gh-90370: Argument Clinic: avoid temporary tuple creation for varargs (#126064)

Avoid temporary tuple creation when all arguments either positional-only
or vararg.

Objects/setobject.c and Modules/gcmodule.c adapted. This fixes slight
performance regression for set methods, introduced by gh-115112.

2 files changed, 24 insertions, 26 deletions

diff --git a/Tools/clinic/libclinic/clanguage.py b/Tools/clinic/libclinic/clanguage.py
index 73d4783..32aba81 100644
--- a/Tools/clinic/libclinic/clanguage.py
+++ b/Tools/clinic/libclinic/clanguage.py
@@ -15,7 +15,7 @@ from libclinic.function import (
     Module, Class, Function, Parameter,
     permute_optional_groups,
     GETTER, SETTER, METHOD_INIT)
-from libclinic.converters import self_converter
+from libclinic.converters import defining_class_converter, self_converter
 from libclinic.parse_args import ParseArgsCodeGen
 if TYPE_CHECKING:
     from libclinic.app import Clinic
@@ -396,6 +396,12 @@ class CLanguage(Language):
         first_optional = len(selfless)
         positional = selfless and selfless[-1].is_positional_only()
         has_option_groups = False
+        requires_defining_class = (len(selfless)
+                                   and isinstance(selfless[0].converter,
+                                                  defining_class_converter))
+        pass_vararg_directly = (all(p.is_positional_only() or p.is_vararg()
+                                    for p in selfless)
+                                and not requires_defining_class)
 
         # offset i by -1 because first_optional needs to ignore self
         for i, p in enumerate(parameters, -1):
@@ -404,7 +410,7 @@ class CLanguage(Language):
             if (i != -1) and (p.default is not unspecified):
                 first_optional = min(first_optional, i)
 
-            if p.is_vararg():
+            if p.is_vararg() and not pass_vararg_directly:
                 data.cleanup.append(f"Py_XDECREF({c.parser_name});")
 
             # insert group variable
@@ -418,6 +424,11 @@ class CLanguage(Language):
                     data.impl_parameters.append("int " + group_name)
                     has_option_groups = True
 
+            if p.is_vararg() and pass_vararg_directly:
+                data.impl_arguments.append('nvararg')
+                data.impl_parameters.append('Py_ssize_t nargs')
+                p.converter.type = 'PyObject *const *'
+
             c.render(p, data)
 
         if has_option_groups and (not positional):
diff --git a/Tools/clinic/libclinic/parse_args.py b/Tools/clinic/libclinic/parse_args.py
index 96c9b91..559d4fb 100644
--- a/Tools/clinic/libclinic/parse_args.py
+++ b/Tools/clinic/libclinic/parse_args.py
@@ -469,7 +469,11 @@ class ParseArgsCodeGen:
                 nargs = 'PyTuple_GET_SIZE(args)'
                 argname_fmt = 'PyTuple_GET_ITEM(args, %d)'
 
-        left_args = f"{nargs} - {self.max_pos}"
+        if self.vararg != NO_VARARG:
+            self.declarations = f"Py_ssize_t nvararg = {nargs} - {self.max_pos};"
+        else:
+            self.declarations = ""
+
         max_args = NO_VARARG if (self.vararg != NO_VARARG) else self.max_pos
         if self.limited_capi:
             parser_code = []
@@ -518,30 +522,13 @@ class ParseArgsCodeGen:
         use_parser_code = True
         for i, p in enumerate(self.parameters):
             if p.is_vararg():
+                var = p.converter.parser_name
                 if self.fastcall:
-                    parser_code.append(libclinic.normalize_snippet("""
-                        %s = PyTuple_New(%s);
-                        if (!%s) {{
-                            goto exit;
-                        }}
-                        for (Py_ssize_t i = 0; i < %s; ++i) {{
-                            PyTuple_SET_ITEM(%s, i, Py_NewRef(args[%d + i]));
-                        }}
-                        """ % (
-                            p.converter.parser_name,
-                            left_args,
-                            p.converter.parser_name,
-                            left_args,
-                            p.converter.parser_name,
-                            self.max_pos
-                        ), indent=4))
+                    code = f"{var} = args + {self.vararg};"
                 else:
-                    parser_code.append(libclinic.normalize_snippet("""
-                        %s = PyTuple_GetSlice(%d, -1);
-                        """ % (
-                            p.converter.parser_name,
-                            self.max_pos
-                        ), indent=4))
+                    code = f"{var} = _PyTuple_CAST(args)->ob_item;"
+                formatted_code = libclinic.normalize_snippet(code, indent=4)
+                parser_code.append(formatted_code)
                 continue
 
             displayname = p.get_displayname(i+1)
@@ -588,7 +575,7 @@ class ParseArgsCodeGen:
                         goto exit;
                     }}
                     """, indent=4)]
-        self.parser_body(*parser_code)
+        self.parser_body(*parser_code, declarations=self.declarations)
 
     def parse_general(self, clang: CLanguage) -> None:
         parsearg: str | None