introduce a @never_allocate class decorator, which ensure that a certain RPython class is never actually instantiated at runtime. Useful to ensure that e.g. it's always constant-folded away

4 files changed, 50 insertions, 3 deletions

diff --git a/rpython/memory/gctransform/transform.py b/rpython/memory/gctransform/transform.py
index d793f91ef2..d3dd64702f 100644
--- a/rpython/memory/gctransform/transform.py
+++ b/rpython/memory/gctransform/transform.py
@@ -19,6 +19,8 @@ from rpython.rtyper.lltypesystem.lloperation import llop
 from rpython.translator.simplify import cleanup_graph
 from rpython.memory.gctransform.log import log
 
+class GCTransformError(Exception):
+    pass
 
 class GcHighLevelOp(object):
     def __init__(self, gct, op, index, llops):
@@ -236,8 +238,12 @@ class BaseGCTransformer(object):
             inserted_empty_startblock = True
         is_borrowed = self.compute_borrowed_vars(graph)
 
-        for block in graph.iterblocks():
-            self.transform_block(block, is_borrowed)
+        try:
+            for block in graph.iterblocks():
+                self.transform_block(block, is_borrowed)
+        except GCTransformError as e:
+            e.args = ('[function %s]: %s' % (graph.name, e.message),)
+            raise
 
         for link, livecounts in self.links_to_split.iteritems():
             llops = LowLevelOpList()
@@ -519,6 +525,12 @@ class GCTransformer(BaseGCTransformer):
 
     def gct_malloc(self, hop, add_flags=None):
         TYPE = hop.spaceop.result.concretetype.TO
+        if TYPE._hints.get('never_allocate'):
+            raise GCTransformError(
+                "struct %s was marked as @never_allocate but a call to malloc() "
+                "was found. This probably means that the corresponding class is "
+                "supposed to be constant-folded away, but for some reason it was not."
+                % TYPE._name)
         assert not TYPE._is_varsize()
         flags = hop.spaceop.args[1].value
         flavor = flags['flavor']
diff --git a/rpython/rlib/objectmodel.py b/rpython/rlib/objectmodel.py
index 8e59b387ec..0b3647ae2c 100644
--- a/rpython/rlib/objectmodel.py
+++ b/rpython/rlib/objectmodel.py
@@ -1058,3 +1058,13 @@ def import_from_mixin(M, special_methods=['__init__', '__del__']):
         target[key] = value
     if immutable_fields:
         target['_immutable_fields_'] = target.get('_immutable_fields_', []) + immutable_fields
+
+def never_allocate(cls):
+    """
+    Class decorator to ensure that a class is NEVER instantiated at runtime.
+
+    Useful e.g for context manager which are expected to be constant-folded
+    away.
+    """
+    cls._rpython_never_allocate_ = True
+    return cls
diff --git a/rpython/rlib/test/test_objectmodel.py b/rpython/rlib/test/test_objectmodel.py
index 6bcb9d60c6..8aef08cdd1 100644
--- a/rpython/rlib/test/test_objectmodel.py
+++ b/rpython/rlib/test/test_objectmodel.py
@@ -7,7 +7,8 @@ from rpython.rlib.objectmodel import (
     resizelist_hint, is_annotation_constant, always_inline, NOT_CONSTANT,
     iterkeys_with_hash, iteritems_with_hash, contains_with_hash,
     setitem_with_hash, getitem_with_hash, delitem_with_hash, import_from_mixin,
-    fetch_translated_config, try_inline, delitem_if_value_is, move_to_end)
+    fetch_translated_config, try_inline, delitem_if_value_is, move_to_end,
+    never_allocate, dont_inline)
 from rpython.translator.translator import TranslationContext, graphof
 from rpython.rtyper.test.tool import BaseRtypingTest
 from rpython.rtyper.test.test_llinterp import interpret
@@ -851,3 +852,24 @@ def test_import_from_mixin_immutable_fields():
         import_from_mixin(C)
 
     assert BA._immutable_fields_ == ['c', 'a']
+
+
+def test_never_allocate():
+    from rpython.translator.c.test.test_genc import compile as c_compile
+    from rpython.memory.gctransform.transform import GCTransformError
+
+    @never_allocate
+    class MyClass(object):
+        pass
+
+    @dont_inline
+    def allocate_MyClass():
+        return MyClass()
+
+    def f():
+        allocate_MyClass()
+
+    with py.test.raises(GCTransformError) as exc:
+        c_compile(f, [])
+    assert '[function allocate_MyClass]' in str(exc)
+    assert 'was marked as @never_allocate' in str(exc)
diff --git a/rpython/rtyper/rclass.py b/rpython/rtyper/rclass.py
index 0d09123617..a98a04649b 100644
--- a/rpython/rtyper/rclass.py
+++ b/rpython/rtyper/rclass.py
@@ -527,6 +527,9 @@ class InstanceRepr(Repr):
             if hints is None:
                 hints = {}
             hints = self._check_for_immutable_hints(hints)
+            if self.classdef.classdesc.get_param('_rpython_never_allocate_'):
+                hints['never_allocate'] = True
+
             kwds = {}
             if self.gcflavor == 'gc':
                 kwds['rtti'] = True