GPU binning and compression. (#3319)

* GPU binning and compression.

- binning and index compression are done inside the DeviceShard constructor
- in case of a DMatrix with multiple row batches, it is first converted into a single row batch

src/common/compressed_iterator.h

@@ -8,6 +8,10 @@
 #include <cstddef>
 #include <algorithm>
 
+#ifdef __CUDACC__
+#include "device_helpers.cuh"
+#endif
+
 namespace xgboost {
 namespace common {
 
@@ -96,6 +100,23 @@ class CompressedBufferWriter {
       }
     }
   }
+
+#ifdef __CUDACC__
+  __device__ void AtomicWriteSymbol
+    (CompressedByteT* buffer, uint64_t symbol, size_t offset) {
+    size_t ibit_start = offset * symbol_bits_;
+    size_t ibit_end = (offset + 1) * symbol_bits_ - 1;
+    size_t ibyte_start = ibit_start / 8, ibyte_end = ibit_end / 8;
+
+    symbol <<= 7 - ibit_end % 8;
+    for (ptrdiff_t ibyte = ibyte_end; ibyte >= (ptrdiff_t)ibyte_start; --ibyte) {
+      dh::AtomicOrByte(reinterpret_cast<unsigned int*>(buffer + detail::kPadding),
+                   ibyte, symbol & 0xff);
+      symbol >>= 8;
+    }
+  }
+#endif
+
   template <typename IterT>
   void Write(CompressedByteT *buffer, IterT input_begin, IterT input_end) {
     uint64_t tmp = 0;