Add optional headers to `mx.fast.metal_kernel` (#1358)

mlx/fast.cpp

@@ -1112,7 +1112,6 @@ std::map<std::string, array> MetalKernel::operator()(
         "[metal_kernel] MetalKernel only works on GPU.");
   }
 
-  std::ostringstream kernel_source;
   std::ostringstream func_name;
 
   std::string template_def = "";
@@ -1128,6 +1127,9 @@ std::map<std::string, array> MetalKernel::operator()(
   func_name << "custom_kernel_" << name_ << hash_key;
   std::string kernel_name = func_name.str();
 
+  std::ostringstream kernel_source;
+  kernel_source << header_ << std::endl;
+
   std::vector<CustomKernelShapeInfo> shape_infos;
   write_signature(
       func_name.str(),

mlx/fast.h

@@ -71,10 +71,12 @@ class MetalKernel {
   MetalKernel(
       const std::string& name,
       const std::string& source,
-      bool ensure_row_contiguous,
-      bool atomic_outputs)
+      const std::string& header = "",
+      bool ensure_row_contiguous = true,
+      bool atomic_outputs = false)
       : name_(name),
         source_(source),
+        header_(header),
         ensure_row_contiguous_(ensure_row_contiguous),
         atomic_outputs_(atomic_outputs) {}
 
@@ -93,7 +95,8 @@ class MetalKernel {
  private:
   std::string name_;
   std::string source_;
-  bool ensure_row_contiguous_ = true;
-  bool atomic_outputs_ = false;
+  std::string header_;
+  bool ensure_row_contiguous_;
+  bool atomic_outputs_;
 };
 } // namespace mlx::core::fast

python/src/fast.cpp

@@ -200,9 +200,15 @@ void init_fast(nb::module_& parent_module) {
       A jit-compiled custom Metal kernel defined from a source string.
       )pbdoc")
       .def(
-          nb::init<const std::string&, const std::string&, bool, bool>(),
+          nb::init<
+              const std::string&,
+              const std::string&,
+              const std::string&,
+              bool,
+              bool>(),
           "name"_a,
           "source"_a,
+          "header"_a = "",
           "ensure_row_contiguous"_a = true,
           "atomic_outputs"_a = false,
           R"pbdoc(
@@ -214,6 +220,8 @@ void init_fast(nb::module_& parent_module) {
             the function signature will be generated for you. The names of the inputs/outputs
             are determined by the ``inputs`` and ``output_shapes``/``output_dtypes``
             used when the kernel is called.
+        header (str): Header source code to include before the main function.
+            Useful for helper functions or includes that should live outside of the main function body.
         ensure_row_contiguous (bool): Whether to ensure the inputs are row contiguous
             before the kernel runs. Default: ``True``.
         atomic_outputs (bool): Whether to use atomic outputs in the function signature

python/tests/test_fast.py

@@ -551,7 +551,7 @@ class TestFast(mlx_tests.MLXTestCase):
     @unittest.skipIf(not mx.metal.is_available(), "Metal is not available")
     def test_custom_kernel_basic(self):
         mx.random.seed(7)
-        a = mx.random.normal(shape=(3, 6))
+        a = mx.random.normal(shape=(2, 2))
         kernel = mx.fast.metal_kernel(
             name="basic",
             source="""
@@ -567,7 +567,7 @@ class TestFast(mlx_tests.MLXTestCase):
             output_dtypes={"out1": mx.float32},
             stream=mx.gpu,
         )
-        mx.allclose(out["out1"], a[:2, :2])
+        self.assertTrue(mx.allclose(out["out1"], a))
 
     @unittest.skipIf(not mx.metal.is_available(), "Metal is not available")
     def test_custom_kernel_args(self):
@@ -618,12 +618,12 @@ class TestFast(mlx_tests.MLXTestCase):
             uint elem = thread_position_in_grid.x;
             uint loc = elem_to_loc(elem, inp_shape, inp_strides, inp_ndim);
             T tmp = inp[loc];
-            out[elem] = metal::exp(tmp) * threads_per_simdgroup;
+            out[elem] = metal::precise::exp(tmp) * threads_per_simdgroup;
         """
         source_contig = """
             uint elem = thread_position_in_grid.x;
             T tmp = inp[elem];
-            out[elem] = metal::exp(tmp) * threads_per_simdgroup;
+            out[elem] = metal::precise::exp(tmp) * threads_per_simdgroup;
         """
 
         # non contiguous
@@ -646,6 +646,33 @@ class TestFast(mlx_tests.MLXTestCase):
             )
             self.assertTrue(mx.allclose(mx.exp(a) * 32, outputs["out"]))
 
+    @unittest.skipIf(not mx.metal.is_available(), "Metal is not available")
+    def test_custom_kernel_helper(self):
+        mx.random.seed(7)
+        a = mx.random.normal(shape=(2, 2))
+        kernel = mx.fast.metal_kernel(
+            name="helper",
+            header="""
+            template <typename T>
+            T do_exp(T x) {
+                return metal::precise::exp(x);
+            }
+            """,
+            source="""
+                uint elem = thread_position_in_grid.x;
+                out1[elem] = do_exp(a[elem]);
+            """,
+        )
+        out = kernel(
+            inputs={"a": a},
+            grid=(4, 1, 1),
+            threadgroup=(2, 1, 1),
+            output_shapes={"out1": (2, 2)},
+            output_dtypes={"out1": mx.float32},
+            stream=mx.gpu,
+        )
+        self.assertTrue(mx.allclose(out["out1"], mx.exp(a)))
+
 
 if __name__ == "__main__":
     unittest.main()