Fix gather algorithm

Author: jszuppe

include/boost/compute/algorithm/gather.hpp

@@ -36,7 +36,6 @@ class gather_kernel : public meta_kernel
                    OutputIterator result)
     {
         m_count = iterator_range_size(first, last);
-        m_offset = first.get_index();
 
         *this <<
             "const uint i = get_global_id(0);\n" <<
@@ -50,12 +49,11 @@ class gather_kernel : public meta_kernel
             return event();
         }
 
-        return exec_1d(queue, m_offset, m_count);
+        return exec_1d(queue, 0, m_count);
     }
 
 private:
     size_t m_count;
-    size_t m_offset;
 };
 
 } // end detail namespace

test/test_gather.cpp

@@ -34,6 +34,11 @@ BOOST_AUTO_TEST_CASE(gather_int)
         indices.begin(), indices.end(), input.begin(), output.begin(), queue
     );
     CHECK_RANGE_EQUAL(int, 5, output, (1, 5, 2, 4, 3));
+
+    compute::gather(
+        indices.begin() + 1, indices.end(), input.begin(), output.begin(), queue
+    );
+    CHECK_RANGE_EQUAL(int, 5, output, (5, 2, 4, 3, 3));
 }
 
 BOOST_AUTO_TEST_CASE(copy_index_then_gather)