Specialized function dispatchers for very simple functions (#944)

Nanobind previously distinguished between a "complex" and a "simple"
function dispatcher. This PR adds variants of the simple dispatcher that
further specializes to 0 and 1-argument functions without overloads. One
common class of functions that benefits are property getters.

The speedup is pretty small (~2%), but we will take it :-).

src/nb_func.cpp

@@ -29,6 +29,10 @@ NAMESPACE_BEGIN(detail)
 // Forward/external declarations
 extern Buffer buf;
 
+static PyObject *nb_func_vectorcall_simple_0(PyObject *, PyObject *const *,
+                                             size_t, PyObject *) noexcept;
+static PyObject *nb_func_vectorcall_simple_1(PyObject *, PyObject *const *,
+                                             size_t, PyObject *) noexcept;
 static PyObject *nb_func_vectorcall_simple(PyObject *, PyObject *const *,
                                            size_t, PyObject *) noexcept;
 static PyObject *nb_func_vectorcall_complex(PyObject *, PyObject *const *,
@@ -335,8 +339,20 @@ PyObject *nb_func_new(const void *in_) noexcept {
 
     func->max_nargs = max_nargs;
     func->complex_call = complex_call;
-    func->vectorcall = complex_call ? nb_func_vectorcall_complex
-                                    : nb_func_vectorcall_simple;
+
+
+    PyObject* (*vectorcall)(PyObject *, PyObject * const*, size_t, PyObject *);
+    if (complex_call) {
+        vectorcall = nb_func_vectorcall_complex;
+    } else {
+        if (f->nargs == 0 && !prev_overloads)
+            vectorcall = nb_func_vectorcall_simple_0;
+        else if (f->nargs == 1 && !prev_overloads)
+            vectorcall = nb_func_vectorcall_simple_1;
+        else
+            vectorcall = nb_func_vectorcall_simple;
+    }
+    func->vectorcall = vectorcall;
 
 #if !defined(NB_FREE_THREADED)
     // Register the function
@@ -954,6 +970,104 @@ static PyObject *nb_func_vectorcall_simple(PyObject *self,
     return result;
 }
 
+/// Simplified nb_func_vectorcall variant for non-overloaded functions with 0 args
+static PyObject *nb_func_vectorcall_simple_0(PyObject *self,
+                                             PyObject *const *args_in,
+                                             size_t nargsf,
+                                             PyObject *kwargs_in) noexcept {
+    func_data *fr = nb_func_data(self);
+    const size_t nargs_in = (size_t) NB_VECTORCALL_NARGS(nargsf);
+
+    // Handler routine that will be invoked in case of an error condition
+    PyObject *(*error_handler)(PyObject *, PyObject *const *, size_t,
+                               PyObject *) noexcept = nullptr;
+
+    PyObject *result = nullptr;
+
+    if (kwargs_in == nullptr && nargs_in == 0) {
+        try {
+            result = fr->impl((void *) fr->capture, (PyObject **) args_in,
+                              nullptr, (rv_policy) (fr->flags & 0b111), nullptr);
+            if (result == NB_NEXT_OVERLOAD)
+                error_handler = nb_func_error_overload;
+            else if (!result)
+                error_handler = nb_func_error_noconvert;
+        } catch (builtin_exception &e) {
+            if (!set_builtin_exception_status(e))
+                error_handler = nb_func_error_overload;
+        } catch (python_error &e) {
+            e.restore();
+        } catch (...) {
+            nb_func_convert_cpp_exception();
+        }
+    } else {
+        error_handler = nb_func_error_overload;
+    }
+
+    if (NB_UNLIKELY(error_handler))
+        result = error_handler(self, args_in, nargs_in, kwargs_in);
+
+    return result;
+}
+
+/// Simplified nb_func_vectorcall variant for non-overloaded functions with 1 arg
+static PyObject *nb_func_vectorcall_simple_1(PyObject *self,
+                                             PyObject *const *args_in,
+                                             size_t nargsf,
+                                             PyObject *kwargs_in) noexcept {
+    func_data *fr = nb_func_data(self);
+    const size_t nargs_in = (size_t) NB_VECTORCALL_NARGS(nargsf);
+    bool is_constructor = fr->flags & (uint32_t) func_flags::is_constructor;
+
+    // Handler routine that will be invoked in case of an error condition
+    PyObject *(*error_handler)(PyObject *, PyObject *const *, size_t,
+                               PyObject *) noexcept = nullptr;
+
+    PyObject *result = nullptr;
+
+    if (kwargs_in == nullptr && nargs_in == 1 && args_in[0] != Py_None) {
+        PyObject *arg = args_in[0];
+        cleanup_list cleanup(arg);
+        uint8_t args_flags[1] = {
+            (uint8_t) (is_constructor ? (1 | (uint8_t) cast_flags::construct) : 1)
+        };
+
+        try {
+            result = fr->impl((void *) fr->capture, (PyObject **) args_in,
+                              args_flags, (rv_policy) (fr->flags & 0b111), &cleanup);
+            if (result == NB_NEXT_OVERLOAD) {
+                error_handler = nb_func_error_overload;
+            } else if (!result) {
+                error_handler = nb_func_error_noconvert;
+            } else if (is_constructor) {
+                nb_inst *arg_nb = (nb_inst *) arg;
+                arg_nb->destruct = true;
+                arg_nb->state = nb_inst::state_ready;
+                if (NB_UNLIKELY(arg_nb->intrusive))
+                    nb_type_data(Py_TYPE(arg))
+                        ->set_self_py(inst_ptr(arg_nb), arg);
+            }
+        } catch (builtin_exception &e) {
+            if (!set_builtin_exception_status(e))
+                error_handler = nb_func_error_overload;
+        } catch (python_error &e) {
+            e.restore();
+        } catch (...) {
+            nb_func_convert_cpp_exception();
+        }
+
+        if (NB_UNLIKELY(cleanup.used()))
+            cleanup.release();
+    } else {
+        error_handler = nb_func_error_overload;
+    }
+
+    if (NB_UNLIKELY(error_handler))
+        result = error_handler(self, args_in, nargs_in, kwargs_in);
+
+    return result;
+}
+
 static PyObject *nb_bound_method_vectorcall(PyObject *self,
                                             PyObject *const *args_in,
                                             size_t nargsf,