GPUTreeShap (#6038)

src/common/device_helpers.cuh

@@ -474,8 +474,18 @@ class TemporaryArray {
   using AllocT = XGBCachingDeviceAllocator<T>;
   using value_type = T;  // NOLINT
   explicit TemporaryArray(size_t n) : size_(n) { ptr_ = AllocT().allocate(n); }
+  TemporaryArray(size_t n, T val) : size_(n) {
+    ptr_ = AllocT().allocate(n);
+    this->fill(val);
+  }
   ~TemporaryArray() { AllocT().deallocate(ptr_, this->size()); }
-
+  void fill(T val)  // NOLINT
+  {
+    int device = 0;
+    dh::safe_cuda(cudaGetDevice(&device));
+    auto d_data = ptr_.get();
+    LaunchN(device, this->size(), [=] __device__(size_t idx) { d_data[idx] = val; });
+  }
   thrust::device_ptr<T> data() { return ptr_; }  // NOLINT
   size_t size() { return size_; }  // NOLINT