) {

Review comment:
       Are these tests useful? `int8` should already be tested in `TestIntegerModeKernel`, right?




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