allow compiling lambdas in C++ (#1650)

* allow compiling lambdas in C++

* fix test

* more tests

* auto detect capture-less lambda

mlx/compile.cpp

@@ -208,8 +208,7 @@ std::uintptr_t get_function_address(const std::function<T(U...)>& fun) {
   using FunType = T (*)(U...);
   const FunType* fun_ptr = fun.template target<FunType>();
   if (fun_ptr == nullptr) {
-    throw std::invalid_argument(
-        "[compile] Cannot compile a non-addressable function.");
+    return 0;
   }
   return reinterpret_cast<std::uintptr_t>(*fun_ptr);
 }
@@ -817,17 +816,28 @@ void compile_validate_shapeless(const std::vector<array>& tape) {
   }
 }
 
+bool skip_compile() {
+  return compile_mode() == CompileMode::disabled ||
+      !(compile_available_for_device(default_device()));
+}
+
 std::function<std::vector<array>(const std::vector<array>&)> compile(
-    const std::function<std::vector<array>(const std::vector<array>&)>& fun,
+    std::function<std::vector<array>(const std::vector<array>&)> fun,
     std::uintptr_t fun_id,
     bool shapeless /* = false */,
     std::vector<uint64_t> constants /* = {} */) {
-  if (compile_mode() == CompileMode::disabled ||
-      !(compile_available_for_device(default_device()))) {
+  if (skip_compile()) {
     return fun;
   }
-  return [fun, fun_id, shapeless, constants = std::move(constants)](
-             const std::vector<array>& inputs) {
+  if (!fun) {
+    throw std::invalid_argument(
+        "[compile] Cannot compile a function without a target.");
+  }
+
+  return [fun = std::move(fun),
+          fun_id,
+          shapeless,
+          constants = std::move(constants)](const std::vector<array>& inputs) {
     // If the inputs are tracers, trace the original graph
     if (std::any_of(inputs.begin(), inputs.end(), [](auto& in) {
           return in.is_tracer();
@@ -889,13 +899,41 @@ void compile_clear_cache() {
 } // namespace detail
 
 std::function<std::vector<array>(const std::vector<array>&)> compile(
-    const std::function<std::vector<array>(const std::vector<array>&)>& fun,
+    std::function<std::vector<array>(const std::vector<array>&)> fun,
     bool shapeless /* false */) {
-  if (detail::compile_mode() == CompileMode::disabled) {
+  if (detail::skip_compile()) {
     return fun;
   }
   auto fun_id = detail::get_function_address(fun);
-  return detail::compile(fun, fun_id, shapeless);
+  if (fun_id) {
+    // If the function has an addressable target then no need to manage it's
+    // lifetime
+    return detail::compile(std::move(fun), fun_id, shapeless);
+  } else {
+    auto pfun = std::shared_ptr<
+        std::function<std::vector<array>(const std::vector<array>&)>>(
+        new std::function<std::vector<array>(const std::vector<array>&)>{fun},
+        [](auto p) {
+          detail::compile_erase(reinterpret_cast<std::uintptr_t>(p));
+          delete p;
+        });
+    fun_id = reinterpret_cast<std::uintptr_t>(pfun.get());
+    return detail::compile(
+        [pfun = std::move(pfun)](const auto& inputs) {
+          return (*pfun)(inputs);
+        },
+        fun_id,
+        shapeless);
+  }
+}
+
+std::function<std::vector<array>(const std::vector<array>&)> compile(
+    std::vector<array>(fun)(const std::vector<array>&),
+    bool shapeless /* = false */) {
+  if (detail::skip_compile()) {
+    return fun;
+  }
+  return detail::compile(fun, reinterpret_cast<std::uintptr_t>(fun), shapeless);
 }
 
 void disable_compile() {

mlx/compile.h

@@ -10,9 +10,24 @@ enum class CompileMode { disabled, no_simplify, no_fuse, enabled };
 
 /** Compile takes a function and returns a compiled function. */
 std::function<std::vector<array>(const std::vector<array>&)> compile(
-    const std::function<std::vector<array>(const std::vector<array>&)>& fun,
+    std::function<std::vector<array>(const std::vector<array>&)> fun,
     bool shapeless = false);
 
+std::function<std::vector<array>(const std::vector<array>&)> compile(
+    std::vector<array>(fun)(const std::vector<array>&),
+    bool shapeless = false);
+
+// Convert capture-less lambdas to function pointers.
+template <
+    typename F,
+    typename = std::enable_if_t<
+        std::is_convertible_v<F, decltype(+std::declval<F>())>>>
+std::function<std::vector<array>(const std::vector<array>&)> compile(
+    F&& f,
+    bool shapeless = false) {
+  return compile(+f, shapeless);
+}
+
 /** Globally disable compilation.
  * Setting the environment variable ``MLX_DISABLE_COMPILE`` can also
  * be used to disable compilation.

mlx/compile_impl.h

@@ -9,7 +9,7 @@ namespace mlx::core::detail {
 // This is not part of the general C++ API as calling with a bad id is a bad
 // idea.
 std::function<std::vector<array>(const std::vector<array>&)> compile(
-    const std::function<std::vector<array>(const std::vector<array>&)>& fun,
+    std::function<std::vector<array>(const std::vector<array>&)> fun,
     std::uintptr_t fun_id,
     bool shapeless = false,
     std::vector<uint64_t> constants = {});

tests/compile_tests.cpp

@@ -730,3 +730,53 @@ TEST_CASE("test compile change streams") {
   out = cfun({array(1.0f), array(2.0f)})[0];
   CHECK_EQ(out.primitive().stream(), s);
 }
+
+TEST_CASE("test compile lambda") {
+  auto fun = [](const std::vector<array>& inputs) {
+    return std::vector<array>{abs(inputs[0])};
+  };
+
+  auto out = compile(fun)({array(-1)});
+  CHECK_EQ(out[0].item<int>(), 1);
+
+  decltype(compile(nullptr)) c_local_fun;
+  {
+    auto local_fun = [](const std::vector<array>& inputs) {
+      return std::vector<array>{abs(inputs[0])};
+    };
+    c_local_fun = compile(local_fun);
+  }
+
+  // This is ok even though local_fun is out of scope
+  out = c_local_fun({array(-1)});
+  CHECK_EQ(out[0].item<int>(), 1);
+
+  {
+    int x = 2;
+    auto local_fun = [x](const std::vector<array>& inputs) {
+      return std::vector<array>{inputs[0] + x};
+    };
+    c_local_fun = compile(local_fun);
+  }
+  // Also ok even though local_fun is out of scope.
+  out = c_local_fun({array(0)});
+  CHECK_EQ(out[0].item<int>(), 2);
+
+  int x = 2;
+  auto fun_with_capture = [&x](const std::vector<array>& inputs) {
+    return std::vector<array>{inputs[0] + x};
+  };
+  auto cfun = compile(fun_with_capture);
+  out = cfun({array(0)});
+  CHECK_EQ(out[0].item<int>(), 2);
+
+  // Doesn't recompile
+  x = 3;
+  out = cfun({array(0)});
+  CHECK_EQ(out[0].item<int>(), 2);
+
+  // Recompiles
+  auto cfun2 = compile(fun_with_capture);
+  out = cfun2({array(0)});
+  CHECK_EQ(out[0].item<int>(), 3);
+}