change a the gather implementation to suppport axis to be negative.

src/include/migraphx/operators.hpp

@@ -640,17 +640,24 @@ struct as_shape
 
 struct gather
 {
-    std::size_t axis = 0;
+    mutable int axis = 0;
     std::string name() const { return "gather"; }
 
     shape compute_shape(std::vector<shape> inputs) const
     {
         check_shapes{inputs, *this}.has(2);
         auto lens = inputs[0].lens();
-        if(axis >= lens.size())
+        if(axis >= lens.size() || axis < -lens.size())
         {
             MIGRAPHX_THROW("Gather, axis is out of range.");
         }
+
+        // negative value means counting dimensions from back
+        if (axis < 0)
+        {
+            axis += lens.size();
+        }
+
         auto type  = inputs[0].type();
         lens[axis] = inputs[1].elements();
 

src/onnx/onnx.cpp

@@ -377,7 +377,7 @@ struct onnx_parser
     instruction_ref
     parse_gather(const std::string&, attribute_map attributes, std::vector<instruction_ref> args)
     {
-        std::size_t axis = 0;
+        int axis = 0;
         if(contains(attributes, "axis"))
         {
             axis = parse_value(attributes.at("axis")).at<int>();

test/onnx/onnx_test.cpp

@@ -417,7 +417,7 @@ TEST_CASE(gather_test)
     migraphx::program p;
     auto l0 = p.add_parameter("data", migraphx::shape{migraphx::shape::float_type, {3, 4, 5, 6}});
     auto l1 = p.add_parameter("indices", migraphx::shape{migraphx::shape::int32_type, {2, 3}});
-    std::size_t axis = 1;
+    int axis = 1;
     p.add_instruction(migraphx::op::gather{axis}, l0, l1);
     auto prog = migraphx::parse_onnx("gather_test.onnx");
 
@@ -432,7 +432,7 @@ TEST_CASE(shape_gather_test)
         p.add_literal(migraphx::shape{migraphx::shape::int64_type, {3}}, l0->get_shape().lens());
     migraphx::shape const_shape{migraphx::shape::int32_type, {1}};
     auto l2          = p.add_literal(migraphx::literal{const_shape, {1}});
-    std::size_t axis = 0;
+    int axis = 0;
     p.add_instruction(migraphx::op::gather{axis}, l1, l2);
     auto prog = migraphx::parse_onnx("shape_gather.onnx");