Modify layernorm to allow higher overflow limit for lower precision (#1534)

src/targets/gpu/kernels/include/migraphx/kernels/layernorm.hpp

@@ -48,12 +48,20 @@ __device__ void generic_binary_layernorm(
 {
     using block         = reduce::auto_block<reduce::reduce_elements_with_axis<Input1, Axis>()>;
     using reduce_output = reduce::with_axis<Input1, Axis>;
+
     block::template run<reduce_output>([&](auto, auto r) {
         auto input       = r.inner([&](auto x1, auto x2) { return op(x1, x2); })(input1, input2);
         using value_type = typename Input1::type;
         constexpr auto relements   = r.template elements<Input1>();
+        constexpr auto relements_r = vec_type<value_type>{1.0 / relements};
+        auto relements_rsqrt       = sqrt(relements_r);
+
         auto means = r.reduce(op::sum{}, make_array<vec_type<value_type>>(0, 0), [&](auto x) {
-            return make_array(x, x * x) * vec_type<value_type>{1.0 / relements};
+            auto x_out = x * relements_r;
+            // dividing x by sqrt(relements) before squaring allows computing higher values
+            // before overflow in low precision
+            auto x2_sqrt = x * relements_rsqrt;
+            return make_array(x_out, x2_sqrt * x2_sqrt);
         })(input);
 
         auto mean_x        = means[0];

test/verify/test_layernorm.cpp

@@ -24,6 +24,7 @@
 
 #include "verify_program.hpp"
 #include <migraphx/program.hpp>
+#include <migraphx/instruction.hpp>
 #include <migraphx/generate.hpp>
 #include <migraphx/make_op.hpp>
 
@@ -34,12 +35,12 @@ migraphx::instruction_ref add_layernorm(migraphx::module& m,
                                         std::vector<size_t> dims,
                                         float eps = 1e-12f)
 {
-    auto scale =
-        m.add_parameter("scale", migraphx::shape{migraphx::shape::float_type, {dims.back()}});
-    auto bias =
-        m.add_parameter("bias", migraphx::shape{migraphx::shape::float_type, {dims.back()}});
-    auto epsilon  = m.add_literal(eps);
-    auto exponent = m.add_literal(2.0f);
+    auto mgx_type = x->get_shape().type();
+    auto scale    = m.add_parameter("scale", migraphx::shape{mgx_type, {dims.back()}});
+    auto bias     = m.add_parameter("bias", migraphx::shape{mgx_type, {dims.back()}});
+
+    auto epsilon  = m.add_literal(migraphx::literal{migraphx::shape{mgx_type}, {eps}});
+    auto exponent = m.add_literal(migraphx::literal{migraphx::shape{mgx_type}, {2.0f}});
 
     auto mean = m.add_instruction(migraphx::op::reduce_mean({2}), x);
     auto mean_mbcast =
@@ -90,6 +91,19 @@ struct test_layernorm2 : verify_program<test_layernorm2>
     }
 };
 
+struct test_layernorm_fp16 : verify_program<test_layernorm_fp16>
+{
+    migraphx::program create_program() const
+    {
+        migraphx::program p;
+        auto* mm                 = p.get_main_module();
+        std::vector<size_t> dims = {1, 24, 64};
+        auto x = mm->add_parameter("x", migraphx::shape{migraphx::shape::half_type, dims});
+        add_layernorm(*mm, x, dims);
+        return p;
+    }
+};
+
 struct test_layernorm_eps : verify_program<test_layernorm_eps>
 {
     migraphx::program create_program() const