Specify the number of threads for parallel sort. (#8735)

* Specify the number of threads for parallel sort.

- Pass context object into argsort.
- Replace macros with inline functions.

src/objective/adaptive.cc

@@ -6,24 +6,25 @@
 #include <limits>
 #include <vector>
 
-#include "../common/common.h"
-#include "../common/numeric.h"
-#include "../common/stats.h"
-#include "../common/threading_utils.h"
+#include "../common/algorithm.h"           // ArgSort
+#include "../common/numeric.h"             // RunLengthEncode
+#include "../common/stats.h"               // Quantile,WeightedQuantile
+#include "../common/threading_utils.h"     // ParallelFor
 #include "../common/transform_iterator.h"  // MakeIndexTransformIter
+#include "xgboost/context.h"               // Context
 #include "xgboost/linalg.h"
 #include "xgboost/tree_model.h"
 
 namespace xgboost {
 namespace obj {
 namespace detail {
-void EncodeTreeLeafHost(RegTree const& tree, std::vector<bst_node_t> const& position,
-                        std::vector<size_t>* p_nptr, std::vector<bst_node_t>* p_nidx,
-                        std::vector<size_t>* p_ridx) {
+void EncodeTreeLeafHost(Context const* ctx, RegTree const& tree,
+                        std::vector<bst_node_t> const& position, std::vector<size_t>* p_nptr,
+                        std::vector<bst_node_t>* p_nidx, std::vector<size_t>* p_ridx) {
   auto& nptr = *p_nptr;
   auto& nidx = *p_nidx;
   auto& ridx = *p_ridx;
-  ridx = common::ArgSort<size_t>(position);
+  ridx = common::ArgSort<size_t>(ctx, position.cbegin(), position.cend());
   std::vector<bst_node_t> sorted_pos(position);
   // permutation
   for (size_t i = 0; i < position.size(); ++i) {
@@ -74,7 +75,7 @@ void UpdateTreeLeafHost(Context const* ctx, std::vector<bst_node_t> const& posit
   std::vector<bst_node_t> nidx;
   std::vector<size_t> nptr;
   std::vector<size_t> ridx;
-  EncodeTreeLeafHost(*p_tree, position, &nptr, &nidx, &ridx);
+  EncodeTreeLeafHost(ctx, *p_tree, position, &nptr, &nidx, &ridx);
   size_t n_leaf = nidx.size();
   if (nptr.empty()) {
     std::vector<float> quantiles;