Extract optional weight. (#8747)

- Extract optional weight from coommon.h to reduce dependency on this header.
- Add test.

src/common/common.h

@@ -1,5 +1,5 @@
-/*!
- * Copyright 2015-2022 by XGBoost Contributors
+/**
+ * Copyright 2015-2023 by XGBoost Contributors
  * \file common.h
  * \brief Common utilities
  */
@@ -199,17 +199,6 @@ std::vector<Idx> ArgSort(Container const &array, Comp comp = std::less<V>{}) {
   return result;
 }
 
-struct OptionalWeights {
-  Span<float const> weights;
-  float dft{1.0f};  // fixme: make this compile time constant
-
-  explicit OptionalWeights(Span<float const> w) : weights{w} {}
-  explicit OptionalWeights(float w) : dft{w} {}
-
-  XGBOOST_DEVICE float operator[](size_t i) const { return weights.empty() ? dft : weights[i]; }
-  auto Empty() const { return weights.empty(); }
-};
-
 /**
  * Last index of a group in a CSR style of index pointer.
  */

src/common/optional_weight.h

@@ -0,0 +1,33 @@
+/**
+ * Copyright 2022-2023 by XGBoost Contributors
+ */
+#ifndef XGBOOST_COMMON_OPTIONAL_WEIGHT_H_
+#define XGBOOST_COMMON_OPTIONAL_WEIGHT_H_
+#include "xgboost/base.h"                // XGBOOST_DEVICE
+#include "xgboost/context.h"             // Context
+#include "xgboost/host_device_vector.h"  // HostDeviceVector
+#include "xgboost/span.h"                // Span
+
+namespace xgboost {
+namespace common {
+struct OptionalWeights {
+  Span<float const> weights;
+  float dft{1.0f};  // fixme: make this compile time constant
+
+  explicit OptionalWeights(Span<float const> w) : weights{w} {}
+  explicit OptionalWeights(float w) : dft{w} {}
+
+  XGBOOST_DEVICE float operator[](size_t i) const { return weights.empty() ? dft : weights[i]; }
+  auto Empty() const { return weights.empty(); }
+};
+
+inline OptionalWeights MakeOptionalWeights(Context const* ctx,
+                                           HostDeviceVector<float> const& weights) {
+  if (ctx->IsCUDA()) {
+    weights.SetDevice(ctx->gpu_id);
+  }
+  return OptionalWeights{ctx->IsCPU() ? weights.ConstHostSpan() : weights.ConstDeviceSpan()};
+}
+}  // namespace common
+}  // namespace xgboost
+#endif  // XGBOOST_COMMON_OPTIONAL_WEIGHT_H_

src/common/quantile.h

@@ -19,6 +19,7 @@
 
 #include "categorical.h"
 #include "common.h"
+#include "optional_weight.h"  // OptionalWeights
 #include "threading_utils.h"
 #include "timer.h"
 

src/common/stats.cu

@@ -6,9 +6,9 @@
 
 #include <cstddef>                              // size_t
 
-#include "common.h"                             // common::OptionalWeights
 #include "cuda_context.cuh"                     // CUDAContext
 #include "device_helpers.cuh"                   // dh::MakeTransformIterator, tcbegin, tcend
+#include "optional_weight.h"                    // common::OptionalWeights
 #include "stats.cuh"          // common::SegmentedQuantile, common::SegmentedWeightedQuantile
 #include "xgboost/base.h"     // XGBOOST_DEVICE
 #include "xgboost/context.h"  // Context

src/common/stats.h

@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "common.h"              // AssertGPUSupport, OptionalWeights
+#include "optional_weight.h"     // OptionalWeights
 #include "transform_iterator.h"  // MakeIndexTransformIter
 #include "xgboost/context.h"     // Context
 #include "xgboost/linalg.h"