merge default

2 files changed, 46 insertions, 1 deletions

diff --git a/rpython/rlib/objectmodel.py b/rpython/rlib/objectmodel.py
index 5faa6850fd..37ccdf9cb1 100644
--- a/rpython/rlib/objectmodel.py
+++ b/rpython/rlib/objectmodel.py
@@ -1067,3 +1067,13 @@ 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..5f14528556 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 +852,37 @@ 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):
+        def __init__(self, x):
+            self.x = x + 1
+
+    @dont_inline
+    def allocate_MyClass(x):
+        return MyClass(x)
+
+    def f(x):
+        # this fails because the allocation of MyClass can't be
+        # constant-folded (because it's inside a @dont_inline function)
+        return allocate_MyClass(x).x
+
+    def g(x):
+        # this works because MyClass is constant folded, so the GC transformer
+        # never sees a malloc(MyClass)
+        return MyClass(x).x
+
+    # test what happens if MyClass escapes
+    with py.test.raises(GCTransformError) as exc:
+        c_compile(f, [int])
+    assert '[function allocate_MyClass]' in str(exc)
+    assert 'was marked as @never_allocate' in str(exc)
+
+    # test that it works in the "normal" case
+    compiled_g = c_compile(g, [int])
+    assert compiled_g(41) == 42