Use quantised gradients in gpu_hist histograms (#8246)

include/xgboost/base.h

@@ -259,10 +259,61 @@ class GradientPairInternal {
 using GradientPair = detail::GradientPairInternal<float>;
 /*! \brief High precision gradient statistics pair */
 using GradientPairPrecise = detail::GradientPairInternal<double>;
-/*! \brief Fixed point representation for gradient pair. */
-using GradientPairInt32 = detail::GradientPairInternal<int>;
-/*! \brief Fixed point representation for high precision gradient pair. */
-using GradientPairInt64 = detail::GradientPairInternal<int64_t>;
+
+/*! \brief Fixed point representation for high precision gradient pair. Has a different interface so
+ * we don't accidentally use it in gain calculations.*/
+class GradientPairInt64 {
+  using T = int64_t;
+  T grad_;
+  T hess_;
+
+ public:
+  using ValueT = T;
+
+  XGBOOST_DEVICE GradientPairInt64(T grad, T hess) : grad_(grad), hess_(hess) {}
+  GradientPairInt64() = default;
+
+  // Copy constructor if of same value type, marked as default to be trivially_copyable
+  GradientPairInt64(const GradientPairInt64 &g) = default;
+
+  XGBOOST_DEVICE T GetQuantisedGrad() const { return grad_; }
+  XGBOOST_DEVICE T GetQuantisedHess() const { return hess_; }
+
+  XGBOOST_DEVICE GradientPairInt64 &operator+=(const GradientPairInt64 &rhs) {
+    grad_ += rhs.grad_;
+    hess_ += rhs.hess_;
+    return *this;
+  }
+
+  XGBOOST_DEVICE GradientPairInt64 operator+(const GradientPairInt64 &rhs) const {
+    GradientPairInt64 g;
+    g.grad_ = grad_ + rhs.grad_;
+    g.hess_ = hess_ + rhs.hess_;
+    return g;
+  }
+
+  XGBOOST_DEVICE GradientPairInt64 &operator-=(const GradientPairInt64 &rhs) {
+    grad_ -= rhs.grad_;
+    hess_ -= rhs.hess_;
+    return *this;
+  }
+
+  XGBOOST_DEVICE GradientPairInt64 operator-(const GradientPairInt64 &rhs) const {
+    GradientPairInt64 g;
+    g.grad_ = grad_ - rhs.grad_;
+    g.hess_ = hess_ - rhs.hess_;
+    return g;
+  }
+
+  XGBOOST_DEVICE bool operator==(const GradientPairInt64 &rhs) const {
+    return grad_ == rhs.grad_ && hess_ == rhs.hess_;
+  }
+  friend std::ostream &operator<<(std::ostream &os,
+                                  const GradientPairInt64 &g) {
+    os << g.GetQuantisedGrad() << "/" << g.GetQuantisedHess();
+    return os;
+  }
+};
 
 using Args = std::vector<std::pair<std::string, std::string> >;