[pypy-svn] pypy default: Merge out-of-line-guards-2. This branch provides a way to specify a field

fijal commits-noreply at bitbucket.org
Sun Apr 17 01:44:22 CEST 2011


Author: Maciej Fijalkowski <fijall at gmail.com>
Branch: 
Changeset: r43414:124ebb7828dd
Date: 2011-04-17 01:43 +0200
http://bitbucket.org/pypy/pypy/changeset/124ebb7828dd/

Log:	Merge out-of-line-guards-2. This branch provides a way to specify a
	field that doesn't change very often by saying

	_immutable_fields_ = ['a?']

	which means when it changes, a new assembler will be compiled.

diff --git a/pypy/rpython/ootypesystem/rclass.py b/pypy/rpython/ootypesystem/rclass.py
--- a/pypy/rpython/ootypesystem/rclass.py
+++ b/pypy/rpython/ootypesystem/rclass.py
@@ -262,6 +262,10 @@
         self.rbase = getinstancerepr(self.rtyper, self.classdef.basedef)
         self.rbase.setup()
 
+        for name, attrdef in selfattrs.iteritems():
+            if not attrdef.readonly and self.is_quasi_immutable(name):
+                ootype.addFields(self.lowleveltype, {'mutable_'+name: OBJECT})
+
         classattributes = {}
         baseInstance = self.lowleveltype._superclass
         classrepr = getclassrepr(self.rtyper, self.classdef)
@@ -476,11 +480,9 @@
         mangled_name = mangle(attr, self.rtyper.getconfig())
         cname = inputconst(ootype.Void, mangled_name)
         self.hook_access_field(vinst, cname, llops, flags)
+        self.hook_setfield(vinst, attr, llops)
         llops.genop('oosetfield', [vinst, cname, vvalue])
 
-    def hook_access_field(self, vinst, cname, llops, flags):
-        pass        # for virtualizables; see rvirtualizable2.py
-
     def rtype_is_true(self, hop):
         vinst, = hop.inputargs(self)
         return hop.genop('oononnull', [vinst], resulttype=ootype.Bool)

diff --git a/pypy/jit/backend/model.py b/pypy/jit/backend/model.py
--- a/pypy/jit/backend/model.py
+++ b/pypy/jit/backend/model.py
@@ -291,6 +291,7 @@
         # that belong to this loop or to a bridge attached to it.
         # Filled by the frontend calling record_faildescr_index().
         self.faildescr_indices = []
+        self.invalidate_positions = []
         debug_start("jit-mem-looptoken-alloc")
         debug_print("allocating Loop #", self.number)
         debug_stop("jit-mem-looptoken-alloc")

diff --git a/pypy/jit/metainterp/history.py b/pypy/jit/metainterp/history.py
--- a/pypy/jit/metainterp/history.py
+++ b/pypy/jit/metainterp/history.py
@@ -791,6 +791,7 @@
     operations = None
     token = None
     call_pure_results = None
+    quasi_immutable_deps = None
 
     def __init__(self, name):
         self.name = name

diff --git a/pypy/rpython/lltypesystem/lltype.py b/pypy/rpython/lltypesystem/lltype.py
--- a/pypy/rpython/lltypesystem/lltype.py
+++ b/pypy/rpython/lltypesystem/lltype.py
@@ -341,13 +341,14 @@
         return _struct(self, n, initialization='example')
 
     def _immutable_field(self, field):
+        if self._hints.get('immutable'):
+            return True
         if 'immutable_fields' in self._hints:
             try:
-                s = self._hints['immutable_fields'].fields[field]
-                return s or True
+                return self._hints['immutable_fields'].fields[field]
             except KeyError:
                 pass
-        return self._hints.get('immutable', False)
+        return False
 
 class RttiStruct(Struct):
     _runtime_type_info = None
@@ -1029,6 +1030,8 @@
         return None   # null pointer
     if type(p._obj0) is int:
         return p      # a pointer obtained by cast_int_to_ptr
+    if getattr(p._obj0, '_carry_around_for_tests', False):
+        return p      # a pointer obtained by cast_instance_to_base_ptr
     container = obj._normalizedcontainer()
     if type(container) is int:
         # this must be an opaque ptr originating from an integer
@@ -1881,8 +1884,8 @@
         if self.__class__ is not other.__class__:
             return NotImplemented
         if hasattr(self, 'container') and hasattr(other, 'container'):
-            obj1 = self.container._normalizedcontainer()
-            obj2 = other.container._normalizedcontainer()
+            obj1 = self._normalizedcontainer()
+            obj2 = other._normalizedcontainer()
             return obj1 == obj2
         else:
             return self is other
@@ -1906,6 +1909,8 @@
             # an integer, cast to a ptr, cast to an opaque    
             if type(self.container) is int:
                 return self.container
+            if getattr(self.container, '_carry_around_for_tests', False):
+                return self.container
             return self.container._normalizedcontainer()
         else:
             return _parentable._normalizedcontainer(self)

diff --git a/pypy/rpython/lltypesystem/ll2ctypes.py b/pypy/rpython/lltypesystem/ll2ctypes.py
--- a/pypy/rpython/lltypesystem/ll2ctypes.py
+++ b/pypy/rpython/lltypesystem/ll2ctypes.py
@@ -578,6 +578,7 @@
 _all_callbacks_results = []
 _int2obj = {}
 _callback_exc_info = None
+_opaque_objs = [None]
 
 def get_rtyper():
     llinterp = LLInterpreter.current_interpreter
@@ -616,6 +617,10 @@
             T = lltype.Ptr(lltype.typeOf(container))
             # otherwise it came from integer and we want a c_void_p with
             # the same valu
+            if getattr(container, 'llopaque', None):
+                no = len(_opaque_objs)
+                _opaque_objs.append(container)
+                return no * 2 + 1
         else:
             container = llobj._obj
         if isinstance(T.TO, lltype.FuncType):
@@ -764,10 +769,14 @@
     if isinstance(T, lltype.Typedef):
         T = T.OF
     if isinstance(T, lltype.Ptr):
-        if not cobj or not ctypes.cast(cobj, ctypes.c_void_p).value:   # NULL pointer
+        ptrval = ctypes.cast(cobj, ctypes.c_void_p).value
+        if not cobj or not ptrval:   # NULL pointer
             # CFunctionType.__nonzero__ is broken before Python 2.6
             return lltype.nullptr(T.TO)
         if isinstance(T.TO, lltype.Struct):
+            if ptrval & 1: # a tagged pointer
+                gcref = _opaque_objs[ptrval // 2].hide()
+                return lltype.cast_opaque_ptr(T, gcref)
             REAL_TYPE = T.TO
             if T.TO._arrayfld is not None:
                 carray = getattr(cobj.contents, T.TO._arrayfld)
@@ -1228,7 +1237,9 @@
         return not self == other
 
     def _cast_to_ptr(self, PTRTYPE):
-         return force_cast(PTRTYPE, self.intval)
+        if self.intval & 1:
+            return _opaque_objs[self.intval // 2]
+        return force_cast(PTRTYPE, self.intval)
 
 ##     def _cast_to_int(self):
 ##         return self.intval

diff --git a/pypy/jit/metainterp/test/test_quasiimmut.py b/pypy/jit/metainterp/test/test_quasiimmut.py
new file mode 100644
--- /dev/null
+++ b/pypy/jit/metainterp/test/test_quasiimmut.py
@@ -0,0 +1,266 @@
+
+import py
+
+from pypy.rpython.lltypesystem import lltype, llmemory, rclass
+from pypy.rpython.rclass import FieldListAccessor, IR_QUASI_IMMUTABLE
+from pypy.jit.metainterp import typesystem
+from pypy.jit.metainterp.quasiimmut import QuasiImmut
+from pypy.jit.metainterp.quasiimmut import get_current_qmut_instance
+from pypy.jit.metainterp.test.test_basic import LLJitMixin
+from pypy.jit.codewriter.policy import StopAtXPolicy
+from pypy.rlib.jit import JitDriver, dont_look_inside
+
+
+def test_get_current_qmut_instance():
+    accessor = FieldListAccessor()
+    accessor.initialize(None, {'inst_x': IR_QUASI_IMMUTABLE})
+    STRUCT = lltype.GcStruct('Foo', ('inst_x', lltype.Signed),
+                             ('mutate_x', rclass.OBJECTPTR),
+                             hints={'immutable_fields': accessor})
+    foo = lltype.malloc(STRUCT, zero=True)
+    foo.inst_x = 42
+    assert not foo.mutate_x
+
+    class FakeCPU:
+        ts = typesystem.llhelper
+
+        def bh_getfield_gc_r(self, gcref, fielddescr):
+            assert fielddescr == mutatefielddescr
+            foo = lltype.cast_opaque_ptr(lltype.Ptr(STRUCT), gcref)
+            result = foo.mutate_x
+            return lltype.cast_opaque_ptr(llmemory.GCREF, result)
+
+        def bh_setfield_gc_r(self, gcref, fielddescr, newvalue_gcref):
+            assert fielddescr == mutatefielddescr
+            foo = lltype.cast_opaque_ptr(lltype.Ptr(STRUCT), gcref)
+            newvalue = lltype.cast_opaque_ptr(rclass.OBJECTPTR, newvalue_gcref)
+            foo.mutate_x = newvalue
+
+    cpu = FakeCPU()
+    mutatefielddescr = ('fielddescr', STRUCT, 'mutate_x')
+
+    foo_gcref = lltype.cast_opaque_ptr(llmemory.GCREF, foo)
+    qmut1 = get_current_qmut_instance(cpu, foo_gcref, mutatefielddescr)
+    assert isinstance(qmut1, QuasiImmut)
+    qmut2 = get_current_qmut_instance(cpu, foo_gcref, mutatefielddescr)
+    assert qmut1 is qmut2
+
+
+class QuasiImmutTests(object):
+
+    def test_simple_1(self):
+        myjitdriver = JitDriver(greens=['foo'], reds=['x', 'total'])
+        class Foo:
+            _immutable_fields_ = ['a?']
+            def __init__(self, a):
+                self.a = a
+        def f(a, x):
+            foo = Foo(a)
+            total = 0
+            while x > 0:
+                myjitdriver.jit_merge_point(foo=foo, x=x, total=total)
+                # read a quasi-immutable field out of a Constant
+                total += foo.a
+                x -= 1
+            return total
+        #
+        res = self.meta_interp(f, [100, 7])
+        assert res == 700
+        self.check_loops(getfield_gc=0, everywhere=True)
+        #
+        from pypy.jit.metainterp.warmspot import get_stats
+        loops = get_stats().loops
+        for loop in loops:
+            assert len(loop.quasi_immutable_deps) == 1
+            assert isinstance(loop.quasi_immutable_deps.keys()[0], QuasiImmut)
+
+    def test_nonopt_1(self):
+        myjitdriver = JitDriver(greens=[], reds=['x', 'total', 'lst'])
+        class Foo:
+            _immutable_fields_ = ['a?']
+            def __init__(self, a):
+                self.a = a
+        def setup(x):
+            return [Foo(100 + i) for i in range(x)]
+        def f(a, x):
+            lst = setup(x)
+            total = 0
+            while x > 0:
+                myjitdriver.jit_merge_point(lst=lst, x=x, total=total)
+                # read a quasi-immutable field out of a variable
+                x -= 1
+                total += lst[x].a
+            return total
+        #
+        assert f(100, 7) == 721
+        res = self.meta_interp(f, [100, 7])
+        assert res == 721
+        self.check_loops(getfield_gc=1)
+        #
+        from pypy.jit.metainterp.warmspot import get_stats
+        loops = get_stats().loops
+        for loop in loops:
+            assert loop.quasi_immutable_deps is None
+
+    def test_change_during_tracing_1(self):
+        myjitdriver = JitDriver(greens=['foo'], reds=['x', 'total'])
+        class Foo:
+            _immutable_fields_ = ['a?']
+            def __init__(self, a):
+                self.a = a
+        @dont_look_inside
+        def residual_call(foo):
+            foo.a += 1
+        def f(a, x):
+            foo = Foo(a)
+            total = 0
+            while x > 0:
+                myjitdriver.jit_merge_point(foo=foo, x=x, total=total)
+                # read a quasi-immutable field out of a Constant
+                total += foo.a
+                residual_call(foo)
+                x -= 1
+            return total
+        #
+        assert f(100, 7) == 721
+        res = self.meta_interp(f, [100, 7])
+        assert res == 721
+        self.check_loops(getfield_gc=1)
+
+    def test_change_during_tracing_2(self):
+        myjitdriver = JitDriver(greens=['foo'], reds=['x', 'total'])
+        class Foo:
+            _immutable_fields_ = ['a?']
+            def __init__(self, a):
+                self.a = a
+        @dont_look_inside
+        def residual_call(foo, difference):
+            foo.a += difference
+        def f(a, x):
+            foo = Foo(a)
+            total = 0
+            while x > 0:
+                myjitdriver.jit_merge_point(foo=foo, x=x, total=total)
+                # read a quasi-immutable field out of a Constant
+                total += foo.a
+                residual_call(foo, +1)
+                residual_call(foo, -1)
+                x -= 1
+            return total
+        #
+        assert f(100, 7) == 700
+        res = self.meta_interp(f, [100, 7])
+        assert res == 700
+        self.check_loops(getfield_gc=1)
+
+    def test_change_invalidate_reentering(self):
+        myjitdriver = JitDriver(greens=['foo'], reds=['x', 'total'])
+        class Foo:
+            _immutable_fields_ = ['a?']
+            def __init__(self, a):
+                self.a = a
+        def f(foo, x):
+            total = 0
+            while x > 0:
+                myjitdriver.jit_merge_point(foo=foo, x=x, total=total)
+                # read a quasi-immutable field out of a Constant
+                total += foo.a
+                x -= 1
+            return total
+        def g(a, x):
+            foo = Foo(a)
+            res1 = f(foo, x)
+            foo.a += 1          # invalidation, while the jit is not running
+            res2 = f(foo, x)    # should still mark the loop as invalid
+            return res1 * 1000 + res2
+        #
+        assert g(100, 7) == 700707
+        res = self.meta_interp(g, [100, 7])
+        assert res == 700707
+        self.check_loops(getfield_gc=0)
+
+    def test_invalidate_while_running(self):
+        jitdriver = JitDriver(greens=['foo'], reds=['i', 'total'])
+
+        class Foo(object):
+            _immutable_fields_ = ['a?']
+            def __init__(self, a):
+                self.a = a
+
+        def external(foo, v):
+            if v:
+                foo.a = 2
+
+        def f(foo):
+            i = 0
+            total = 0
+            while i < 10:
+                jitdriver.jit_merge_point(i=i, foo=foo, total=total)
+                external(foo, i > 7)
+                i += 1
+                total += foo.a
+            return total
+
+        def g():
+            return f(Foo(1))
+
+        assert self.meta_interp(g, [], policy=StopAtXPolicy(external)) == g()
+
+    def test_invalidate_by_setfield(self):
+        py.test.skip("Not implemented")
+        jitdriver = JitDriver(greens=['bc', 'foo'], reds=['i', 'total'])
+
+        class Foo(object):
+            _immutable_fields_ = ['a?']
+            def __init__(self, a):
+                self.a = a
+
+        def f(foo, bc):
+            i = 0
+            total = 0
+            while i < 10:
+                jitdriver.jit_merge_point(bc=bc, i=i, foo=foo, total=total)
+                if bc == 0:
+                    f(foo, 1)
+                if bc == 1:
+                    foo.a = int(i > 5)
+                i += 1
+                total += foo.a
+            return total
+
+        def g():
+            return f(Foo(1), 0)
+
+        assert self.meta_interp(g, []) == g()
+
+    def test_invalidate_bridge(self):
+        jitdriver = JitDriver(greens=['foo'], reds=['i', 'total'])
+
+        class Foo(object):
+            _immutable_fields_ = ['a?']
+
+        def f(foo):
+            i = 0
+            total = 0
+            while i < 10:
+                jitdriver.jit_merge_point(i=i, total=total, foo=foo)
+                if i > 5:
+                    total += foo.a
+                else:
+                    total += 2*foo.a
+                i += 1
+            return total
+
+        def main():
+            foo = Foo()
+            foo.a = 1
+            total = f(foo)
+            foo.a = 2
+            total += f(foo)
+            return total
+
+        res = self.meta_interp(main, [])
+        assert res == main()
+
+class TestLLtypeGreenFieldsTests(QuasiImmutTests, LLJitMixin):
+    pass

diff --git a/pypy/jit/metainterp/quasiimmut.py b/pypy/jit/metainterp/quasiimmut.py
new file mode 100644
--- /dev/null
+++ b/pypy/jit/metainterp/quasiimmut.py
@@ -0,0 +1,116 @@
+import weakref
+from pypy.rpython.rclass import IR_QUASI_IMMUTABLE
+from pypy.rpython.lltypesystem import lltype, rclass
+from pypy.rpython.annlowlevel import cast_base_ptr_to_instance
+from pypy.jit.metainterp.history import AbstractDescr
+
+
+def is_quasi_immutable(STRUCT, fieldname):
+    imm_fields = STRUCT._hints.get('immutable_fields')
+    return (imm_fields is not None and
+            imm_fields.fields.get(fieldname) is IR_QUASI_IMMUTABLE)
+
+def get_mutate_field_name(fieldname):
+    if fieldname.startswith('inst_'):    # lltype
+        return 'mutate_' + fieldname[5:]
+    elif fieldname.startswith('o'):      # ootype
+        return 'mutate_' + fieldname[1:]
+    else:
+        raise AssertionError(fieldname)
+
+def get_current_qmut_instance(cpu, gcref, mutatefielddescr):
+    """Returns the current QuasiImmut instance in the field,
+    possibly creating one.
+    """
+    # XXX this is broken on x86
+    qmut_gcref = cpu.bh_getfield_gc_r(gcref, mutatefielddescr)
+    if qmut_gcref:
+        qmut = QuasiImmut.show(cpu, qmut_gcref)
+    else:
+        qmut = QuasiImmut(cpu)
+        cpu.bh_setfield_gc_r(gcref, mutatefielddescr, qmut.hide())
+    return qmut
+
+def make_invalidation_function(STRUCT, mutatefieldname):
+    #
+    def _invalidate_now(p):
+        qmut_ptr = getattr(p, mutatefieldname)
+        setattr(p, mutatefieldname, lltype.nullptr(rclass.OBJECT))
+        qmut = cast_base_ptr_to_instance(QuasiImmut, qmut_ptr)
+        qmut.invalidate()
+    _invalidate_now._dont_inline_ = True
+    #
+    def invalidation(p):
+        if getattr(p, mutatefieldname):
+            _invalidate_now(p)
+    #
+    return invalidation
+
+
+class QuasiImmut(object):
+    llopaque = True
+    
+    def __init__(self, cpu):
+        self.cpu = cpu
+        # list of weakrefs to the LoopTokens that must be invalidated if
+        # this value ever changes
+        self.looptokens_wrefs = []
+        self.compress_limit = 30
+
+    def hide(self):
+        qmut_ptr = self.cpu.ts.cast_instance_to_base_ref(self)
+        return self.cpu.ts.cast_to_ref(qmut_ptr)
+
+    @staticmethod
+    def show(cpu, qmut_gcref):
+        qmut_ptr = cpu.ts.cast_to_baseclass(qmut_gcref)
+        return cast_base_ptr_to_instance(QuasiImmut, qmut_ptr)
+
+    def register_loop_token(self, wref_looptoken):
+        if len(self.looptokens_wrefs) > self.compress_limit:
+            self.compress_looptokens_list()
+        self.looptokens_wrefs.append(wref_looptoken)
+
+    def compress_looptokens_list(self):
+        self.looptokens_wrefs = [wref for wref in self.looptokens_wrefs
+                                      if wref() is not None]
+        self.compress_limit = (len(self.looptokens_wrefs) + 15) * 2
+
+    def invalidate(self):
+        # When this is called, all the loops that we record become
+        # invalid and must not be called again, nor returned to.
+        wrefs = self.looptokens_wrefs
+        self.looptokens_wrefs = []
+        for wref in wrefs:
+            looptoken = wref()
+            if looptoken is not None:
+                self.cpu.invalidate_loop(looptoken)
+
+
+class QuasiImmutDescr(AbstractDescr):
+    def __init__(self, cpu, structbox, fielddescr, mutatefielddescr):
+        self.cpu = cpu
+        self.structbox = structbox
+        self.fielddescr = fielddescr
+        self.mutatefielddescr = mutatefielddescr
+        gcref = structbox.getref_base()
+        self.qmut = get_current_qmut_instance(cpu, gcref, mutatefielddescr)
+        self.constantfieldbox = self.get_current_constant_fieldvalue()
+
+    def get_current_constant_fieldvalue(self):
+        from pypy.jit.metainterp import executor
+        from pypy.jit.metainterp.resoperation import rop
+        fieldbox = executor.execute(self.cpu, None, rop.GETFIELD_GC,
+                                    self.fielddescr, self.structbox)
+        return fieldbox.constbox()
+
+    def is_still_valid(self):
+        cpu = self.cpu
+        gcref = self.structbox.getref_base()
+        qmut = get_current_qmut_instance(cpu, gcref, self.mutatefielddescr)
+        if qmut is not self.qmut:
+            return False
+        else:
+            currentbox = self.get_current_constant_fieldvalue()
+            assert self.constantfieldbox.same_constant(currentbox)
+            return True

diff --git a/pypy/jit/metainterp/test/test_virtualizable.py b/pypy/jit/metainterp/test/test_virtualizable.py
--- a/pypy/jit/metainterp/test/test_virtualizable.py
+++ b/pypy/jit/metainterp/test/test_virtualizable.py
@@ -2,6 +2,7 @@
 from pypy.rpython.extregistry import ExtRegistryEntry
 from pypy.rpython.lltypesystem import lltype, lloperation, rclass, llmemory
 from pypy.rpython.annlowlevel import llhelper
+from pypy.rpython.rclass import IR_IMMUTABLE, IR_ARRAY_IMMUTABLE
 from pypy.jit.codewriter.policy import StopAtXPolicy
 from pypy.jit.codewriter import heaptracker
 from pypy.rlib.jit import JitDriver, hint, dont_look_inside
@@ -45,7 +46,7 @@
         ('inst_node', lltype.Ptr(LLtypeMixin.NODE)),
         hints = {'virtualizable2_accessor': FieldListAccessor()})
     XY._hints['virtualizable2_accessor'].initialize(
-        XY, {'inst_x' : "", 'inst_node' : ""})
+        XY, {'inst_x' : IR_IMMUTABLE, 'inst_node' : IR_IMMUTABLE})
 
     xy_vtable = lltype.malloc(rclass.OBJECT_VTABLE, immortal=True)
     heaptracker.set_testing_vtable_for_gcstruct(XY, xy_vtable, 'XY')
@@ -210,7 +211,8 @@
         ('inst_l2', lltype.Ptr(lltype.GcArray(lltype.Signed))),
         hints = {'virtualizable2_accessor': FieldListAccessor()})
     XY2._hints['virtualizable2_accessor'].initialize(
-        XY2, {'inst_x' : "", 'inst_l1' : "[*]", 'inst_l2' : "[*]"})
+        XY2, {'inst_x' : IR_IMMUTABLE,
+              'inst_l1' : IR_ARRAY_IMMUTABLE, 'inst_l2' : IR_ARRAY_IMMUTABLE})
 
     xy2_vtable = lltype.malloc(rclass.OBJECT_VTABLE, immortal=True)
     heaptracker.set_testing_vtable_for_gcstruct(XY2, xy2_vtable, 'XY2')

diff --git a/pypy/jit/metainterp/optimizeopt/__init__.py b/pypy/jit/metainterp/optimizeopt/__init__.py
--- a/pypy/jit/metainterp/optimizeopt/__init__.py
+++ b/pypy/jit/metainterp/optimizeopt/__init__.py
@@ -41,7 +41,8 @@
                 # during preamble but to keep it during the loop
                 optimizations.append(o)
 
-    if 'rewrite' not in enable_opts or 'virtualize' not in enable_opts:
+    if ('rewrite' not in enable_opts or 'virtualize' not in enable_opts
+        or 'heap' not in enable_opts):
         optimizations.append(OptSimplify())
 
     if inline_short_preamble:

diff --git a/pypy/jit/codewriter/jtransform.py b/pypy/jit/codewriter/jtransform.py
--- a/pypy/jit/codewriter/jtransform.py
+++ b/pypy/jit/codewriter/jtransform.py
@@ -7,8 +7,9 @@
 from pypy.jit.codewriter.flatten import ListOfKind, IndirectCallTargets
 from pypy.jit.codewriter import support, heaptracker, longlong
 from pypy.jit.codewriter.effectinfo import EffectInfo
-from pypy.jit.codewriter.policy import log
+from pypy.jit.codewriter.policy import log, check_skip_operation
 from pypy.jit.metainterp.typesystem import deref, arrayItem
+from pypy.jit.metainterp import quasiimmut
 from pypy.rlib import objectmodel
 from pypy.rlib.jit import _we_are_jitted
 from pypy.translator.simplify import get_funcobj
@@ -561,7 +562,8 @@
                                                 arraydescr)
             return []
         # check for _immutable_fields_ hints
-        if v_inst.concretetype.TO._immutable_field(c_fieldname.value):
+        immut = v_inst.concretetype.TO._immutable_field(c_fieldname.value)
+        if immut:
             if (self.callcontrol is not None and
                 self.callcontrol.could_be_green_field(v_inst.concretetype.TO,
                                                       c_fieldname.value)):
@@ -574,10 +576,21 @@
         descr = self.cpu.fielddescrof(v_inst.concretetype.TO,
                                       c_fieldname.value)
         kind = getkind(RESULT)[0]
-        return SpaceOperation('getfield_%s_%s%s' % (argname, kind, pure),
-                              [v_inst, descr], op.result)
+        op1 = SpaceOperation('getfield_%s_%s%s' % (argname, kind, pure),
+                             [v_inst, descr], op.result)
+        #
+        if immut is quasiimmut.IR_QUASI_IMMUTABLE:
+            descr1 = self.cpu.fielddescrof(
+                v_inst.concretetype.TO,
+                quasiimmut.get_mutate_field_name(c_fieldname.value))
+            op1 = [SpaceOperation('-live-', [], None),
+                   SpaceOperation('record_quasiimmut_field',
+                                  [v_inst, descr, descr1], None),
+                   op1]
+        return op1
 
     def rewrite_op_setfield(self, op):
+        check_skip_operation(op)        # just to check it doesn't raise
         if self.is_typeptr_getset(op):
             # ignore the operation completely -- instead, it's done by 'new'
             return

diff --git a/pypy/jit/metainterp/optimizeopt/optimizer.py b/pypy/jit/metainterp/optimizeopt/optimizer.py
--- a/pypy/jit/metainterp/optimizeopt/optimizer.py
+++ b/pypy/jit/metainterp/optimizeopt/optimizer.py
@@ -257,6 +257,7 @@
         self.pendingfields = []
         self.posponedop = None
         self.exception_might_have_happened = False
+        self.quasi_immutable_deps = None
         self.newoperations = []
         if loop is not None:
             self.call_pure_results = loop.call_pure_results
@@ -309,6 +310,7 @@
         new.pure_operations = self.pure_operations
         new.producer = self.producer
         assert self.posponedop is None
+        new.quasi_immutable_deps = self.quasi_immutable_deps
 
         return new
 
@@ -410,6 +412,7 @@
             self.first_optimization.propagate_forward(op)
             self.i += 1
         self.loop.operations = self.newoperations
+        self.loop.quasi_immutable_deps = self.quasi_immutable_deps
         # accumulate counters
         self.resumedata_memo.update_counters(self.metainterp_sd.profiler)
 

diff --git a/pypy/jit/metainterp/test/test_optimizeopt.py b/pypy/jit/metainterp/test/test_optimizeopt.py
--- a/pypy/jit/metainterp/test/test_optimizeopt.py
+++ b/pypy/jit/metainterp/test/test_optimizeopt.py
@@ -5717,8 +5717,35 @@
         # not obvious, because of the exception UnicodeDecodeError that
         # can be raised by ll_str2unicode()
 
-
-
+    def test_quasi_immut(self):
+        ops = """
+        [p0, p1, i0]
+        quasiimmut_field(p0, descr=quasiimmutdescr)
+        guard_not_invalidated() []
+        i1 = getfield_gc(p0, descr=quasifielddescr)
+        jump(p1, p0, i1)
+        """
+        expected = """
+        [p0, p1, i0]
+        i1 = getfield_gc(p0, descr=quasifielddescr)
+        jump(p1, p0, i1)
+        """
+        self.optimize_loop(ops, expected)
+
+    def test_quasi_immut_2(self):
+        ops = """
+        []
+        quasiimmut_field(ConstPtr(myptr), descr=quasiimmutdescr)
+        guard_not_invalidated() []
+        i1 = getfield_gc(ConstPtr(myptr), descr=quasifielddescr)
+        jump()
+        """
+        expected = """
+        []
+        guard_not_invalidated() []        
+        jump()
+        """
+        self.optimize_loop(ops, expected, expected)
 
 ##class TestOOtype(OptimizeOptTest, OOtypeMixin):
 

diff --git a/pypy/rpython/test/test_annlowlevel.py b/pypy/rpython/test/test_annlowlevel.py
--- a/pypy/rpython/test/test_annlowlevel.py
+++ b/pypy/rpython/test/test_annlowlevel.py
@@ -4,9 +4,12 @@
 
 from pypy.rpython.test.tool import BaseRtypingTest, LLRtypeMixin, OORtypeMixin
 from pypy.rpython.lltypesystem.rstr import mallocstr, mallocunicode
+from pypy.rpython.lltypesystem import lltype
 from pypy.rpython.ootypesystem import ootype
 from pypy.rpython.annlowlevel import hlstr, llstr, oostr
 from pypy.rpython.annlowlevel import hlunicode, llunicode
+from pypy.rpython import annlowlevel
+
 
 class TestLLType(BaseRtypingTest, LLRtypeMixin):
     def test_hlstr(self):
@@ -53,6 +56,15 @@
         res = self.interpret(f, [self.unicode_to_ll(u"abc")])
         assert res == 3
 
+    def test_cast_instance_to_base_ptr(self):
+        class X(object):
+            pass
+        x = X()
+        ptr = annlowlevel.cast_instance_to_base_ptr(x)
+        assert lltype.typeOf(ptr) == annlowlevel.base_ptr_lltype()
+        y = annlowlevel.cast_base_ptr_to_instance(X, ptr)
+        assert y is x
+
 
 class TestOOType(BaseRtypingTest, OORtypeMixin):
     def test_hlstr(self):
@@ -71,3 +83,12 @@
 
         res = self.interpret(f, [self.string_to_ll("abc")])
         assert res == 3
+
+    def test_cast_instance_to_base_obj(self):
+        class X(object):
+            pass
+        x = X()
+        obj = annlowlevel.cast_instance_to_base_obj(x)
+        assert lltype.typeOf(obj) == annlowlevel.base_obj_ootype()
+        y = annlowlevel.cast_base_ptr_to_instance(X, obj)
+        assert y is x

diff --git a/pypy/jit/backend/llgraph/runner.py b/pypy/jit/backend/llgraph/runner.py
--- a/pypy/jit/backend/llgraph/runner.py
+++ b/pypy/jit/backend/llgraph/runner.py
@@ -286,6 +286,10 @@
             raise ValueError("CALL_ASSEMBLER not supported")
         llimpl.redirect_call_assembler(self, oldlooptoken, newlooptoken)
 
+    def invalidate_loop(self, looptoken):
+        for loop in looptoken.compiled_loop_token.loop_and_bridges:
+            loop._obj.externalobj.invalid = True
+
     # ----------
 
     def sizeof(self, S):

diff --git a/pypy/jit/metainterp/test/test_optimizeutil.py b/pypy/jit/metainterp/test/test_optimizeutil.py
--- a/pypy/jit/metainterp/test/test_optimizeutil.py
+++ b/pypy/jit/metainterp/test/test_optimizeutil.py
@@ -3,6 +3,7 @@
 from pypy.rpython.lltypesystem import lltype, llmemory, rclass, rstr
 from pypy.rpython.ootypesystem import ootype
 from pypy.rpython.lltypesystem.rclass import OBJECT, OBJECT_VTABLE
+from pypy.rpython.rclass import FieldListAccessor, IR_QUASI_IMMUTABLE
 
 from pypy.jit.backend.llgraph import runner
 from pypy.jit.metainterp.history import (BoxInt, BoxPtr, ConstInt, ConstPtr,
@@ -12,6 +13,7 @@
 from pypy.jit.codewriter.effectinfo import EffectInfo
 from pypy.jit.codewriter.heaptracker import register_known_gctype, adr2int
 from pypy.jit.tool.oparser import parse
+from pypy.jit.metainterp.quasiimmut import QuasiImmutDescr
 
 def test_sort_descrs():
     class PseudoDescr(AbstractDescr):
@@ -62,6 +64,18 @@
     nextdescr = cpu.fielddescrof(NODE, 'next')
     otherdescr = cpu.fielddescrof(NODE2, 'other')
 
+    accessor = FieldListAccessor()
+    accessor.initialize(None, {'inst_field': IR_QUASI_IMMUTABLE})
+    QUASI = lltype.GcStruct('QUASIIMMUT', ('inst_field', lltype.Signed),
+                            ('mutate_field', rclass.OBJECTPTR),
+                            hints={'immutable_fields': accessor})
+    quasi = lltype.malloc(QUASI, immortal=True)
+    quasifielddescr = cpu.fielddescrof(QUASI, 'inst_field')
+    quasibox = BoxPtr(lltype.cast_opaque_ptr(llmemory.GCREF, quasi))
+    quasiimmutdescr = QuasiImmutDescr(cpu, quasibox,
+                                      quasifielddescr,
+                                      cpu.fielddescrof(QUASI, 'mutate_field'))
+
     NODEOBJ = lltype.GcStruct('NODEOBJ', ('parent', OBJECT),
                                          ('ref', lltype.Ptr(OBJECT)))
     nodeobj = lltype.malloc(NODEOBJ)

diff --git a/pypy/jit/metainterp/virtualizable.py b/pypy/jit/metainterp/virtualizable.py
--- a/pypy/jit/metainterp/virtualizable.py
+++ b/pypy/jit/metainterp/virtualizable.py
@@ -1,6 +1,7 @@
 from pypy.rpython.lltypesystem import lltype, llmemory
 from pypy.rpython.ootypesystem import ootype
 from pypy.rpython.annlowlevel import cast_base_ptr_to_instance
+from pypy.rpython.rclass import IR_ARRAY_IMMUTABLE, IR_IMMUTABLE
 from pypy.rpython import rvirtualizable2
 from pypy.rlib.objectmodel import we_are_translated
 from pypy.rlib.unroll import unrolling_iterable
@@ -10,7 +11,7 @@
 from pypy.jit.metainterp.warmstate import wrap, unwrap
 from pypy.rlib.objectmodel import specialize
 
-class VirtualizableInfo:
+class VirtualizableInfo(object):
     TOKEN_NONE            = 0      # must be 0 -- see also x86.call_assembler
     TOKEN_TRACING_RESCALL = -1
 
@@ -33,11 +34,13 @@
         all_fields = accessor.fields
         static_fields = []
         array_fields = []
-        for name, suffix in all_fields.iteritems():
-            if suffix == '[*]':
+        for name, tp in all_fields.iteritems():
+            if tp == IR_ARRAY_IMMUTABLE:
                 array_fields.append(name)
+            elif tp == IR_IMMUTABLE:
+                static_fields.append(name)
             else:
-                static_fields.append(name)
+                raise Exception("unknown type: %s" % tp)
         self.static_fields = static_fields
         self.array_fields = array_fields
         #

diff --git a/pypy/jit/backend/x86/regalloc.py b/pypy/jit/backend/x86/regalloc.py
--- a/pypy/jit/backend/x86/regalloc.py
+++ b/pypy/jit/backend/x86/regalloc.py
@@ -492,6 +492,8 @@
     def consider_guard_no_exception(self, op):
         self.perform_guard(op, [], None)
 
+    consider_guard_not_invalidated = consider_guard_no_exception
+
     def consider_guard_exception(self, op):
         loc = self.rm.make_sure_var_in_reg(op.getarg(0))
         box = TempBox()

diff --git a/pypy/rpython/lltypesystem/test/test_ll2ctypes.py b/pypy/rpython/lltypesystem/test/test_ll2ctypes.py
--- a/pypy/rpython/lltypesystem/test/test_ll2ctypes.py
+++ b/pypy/rpython/lltypesystem/test/test_ll2ctypes.py
@@ -1293,6 +1293,28 @@
         rffi.cast(SP, p).x = 0
         lltype.free(chunk, flavor='raw')
 
+    def test_opaque_tagged_pointers(self):
+        from pypy.rpython.annlowlevel import cast_base_ptr_to_instance
+        from pypy.rpython.annlowlevel import cast_instance_to_base_ptr
+        from pypy.rpython.lltypesystem import rclass
+        
+        class Opaque(object):
+            llopaque = True
+
+            def hide(self):
+                ptr = cast_instance_to_base_ptr(self)
+                return lltype.cast_opaque_ptr(llmemory.GCREF, ptr)
+
+            @staticmethod
+            def show(gcref):
+                ptr = lltype.cast_opaque_ptr(lltype.Ptr(rclass.OBJECT), gcref)
+                return cast_base_ptr_to_instance(Opaque, ptr)
+
+        opaque = Opaque()
+        round = ctypes2lltype(llmemory.GCREF, lltype2ctypes(opaque.hide()))
+        assert Opaque.show(round) is opaque
+
+
 class TestPlatform(object):
     def test_lib_on_libpaths(self):
         from pypy.translator.platform import platform

diff --git a/pypy/jit/metainterp/optimizeopt/heap.py b/pypy/jit/metainterp/optimizeopt/heap.py
--- a/pypy/jit/metainterp/optimizeopt/heap.py
+++ b/pypy/jit/metainterp/optimizeopt/heap.py
@@ -119,6 +119,7 @@
         self._lazy_setfields = []
         # cached array items:  {descr: CachedArrayItems}
         self.cached_arrayitems = {}
+        self._remove_guard_not_invalidated = False
 
     def reconstruct_for_next_iteration(self, optimizer, valuemap):
         new = OptHeap()
@@ -378,6 +379,43 @@
         self.cache_arrayitem_value(op.getdescr(), value, indexvalue, fieldvalue,
                                    write=True)
 
+    def optimize_QUASIIMMUT_FIELD(self, op):
+        # Pattern: QUASIIMMUT_FIELD(s, descr=QuasiImmutDescr)
+        #          x = GETFIELD_GC(s, descr='inst_x')
+        # If 's' is a constant (after optimizations), then we make 's.inst_x'
+        # a constant too, and we rely on the rest of the optimizations to
+        # constant-fold the following getfield_gc.
+        structvalue = self.getvalue(op.getarg(0))
+        if not structvalue.is_constant():
+            self._remove_guard_not_invalidated = True
+            return    # not a constant at all; ignore QUASIIMMUT_FIELD
+        #
+        from pypy.jit.metainterp.quasiimmut import QuasiImmutDescr
+        qmutdescr = op.getdescr()
+        assert isinstance(qmutdescr, QuasiImmutDescr)
+        # check that the value is still correct; it could have changed
+        # already between the tracing and now.  In this case, we are
+        # simply ignoring the QUASIIMMUT_FIELD hint and compiling it
+        # as a regular getfield.
+        if not qmutdescr.is_still_valid():
+            self._remove_guard_not_invalidated = True
+            return
+        # record as an out-of-line guard
+        if self.optimizer.quasi_immutable_deps is None:
+            self.optimizer.quasi_immutable_deps = {}
+        self.optimizer.quasi_immutable_deps[qmutdescr.qmut] = None
+        # perform the replacement in the list of operations
+        fieldvalue = self.getvalue(qmutdescr.constantfieldbox)
+        cf = self.field_cache(qmutdescr.fielddescr)
+        cf.remember_field_value(structvalue, fieldvalue)
+        self._remove_guard_not_invalidated = False
+
+    def optimize_GUARD_NOT_INVALIDATED(self, op):
+        if self._remove_guard_not_invalidated:
+            return
+        self._remove_guard_not_invalidated = False
+        self.emit_operation(op)
+
     def propagate_forward(self, op):
         opnum = op.getopnum()
         for value, func in optimize_ops:

diff --git a/pypy/jit/metainterp/executor.py b/pypy/jit/metainterp/executor.py
--- a/pypy/jit/metainterp/executor.py
+++ b/pypy/jit/metainterp/executor.py
@@ -312,6 +312,7 @@
                          rop.DEBUG_MERGE_POINT,
                          rop.JIT_DEBUG,
                          rop.SETARRAYITEM_RAW,
+                         rop.QUASIIMMUT_FIELD,
                          ):      # list of opcodes never executed by pyjitpl
                 continue
             raise AssertionError("missing %r" % (key,))

diff --git a/pypy/rpython/rclass.py b/pypy/rpython/rclass.py
--- a/pypy/rpython/rclass.py
+++ b/pypy/rpython/rclass.py
@@ -3,7 +3,8 @@
 #from pypy.annotation.classdef import isclassdef
 from pypy.annotation import description
 from pypy.rpython.error import TyperError
-from pypy.rpython.rmodel import Repr, getgcflavor
+from pypy.rpython.rmodel import Repr, getgcflavor, inputconst
+from pypy.rpython.lltypesystem.lltype import Void
 
 
 class FieldListAccessor(object):
@@ -12,6 +13,8 @@
         assert type(fields) is dict
         self.TYPE = TYPE
         self.fields = fields
+        for x in fields.itervalues():
+            assert isinstance(x, ImmutableRanking)
 
     def __repr__(self):
         return '<FieldListAccessor for %s>' % getattr(self, 'TYPE', '?')
@@ -19,6 +22,20 @@
     def _freeze_(self):
         return True
 
+class ImmutableRanking(object):
+    def __init__(self, name, is_immutable):
+        self.name = name
+        self.is_immutable = is_immutable
+    def __nonzero__(self):
+        return self.is_immutable
+    def __repr__(self):
+        return '<%s>' % self.name
+
+IR_MUTABLE         = ImmutableRanking('mutable', False)
+IR_IMMUTABLE       = ImmutableRanking('immutable', True)
+IR_ARRAY_IMMUTABLE = ImmutableRanking('array_immutable', True)
+IR_QUASI_IMMUTABLE = ImmutableRanking('quasi_immutable', False)
+
 class ImmutableConflictError(Exception):
     """Raised when the _immutable_ or _immutable_fields_ hints are
     not consistent across a class hierarchy."""
@@ -155,7 +172,8 @@
         self.classdef = classdef
 
     def _setup_repr(self):
-        pass
+        if self.classdef is None:
+            self.immutable_field_set = set()
 
     def _check_for_immutable_hints(self, hints):
         loc = self.classdef.classdesc.lookup('_immutable_')
@@ -167,13 +185,13 @@
                     self.classdef,))
             hints = hints.copy()
             hints['immutable'] = True
-        self.immutable_field_list = []  # unless overwritten below
+        self.immutable_field_set = set()  # unless overwritten below
         if self.classdef.classdesc.lookup('_immutable_fields_') is not None:
             hints = hints.copy()
             immutable_fields = self.classdef.classdesc.classdict.get(
                 '_immutable_fields_')
             if immutable_fields is not None:
-                self.immutable_field_list = immutable_fields.value
+                self.immutable_field_set = set(immutable_fields.value)
             accessor = FieldListAccessor()
             hints['immutable_fields'] = accessor
         return hints
@@ -201,33 +219,35 @@
         if "immutable_fields" in hints:
             accessor = hints["immutable_fields"]
             if not hasattr(accessor, 'fields'):
-                immutable_fields = []
+                immutable_fields = set()
                 rbase = self
                 while rbase.classdef is not None:
-                    immutable_fields += rbase.immutable_field_list
+                    immutable_fields.update(rbase.immutable_field_set)
                     rbase = rbase.rbase
                 self._parse_field_list(immutable_fields, accessor)
 
     def _parse_field_list(self, fields, accessor):
-        with_suffix = {}
+        ranking = {}
         for name in fields:
-            if name.endswith('[*]'):
+            if name.endswith('[*]'):    # for virtualizables' lists
                 name = name[:-3]
-                suffix = '[*]'
-            else:
-                suffix = ''
+                rank = IR_ARRAY_IMMUTABLE
+            elif name.endswith('?'):    # a quasi-immutable field
+                name = name[:-1]
+                rank = IR_QUASI_IMMUTABLE
+            else:                       # a regular immutable/green field
+                rank = IR_IMMUTABLE
             try:
                 mangled_name, r = self._get_field(name)
             except KeyError:
                 continue
-            with_suffix[mangled_name] = suffix
-        accessor.initialize(self.object_type, with_suffix)
-        return with_suffix
+            ranking[mangled_name] = rank
+        accessor.initialize(self.object_type, ranking)
+        return ranking
 
     def _check_for_immutable_conflicts(self):
         # check for conflicts, i.e. a field that is defined normally as
         # mutable in some parent class but that is now declared immutable
-        from pypy.rpython.lltypesystem.lltype import Void
         is_self_immutable = "immutable" in self.object_type._hints
         base = self
         while base.classdef is not None:
@@ -248,12 +268,30 @@
                         "class %r has _immutable_=True, but parent class %r "
                         "defines (at least) the mutable field %r" % (
                         self, base, fieldname))
-                if fieldname in self.immutable_field_list:
+                if (fieldname in self.immutable_field_set or
+                    (fieldname + '?') in self.immutable_field_set):
                     raise ImmutableConflictError(
                         "field %r is defined mutable in class %r, but "
                         "listed in _immutable_fields_ in subclass %r" % (
                         fieldname, base, self))
 
+    def hook_access_field(self, vinst, cname, llops, flags):
+        pass        # for virtualizables; see rvirtualizable2.py
+
+    def hook_setfield(self, vinst, fieldname, llops):
+        if self.is_quasi_immutable(fieldname):
+            c_fieldname = inputconst(Void, 'mutate_' + fieldname)
+            llops.genop('jit_force_quasi_immutable', [vinst, c_fieldname])
+
+    def is_quasi_immutable(self, fieldname):
+        search = fieldname + '?'
+        rbase = self
+        while rbase.classdef is not None:
+            if search in rbase.immutable_field_set:
+                return True
+            rbase = rbase.rbase
+        return False
+
     def new_instance(self, llops, classcallhop=None):
         raise NotImplementedError
 

diff --git a/pypy/jit/codewriter/policy.py b/pypy/jit/codewriter/policy.py
--- a/pypy/jit/codewriter/policy.py
+++ b/pypy/jit/codewriter/policy.py
@@ -1,5 +1,5 @@
 from pypy.translator.simplify import get_funcobj
-from pypy.jit.metainterp import history
+from pypy.jit.metainterp import history, quasiimmut
 from pypy.rpython.lltypesystem import lltype, rclass
 from pypy.tool.udir import udir
 
@@ -85,12 +85,20 @@
                     getkind(v.concretetype, supports_floats, supports_longlong)
                 v = op.result
                 getkind(v.concretetype, supports_floats, supports_longlong)
+                check_skip_operation(op)
     except NotImplementedError, e:
         log.WARNING('%s, ignoring graph' % (e,))
         log.WARNING('  %s' % (graph,))
         return True
     return False
 
+def check_skip_operation(op):
+    if op.opname == 'setfield':
+        if quasiimmut.is_quasi_immutable(op.args[0].concretetype.TO,
+                                         op.args[1].value):
+            raise NotImplementedError("write to quasi-immutable field %r"
+                                      % (op.args[1].value,))
+
 # ____________________________________________________________
 
 class StopAtXPolicy(JitPolicy):

diff --git a/pypy/jit/backend/llgraph/llimpl.py b/pypy/jit/backend/llgraph/llimpl.py
--- a/pypy/jit/backend/llgraph/llimpl.py
+++ b/pypy/jit/backend/llgraph/llimpl.py
@@ -167,6 +167,7 @@
 
 class CompiledLoop(object):
     has_been_freed = False
+    invalid = False
 
     def __init__(self):
         self.inputargs = []
@@ -933,6 +934,9 @@
         if forced:
             raise GuardFailed
 
+    def op_guard_not_invalidated(self, descr):
+        if self.loop.invalid:
+            raise GuardFailed
 
 class OOFrame(Frame):
 

diff --git a/pypy/jit/codewriter/test/test_jtransform.py b/pypy/jit/codewriter/test/test_jtransform.py
--- a/pypy/jit/codewriter/test/test_jtransform.py
+++ b/pypy/jit/codewriter/test/test_jtransform.py
@@ -947,3 +947,43 @@
     assert op1.args[1] == 'calldescr-%d' % effectinfo.EffectInfo.OS_ARRAYCOPY
     assert op1.args[2] == ListOfKind('int', [v3, v4, v5])
     assert op1.args[3] == ListOfKind('ref', [v1, v2])
+
+def test_quasi_immutable():
+    from pypy.rpython.rclass import FieldListAccessor, IR_QUASI_IMMUTABLE
+    accessor = FieldListAccessor()
+    accessor.initialize(None, {'inst_x': IR_QUASI_IMMUTABLE})
+    v2 = varoftype(lltype.Signed)
+    STRUCT = lltype.GcStruct('struct', ('inst_x', lltype.Signed),
+                             ('mutate_x', rclass.OBJECTPTR),
+                             hints={'immutable_fields': accessor})
+    for v_x in [const(lltype.malloc(STRUCT)), varoftype(lltype.Ptr(STRUCT))]:
+        op = SpaceOperation('getfield', [v_x, Constant('inst_x', lltype.Void)],
+                            v2)
+        tr = Transformer(FakeCPU())
+        [_, op1, op2] = tr.rewrite_operation(op)
+        assert op1.opname == 'record_quasiimmut_field'
+        assert len(op1.args) == 3
+        assert op1.args[0] == v_x
+        assert op1.args[1] == ('fielddescr', STRUCT, 'inst_x')
+        assert op1.args[2] == ('fielddescr', STRUCT, 'mutate_x')
+        assert op1.result is None
+        assert op2.opname == 'getfield_gc_i'
+        assert len(op2.args) == 2
+        assert op2.args[0] == v_x
+        assert op2.args[1] == ('fielddescr', STRUCT, 'inst_x')
+        assert op2.result is op.result
+
+def test_quasi_immutable_setfield():
+    from pypy.rpython.rclass import FieldListAccessor, IR_QUASI_IMMUTABLE
+    accessor = FieldListAccessor()
+    accessor.initialize(None, {'inst_x': IR_QUASI_IMMUTABLE})
+    v1 = varoftype(lltype.Signed)
+    STRUCT = lltype.GcStruct('struct', ('inst_x', lltype.Signed),
+                             ('mutate_x', rclass.OBJECTPTR),
+                             hints={'immutable_fields': accessor})
+    for v_x in [const(lltype.malloc(STRUCT)), varoftype(lltype.Ptr(STRUCT))]:
+        op = SpaceOperation('setfield',
+                            [v_x, Constant('inst_x', lltype.Void), v1],
+                            varoftype(lltype.Void))
+        tr = Transformer(FakeCPU())
+        raises(NotImplementedError, tr.rewrite_operation, op)

diff --git a/pypy/jit/codewriter/effectinfo.py b/pypy/jit/codewriter/effectinfo.py
--- a/pypy/jit/codewriter/effectinfo.py
+++ b/pypy/jit/codewriter/effectinfo.py
@@ -171,7 +171,8 @@
 class VirtualizableAnalyzer(BoolGraphAnalyzer):
     def analyze_simple_operation(self, op, graphinfo):
         return op.opname in ('jit_force_virtualizable',
-                             'jit_force_virtual')
+                             'jit_force_virtual',
+                             'jit_force_quasi_immutable')
 
 # ____________________________________________________________
 

diff --git a/pypy/jit/metainterp/pyjitpl.py b/pypy/jit/metainterp/pyjitpl.py
--- a/pypy/jit/metainterp/pyjitpl.py
+++ b/pypy/jit/metainterp/pyjitpl.py
@@ -555,6 +555,16 @@
     opimpl_setfield_raw_r = _opimpl_setfield_raw_any
     opimpl_setfield_raw_f = _opimpl_setfield_raw_any
 
+    @arguments("box", "descr", "descr", "orgpc")
+    def opimpl_record_quasiimmut_field(self, box, fielddescr,
+                                       mutatefielddescr, orgpc):
+        from pypy.jit.metainterp.quasiimmut import QuasiImmutDescr
+        cpu = self.metainterp.cpu
+        descr = QuasiImmutDescr(cpu, box, fielddescr, mutatefielddescr)
+        self.metainterp.history.record(rop.QUASIIMMUT_FIELD, [box],
+                                       None, descr=descr)
+        self.generate_guard(rop.GUARD_NOT_INVALIDATED, resumepc=orgpc)
+
     def _nonstandard_virtualizable(self, pc, box):
         # returns True if 'box' is actually not the "standard" virtualizable
         # that is stored in metainterp.virtualizable_boxes[-1]
@@ -1076,6 +1086,8 @@
         if opnum == rop.GUARD_NOT_FORCED:
             resumedescr = compile.ResumeGuardForcedDescr(metainterp_sd,
                                                    metainterp.jitdriver_sd)
+        elif opnum == rop.GUARD_NOT_INVALIDATED:
+            resumedescr = compile.ResumeGuardNotInvalidated()
         else:
             resumedescr = compile.ResumeGuardDescr()
         guard_op = metainterp.history.record(opnum, moreargs, None,
@@ -1848,6 +1860,9 @@
                 self.handle_possible_exception()
             except ChangeFrame:
                 pass
+        elif opnum == rop.GUARD_NOT_INVALIDATED:
+            pass # XXX we want to do something special in resume descr,
+                 # but not now
         elif opnum == rop.GUARD_NO_OVERFLOW:   # an overflow now detected
             self.execute_raised(OverflowError(), constant=True)
             try:

diff --git a/pypy/jit/metainterp/compile.py b/pypy/jit/metainterp/compile.py
--- a/pypy/jit/metainterp/compile.py
+++ b/pypy/jit/metainterp/compile.py
@@ -76,6 +76,11 @@
             op.setdescr(None)    # clear reference, mostly for tests
             if not we_are_translated():
                 op._jumptarget_number = descr.number
+    # record this looptoken on the QuasiImmut used in the code
+    if loop.quasi_immutable_deps is not None:
+        for qmut in loop.quasi_immutable_deps:
+            qmut.register_loop_token(wref)
+        # XXX maybe we should clear the dictionary here
     # mostly for tests: make sure we don't keep a reference to the LoopToken
     loop.token = None
     if not we_are_translated():
@@ -396,6 +401,12 @@
         self.copy_all_attributes_into(res)
         return res
 
+class ResumeGuardNotInvalidated(ResumeGuardDescr):
+    def _clone_if_mutable(self):
+        res = ResumeGuardNotInvalidated()
+        self.copy_all_attributes_into(res)
+        return res
+
 class ResumeAtPositionDescr(ResumeGuardDescr):
     def _clone_if_mutable(self):
         res = ResumeAtPositionDescr()

diff --git a/pypy/rpython/lltypesystem/test/test_lloperation.py b/pypy/rpython/lltypesystem/test/test_lloperation.py
--- a/pypy/rpython/lltypesystem/test/test_lloperation.py
+++ b/pypy/rpython/lltypesystem/test/test_lloperation.py
@@ -54,6 +54,7 @@
 
 def test_is_pure():
     from pypy.objspace.flow.model import Variable, Constant
+    from pypy.rpython import rclass
     assert llop.bool_not.is_pure([Variable()])
     assert llop.debug_assert.is_pure([Variable()])
     assert not llop.int_add_ovf.is_pure([Variable(), Variable()])
@@ -85,38 +86,50 @@
     assert llop.getarrayitem.is_pure([v_a2, Variable()])
     assert llop.getarraysize.is_pure([v_a2])
     #
-    accessor = rclass.FieldListAccessor()
-    S3 = lltype.GcStruct('S', ('x', lltype.Signed), ('y', lltype.Signed),
-                         hints={'immutable_fields': accessor})
-    accessor.initialize(S3, {'x': ''})
-    v_s3 = Variable()
-    v_s3.concretetype = lltype.Ptr(S3)
-    assert not llop.setfield.is_pure([v_s3, Constant('x'), Variable()])
-    assert not llop.setfield.is_pure([v_s3, Constant('y'), Variable()])
-    assert llop.getfield.is_pure([v_s3, Constant('x')])
-    assert not llop.getfield.is_pure([v_s3, Constant('y')])
+    for kind in [rclass.IR_MUTABLE, rclass.IR_IMMUTABLE,
+                 rclass.IR_ARRAY_IMMUTABLE, rclass.IR_QUASI_IMMUTABLE]:
+        accessor = rclass.FieldListAccessor()
+        S3 = lltype.GcStruct('S', ('x', lltype.Signed), ('y', lltype.Signed),
+                             hints={'immutable_fields': accessor})
+        accessor.initialize(S3, {'x': kind})
+        v_s3 = Variable()
+        v_s3.concretetype = lltype.Ptr(S3)
+        assert not llop.setfield.is_pure([v_s3, Constant('x'), Variable()])
+        assert not llop.setfield.is_pure([v_s3, Constant('y'), Variable()])
+        assert llop.getfield.is_pure([v_s3, Constant('x')]) is kind
+        assert not llop.getfield.is_pure([v_s3, Constant('y')])
 
 def test_getfield_pure():
     S1 = lltype.GcStruct('S', ('x', lltype.Signed), ('y', lltype.Signed))
     S2 = lltype.GcStruct('S', ('x', lltype.Signed), ('y', lltype.Signed),
                          hints={'immutable': True})
     accessor = rclass.FieldListAccessor()
-    S3 = lltype.GcStruct('S', ('x', lltype.Signed), ('y', lltype.Signed),
-                         hints={'immutable_fields': accessor})
-    accessor.initialize(S3, {'x': ''})
     #
     s1 = lltype.malloc(S1); s1.x = 45
     py.test.raises(TypeError, llop.getfield, lltype.Signed, s1, 'x')
     s2 = lltype.malloc(S2); s2.x = 45
     assert llop.getfield(lltype.Signed, s2, 'x') == 45
-    s3 = lltype.malloc(S3); s3.x = 46; s3.y = 47
-    assert llop.getfield(lltype.Signed, s3, 'x') == 46
-    py.test.raises(TypeError, llop.getfield, lltype.Signed, s3, 'y')
     #
     py.test.raises(TypeError, llop.getinteriorfield, lltype.Signed, s1, 'x')
     assert llop.getinteriorfield(lltype.Signed, s2, 'x') == 45
-    assert llop.getinteriorfield(lltype.Signed, s3, 'x') == 46
-    py.test.raises(TypeError, llop.getinteriorfield, lltype.Signed, s3, 'y')
+    #
+    for kind in [rclass.IR_MUTABLE, rclass.IR_IMMUTABLE,
+                 rclass.IR_ARRAY_IMMUTABLE, rclass.IR_QUASI_IMMUTABLE]:
+        #
+        S3 = lltype.GcStruct('S', ('x', lltype.Signed), ('y', lltype.Signed),
+                             hints={'immutable_fields': accessor})
+        accessor.initialize(S3, {'x': kind})
+        s3 = lltype.malloc(S3); s3.x = 46; s3.y = 47
+        if kind in [rclass.IR_IMMUTABLE, rclass.IR_ARRAY_IMMUTABLE]:
+            assert llop.getfield(lltype.Signed, s3, 'x') == 46
+            assert llop.getinteriorfield(lltype.Signed, s3, 'x') == 46
+        else:
+            py.test.raises(TypeError, llop.getfield, lltype.Signed, s3, 'x')
+            py.test.raises(TypeError, llop.getinteriorfield,
+                           lltype.Signed, s3, 'x')
+        py.test.raises(TypeError, llop.getfield, lltype.Signed, s3, 'y')
+        py.test.raises(TypeError, llop.getinteriorfield,
+                       lltype.Signed, s3, 'y')
 
 # ___________________________________________________________________________
 # This tests that the LLInterpreter and the LL_OPERATIONS tables are in sync.

diff --git a/pypy/jit/backend/x86/runner.py b/pypy/jit/backend/x86/runner.py
--- a/pypy/jit/backend/x86/runner.py
+++ b/pypy/jit/backend/x86/runner.py
@@ -145,6 +145,14 @@
     def redirect_call_assembler(self, oldlooptoken, newlooptoken):
         self.assembler.redirect_call_assembler(oldlooptoken, newlooptoken)
 
+    def invalidate_loop(self, looptoken):
+        from pypy.jit.backend.x86 import codebuf
+        
+        for addr, tgt in looptoken.compiled_loop_token.invalidate_positions:
+            mc = codebuf.MachineCodeBlockWrapper()
+            mc.JMP_l(tgt)
+            mc.copy_to_raw_memory(addr - 1)
+
 class CPU386(AbstractX86CPU):
     WORD = 4
     NUM_REGS = 8

diff --git a/pypy/rpython/lltypesystem/opimpl.py b/pypy/rpython/lltypesystem/opimpl.py
--- a/pypy/rpython/lltypesystem/opimpl.py
+++ b/pypy/rpython/lltypesystem/opimpl.py
@@ -525,6 +525,9 @@
 def op_jit_force_virtual(x):
     return x
 
+def op_jit_force_quasi_immutable(*args):
+    pass
+
 def op_get_group_member(TYPE, grpptr, memberoffset):
     from pypy.rpython.lltypesystem import llgroup
     assert isinstance(memberoffset, llgroup.GroupMemberOffset)

diff --git a/pypy/rpython/test/test_rvirtualizable2.py b/pypy/rpython/test/test_rvirtualizable2.py
--- a/pypy/rpython/test/test_rvirtualizable2.py
+++ b/pypy/rpython/test/test_rvirtualizable2.py
@@ -5,6 +5,7 @@
 from pypy.rlib.jit import hint
 from pypy.objspace.flow.model import summary
 from pypy.rpython.llinterp import LLInterpreter
+from pypy.rpython.rclass import IR_IMMUTABLE, IR_ARRAY_IMMUTABLE
 from pypy import conftest
 
 
@@ -116,8 +117,8 @@
         TYPE = self.gettype(v_inst)
         accessor = TYPE._hints['virtualizable2_accessor']
         assert accessor.TYPE == TYPE
-        assert accessor.fields == {self.prefix + 'v1' : "",
-                                   self.prefix + 'v2': "[*]"}
+        assert accessor.fields == {self.prefix + 'v1': IR_IMMUTABLE,
+                                   self.prefix + 'v2': IR_ARRAY_IMMUTABLE}
         #
         def fn2(n):
             Base().base1 = 42

diff --git a/pypy/jit/metainterp/blackhole.py b/pypy/jit/metainterp/blackhole.py
--- a/pypy/jit/metainterp/blackhole.py
+++ b/pypy/jit/metainterp/blackhole.py
@@ -1166,6 +1166,11 @@
     def bhimpl_setfield_raw_f(cpu, struct, fielddescr, newvalue):
         cpu.bh_setfield_raw_f(struct, fielddescr, newvalue)
 
+    @arguments("cpu", "r", "d", "d")
+    def bhimpl_record_quasiimmut_field(self, struct, fielddescr,
+                                       mutatefielddescr):
+        pass
+
     @arguments("cpu", "d", returns="r")
     def bhimpl_new(cpu, descr):
         return cpu.bh_new(descr)
@@ -1287,6 +1292,8 @@
             # We get here because it used to overflow, but now it no longer
             # does.
             pass
+        elif opnum == rop.GUARD_NOT_INVALIDATED:
+            pass
         else:
             from pypy.jit.metainterp.resoperation import opname
             raise NotImplementedError(opname[opnum])

diff --git a/pypy/rpython/lltypesystem/lloperation.py b/pypy/rpython/lltypesystem/lloperation.py
--- a/pypy/rpython/lltypesystem/lloperation.py
+++ b/pypy/rpython/lltypesystem/lloperation.py
@@ -433,6 +433,7 @@
     'jit_marker':           LLOp(),
     'jit_force_virtualizable':LLOp(canrun=True),
     'jit_force_virtual':    LLOp(canrun=True),
+    'jit_force_quasi_immutable': LLOp(canrun=True),
     'get_exception_addr':   LLOp(),
     'get_exc_value_addr':   LLOp(),
     'do_malloc_fixedsize_clear':LLOp(canraise=(MemoryError,),canunwindgc=True),

diff --git a/pypy/jit/backend/x86/test/test_quasiimmut.py b/pypy/jit/backend/x86/test/test_quasiimmut.py
new file mode 100644
--- /dev/null
+++ b/pypy/jit/backend/x86/test/test_quasiimmut.py
@@ -0,0 +1,9 @@
+
+import py
+from pypy.jit.backend.x86.test.test_basic import Jit386Mixin
+from pypy.jit.metainterp.test import test_quasiimmut
+
+class TestLoopSpec(Jit386Mixin, test_quasiimmut.QuasiImmutTests):
+    # for the individual tests see
+    # ====> ../../../metainterp/test/test_loop.py
+    pass

diff --git a/pypy/rpython/rvirtualizable2.py b/pypy/rpython/rvirtualizable2.py
--- a/pypy/rpython/rvirtualizable2.py
+++ b/pypy/rpython/rvirtualizable2.py
@@ -50,7 +50,7 @@
 
     def hook_access_field(self, vinst, cname, llops, flags):
         #if not flags.get('access_directly'):
-        if cname.value in self.my_redirected_fields:
+        if self.my_redirected_fields.get(cname.value):
             cflags = inputconst(lltype.Void, flags)
             llops.genop('jit_force_virtualizable', [vinst, cname, cflags])
 

diff --git a/pypy/jit/metainterp/resoperation.py b/pypy/jit/metainterp/resoperation.py
--- a/pypy/jit/metainterp/resoperation.py
+++ b/pypy/jit/metainterp/resoperation.py
@@ -380,6 +380,7 @@
     'GUARD_NO_OVERFLOW/0d',
     'GUARD_OVERFLOW/0d',
     'GUARD_NOT_FORCED/0d',
+    'GUARD_NOT_INVALIDATED/0d',
     '_GUARD_LAST', # ----- end of guard operations -----
 
     '_NOSIDEEFFECT_FIRST', # ----- start of no_side_effect operations -----
@@ -475,6 +476,7 @@
     'VIRTUAL_REF_FINISH/2',   # removed before it's passed to the backend
     'COPYSTRCONTENT/5',       # src, dst, srcstart, dststart, length
     'COPYUNICODECONTENT/5',
+    'QUASIIMMUT_FIELD/1d',    # [objptr], descr=SlowMutateDescr
 
     '_CANRAISE_FIRST', # ----- start of can_raise operations -----
     '_CALL_FIRST',

diff --git a/pypy/rpython/lltypesystem/test/test_lltype.py b/pypy/rpython/lltypesystem/test/test_lltype.py
--- a/pypy/rpython/lltypesystem/test/test_lltype.py
+++ b/pypy/rpython/lltypesystem/test/test_lltype.py
@@ -794,15 +794,8 @@
         def __init__(self, fields):
             self.fields = fields
     S = GcStruct('S', ('x', lltype.Signed),
-                 hints={'immutable_fields': FieldListAccessor({'x':''})})
-    assert S._immutable_field('x') == True
-    #
-    class FieldListAccessor(object):
-        def __init__(self, fields):
-            self.fields = fields
-    S = GcStruct('S', ('x', lltype.Signed),
-                 hints={'immutable_fields': FieldListAccessor({'x':'[*]'})})
-    assert S._immutable_field('x') == '[*]'
+                 hints={'immutable_fields': FieldListAccessor({'x': 1234})})
+    assert S._immutable_field('x') == 1234
 
 def test_typedef():
     T = Typedef(Signed, 'T')

diff --git a/pypy/jit/metainterp/optimizeopt/unroll.py b/pypy/jit/metainterp/optimizeopt/unroll.py
--- a/pypy/jit/metainterp/optimizeopt/unroll.py
+++ b/pypy/jit/metainterp/optimizeopt/unroll.py
@@ -267,6 +267,8 @@
             virtual_state = modifier.get_virtual_state(jump_args)
 
             loop.preamble.operations = self.optimizer.newoperations
+            loop.preamble.quasi_immutable_deps = (
+                self.optimizer.quasi_immutable_deps)
             self.optimizer = self.optimizer.reconstruct_for_next_iteration()
             inputargs = self.inline(self.cloned_operations,
                                     loop.inputargs, jump_args)
@@ -276,6 +278,7 @@
             loop.preamble.operations.append(jmp)
 
             loop.operations = self.optimizer.newoperations
+            loop.quasi_immutable_deps = self.optimizer.quasi_immutable_deps
 
             start_resumedescr = loop.preamble.start_resumedescr.clone_if_mutable()
             assert isinstance(start_resumedescr, ResumeGuardDescr)

diff --git a/pypy/objspace/std/typeobject.py b/pypy/objspace/std/typeobject.py
--- a/pypy/objspace/std/typeobject.py
+++ b/pypy/objspace/std/typeobject.py
@@ -85,6 +85,7 @@
                           'nslots',
                           'instancetypedef',
                           'terminator',
+                          '_version_tag?',
                           ]
 
     # for config.objspace.std.getattributeshortcut

diff --git a/pypy/jit/metainterp/warmspot.py b/pypy/jit/metainterp/warmspot.py
--- a/pypy/jit/metainterp/warmspot.py
+++ b/pypy/jit/metainterp/warmspot.py
@@ -131,6 +131,16 @@
 def find_set_param(graphs):
     return _find_jit_marker(graphs, 'set_param')
 
+def find_force_quasi_immutable(graphs):
+    results = []
+    for graph in graphs:
+        for block in graph.iterblocks():
+            for i in range(len(block.operations)):
+                op = block.operations[i]
+                if op.opname == 'jit_force_quasi_immutable':
+                    results.append((graph, block, i))
+    return results
+
 def get_stats():
     return pyjitpl._warmrunnerdesc.stats
 
@@ -187,6 +197,7 @@
         self.rewrite_can_enter_jits()
         self.rewrite_set_param()
         self.rewrite_force_virtual(vrefinfo)
+        self.rewrite_force_quasi_immutable()
         self.add_finish()
         self.metainterp_sd.finish_setup(self.codewriter)
 
@@ -842,6 +853,28 @@
         all_graphs = self.translator.graphs
         vrefinfo.replace_force_virtual_with_call(all_graphs)
 
+    def replace_force_quasiimmut_with_direct_call(self, op):
+        ARG = op.args[0].concretetype
+        mutatefieldname = op.args[1].value
+        key = (ARG, mutatefieldname)
+        if key in self._cache_force_quasiimmed_funcs:
+            cptr = self._cache_force_quasiimmed_funcs[key]
+        else:
+            from pypy.jit.metainterp import quasiimmut
+            func = quasiimmut.make_invalidation_function(ARG, mutatefieldname)
+            FUNC = lltype.Ptr(lltype.FuncType([ARG], lltype.Void))
+            llptr = self.helper_func(FUNC, func)
+            cptr = Constant(llptr, FUNC)
+            self._cache_force_quasiimmed_funcs[key] = cptr
+        op.opname = 'direct_call'
+        op.args = [cptr, op.args[0]]
+
+    def rewrite_force_quasi_immutable(self):
+        self._cache_force_quasiimmed_funcs = {}
+        graphs = self.translator.graphs
+        for graph, block, i in find_force_quasi_immutable(graphs):
+            self.replace_force_quasiimmut_with_direct_call(block.operations[i])
+
     # ____________________________________________________________
 
     def execute_token(self, loop_token):

diff --git a/pypy/rpython/lltypesystem/rclass.py b/pypy/rpython/lltypesystem/rclass.py
--- a/pypy/rpython/lltypesystem/rclass.py
+++ b/pypy/rpython/lltypesystem/rclass.py
@@ -322,6 +322,7 @@
         #       before they are fully built, to avoid strange bugs in case
         #       of recursion where other code would uses these
         #       partially-initialized dicts.
+        AbstractInstanceRepr._setup_repr(self)
         self.rclass = getclassrepr(self.rtyper, self.classdef)
         fields = {}
         allinstancefields = {}
@@ -370,6 +371,11 @@
             kwds = {}
             if self.gcflavor == 'gc':
                 kwds['rtti'] = True
+
+            for name, attrdef in attrs:
+                if not attrdef.readonly and self.is_quasi_immutable(name):
+                    llfields.append(('mutate_' + name, OBJECTPTR))
+
             object_type = MkStruct(self.classdef.name,
                                    ('super', self.rbase.object_type),
                                    hints=hints,
@@ -488,6 +494,7 @@
             if force_cast:
                 vinst = llops.genop('cast_pointer', [vinst], resulttype=self)
             self.hook_access_field(vinst, cname, llops, flags)
+            self.hook_setfield(vinst, attr, llops)
             llops.genop('setfield', [vinst, cname, vvalue])
         else:
             if self.classdef is None:
@@ -495,9 +502,6 @@
             self.rbase.setfield(vinst, attr, vvalue, llops, force_cast=True,
                                 flags=flags)
 
-    def hook_access_field(self, vinst, cname, llops, flags):
-        pass        # for virtualizables; see rvirtualizable2.py
-
     def new_instance(self, llops, classcallhop=None):
         """Build a new instance, without calling __init__."""
         flavor = self.gcflavor

diff --git a/pypy/rpython/annlowlevel.py b/pypy/rpython/annlowlevel.py
--- a/pypy/rpython/annlowlevel.py
+++ b/pypy/rpython/annlowlevel.py
@@ -480,7 +480,26 @@
 # ____________________________________________________________
 
 def cast_object_to_ptr(PTR, object):
-    raise NotImplementedError("cast_object_to_ptr")
+    """NOT_RPYTHON: hack. The object may be disguised as a PTR now.
+    Limited to casting a given object to a single type.
+    """
+    if isinstance(PTR, lltype.Ptr):
+        TO = PTR.TO
+    else:
+        TO = PTR
+    if not hasattr(object, '_carry_around_for_tests'):
+        assert not hasattr(object, '_TYPE')
+        object._carry_around_for_tests = True
+        object._TYPE = TO
+    else:
+        assert object._TYPE == TO
+    #
+    if isinstance(PTR, lltype.Ptr):
+        return lltype._ptr(PTR, object, True)
+    elif isinstance(PTR, ootype.Instance):
+        return object
+    else:
+        raise NotImplementedError("cast_object_to_ptr(%r, ...)" % PTR)
 
 def cast_instance_to_base_ptr(instance):
     return cast_object_to_ptr(base_ptr_lltype(), instance)
@@ -535,7 +554,13 @@
 # ____________________________________________________________
 
 def cast_base_ptr_to_instance(Class, ptr):
-    raise NotImplementedError("cast_base_ptr_to_instance")
+    """NOT_RPYTHON: hack. Reverse the hacking done in cast_object_to_ptr()."""
+    if isinstance(lltype.typeOf(ptr), lltype.Ptr):
+        ptr = ptr._as_obj()
+    if not isinstance(ptr, Class):
+        raise NotImplementedError("cast_base_ptr_to_instance: casting %r to %r"
+                                  % (ptr, Class))
+    return ptr
 
 class CastBasePtrToInstanceEntry(extregistry.ExtRegistryEntry):
     _about_ = cast_base_ptr_to_instance

diff --git a/pypy/jit/metainterp/optimizeopt/simplify.py b/pypy/jit/metainterp/optimizeopt/simplify.py
--- a/pypy/jit/metainterp/optimizeopt/simplify.py
+++ b/pypy/jit/metainterp/optimizeopt/simplify.py
@@ -20,6 +20,9 @@
         op = ResOperation(rop.SAME_AS, [op.getarg(0)], op.result)
         self.emit_operation(op)
 
+    def optimize_QUASIIMMUT_FIELD(self, op):
+        pass
+
     def propagate_forward(self, op):
         opnum = op.getopnum()
         for value, func in optimize_ops:

diff --git a/pypy/rpython/ootypesystem/ootype.py b/pypy/rpython/ootypesystem/ootype.py
--- a/pypy/rpython/ootypesystem/ootype.py
+++ b/pypy/rpython/ootypesystem/ootype.py
@@ -268,13 +268,14 @@
         return self._superclass._get_fields_with_default() + self._fields_with_default
 
     def _immutable_field(self, field):
+        if self._hints.get('immutable'):
+            return True
         if 'immutable_fields' in self._hints:
             try:
-                s = self._hints['immutable_fields'].fields[field]
-                return s or True
+                return self._hints['immutable_fields'].fields[field]
             except KeyError:
                 pass
-        return self._hints.get('immutable', False)
+        return False
 
 
 class SpecializableType(OOType):

diff --git a/pypy/rpython/test/test_rclass.py b/pypy/rpython/test/test_rclass.py
--- a/pypy/rpython/test/test_rclass.py
+++ b/pypy/rpython/test/test_rclass.py
@@ -5,6 +5,8 @@
 from pypy.rpython.ootypesystem import ootype
 from pypy.rlib.rarithmetic import intmask, r_longlong
 from pypy.rpython.test.tool import BaseRtypingTest, LLRtypeMixin, OORtypeMixin
+from pypy.rpython.rclass import IR_IMMUTABLE, IR_ARRAY_IMMUTABLE
+from pypy.rpython.rclass import IR_QUASI_IMMUTABLE
 from pypy.objspace.flow.model import summary
 
 class EmptyBase(object):
@@ -746,8 +748,10 @@
         t, typer, graph = self.gengraph(f, [])
         A_TYPE = deref(graph.getreturnvar().concretetype)
         accessor = A_TYPE._hints["immutable_fields"]
-        assert accessor.fields == {"inst_x" : "", "inst_y" : "[*]"} or \
-               accessor.fields == {"ox" : "", "oy" : "[*]"} # for ootype
+        assert accessor.fields == {"inst_x": IR_IMMUTABLE,
+                                   "inst_y": IR_ARRAY_IMMUTABLE} or \
+               accessor.fields == {"ox": IR_IMMUTABLE,
+                                   "oy": IR_ARRAY_IMMUTABLE} # for ootype
 
     def test_immutable_fields_subclass_1(self):
         from pypy.jit.metainterp.typesystem import deref
@@ -765,8 +769,8 @@
         t, typer, graph = self.gengraph(f, [])
         B_TYPE = deref(graph.getreturnvar().concretetype)
         accessor = B_TYPE._hints["immutable_fields"]
-        assert accessor.fields == {"inst_x" : ""} or \
-               accessor.fields == {"ox" : ""} # for ootype
+        assert accessor.fields == {"inst_x": IR_IMMUTABLE} or \
+               accessor.fields == {"ox": IR_IMMUTABLE} # for ootype
 
     def test_immutable_fields_subclass_2(self):
         from pypy.jit.metainterp.typesystem import deref
@@ -785,8 +789,10 @@
         t, typer, graph = self.gengraph(f, [])
         B_TYPE = deref(graph.getreturnvar().concretetype)
         accessor = B_TYPE._hints["immutable_fields"]
-        assert accessor.fields == {"inst_x" : "", "inst_y" : ""} or \
-               accessor.fields == {"ox" : "", "oy" : ""} # for ootype
+        assert accessor.fields == {"inst_x": IR_IMMUTABLE,
+                                   "inst_y": IR_IMMUTABLE} or \
+               accessor.fields == {"ox": IR_IMMUTABLE,
+                                   "oy": IR_IMMUTABLE} # for ootype
 
     def test_immutable_fields_only_in_subclass(self):
         from pypy.jit.metainterp.typesystem import deref
@@ -804,8 +810,8 @@
         t, typer, graph = self.gengraph(f, [])
         B_TYPE = deref(graph.getreturnvar().concretetype)
         accessor = B_TYPE._hints["immutable_fields"]
-        assert accessor.fields == {"inst_y" : ""} or \
-               accessor.fields == {"oy" : ""} # for ootype
+        assert accessor.fields == {"inst_y": IR_IMMUTABLE} or \
+               accessor.fields == {"oy": IR_IMMUTABLE} # for ootype
 
     def test_immutable_forbidden_inheritance_1(self):
         from pypy.rpython.rclass import ImmutableConflictError
@@ -849,8 +855,8 @@
         except AttributeError:
             A_TYPE = B_TYPE._superclass  # for ootype
         accessor = A_TYPE._hints["immutable_fields"]
-        assert accessor.fields == {"inst_v" : ""} or \
-               accessor.fields == {"ov" : ""} # for ootype
+        assert accessor.fields == {"inst_v": IR_IMMUTABLE} or \
+               accessor.fields == {"ov": IR_IMMUTABLE} # for ootype
 
     def test_immutable_subclass_1(self):
         from pypy.rpython.rclass import ImmutableConflictError
@@ -895,6 +901,37 @@
         B_TYPE = deref(graph.getreturnvar().concretetype)
         assert B_TYPE._hints["immutable"]
 
+    def test_quasi_immutable(self):
+        from pypy.jit.metainterp.typesystem import deref
+        class A(object):
+            _immutable_fields_ = ['x', 'y', 'a?', 'b?']
+        class B(A):
+            pass
+        def f():
+            a = A()
+            a.x = 42
+            a.a = 142
+            b = B()
+            b.x = 43
+            b.y = 41
+            b.a = 44
+            b.b = 45
+            return B()
+        t, typer, graph = self.gengraph(f, [])
+        B_TYPE = deref(graph.getreturnvar().concretetype)
+        accessor = B_TYPE._hints["immutable_fields"]
+        assert accessor.fields == {"inst_y": IR_IMMUTABLE,
+                                   "inst_b": IR_QUASI_IMMUTABLE} or \
+               accessor.fields == {"ox": IR_IMMUTABLE,
+                                   "oy": IR_IMMUTABLE,
+                                   "oa": IR_QUASI_IMMUTABLE,
+                                   "ob": IR_QUASI_IMMUTABLE} # for ootype
+        found = []
+        for op in graph.startblock.operations:
+            if op.opname == 'jit_force_quasi_immutable':
+                found.append(op.args[1].value)
+        assert found == ['mutate_a', 'mutate_a', 'mutate_b']
+
 
 class TestLLtype(BaseTestRclass, LLRtypeMixin):
 

diff --git a/pypy/jit/backend/x86/assembler.py b/pypy/jit/backend/x86/assembler.py
--- a/pypy/jit/backend/x86/assembler.py
+++ b/pypy/jit/backend/x86/assembler.py
@@ -48,11 +48,12 @@
 
 
 class GuardToken(object):
-    def __init__(self, faildescr, failargs, fail_locs, exc):
+    def __init__(self, faildescr, failargs, fail_locs, exc, has_jump):
         self.faildescr = faildescr
         self.failargs = failargs
         self.fail_locs = fail_locs
         self.exc = exc
+        self.has_jump = has_jump
 
 DEBUG_COUNTER = lltype.Struct('DEBUG_COUNTER', ('i', lltype.Signed))
 
@@ -133,6 +134,7 @@
     def setup(self, looptoken):
         assert self.memcpy_addr != 0, "setup_once() not called?"
         self.current_clt = looptoken.compiled_loop_token
+        self.invalidate_positions = []
         self.pending_guard_tokens = []
         self.mc = codebuf.MachineCodeBlockWrapper()
         if self.datablockwrapper is None:
@@ -141,6 +143,7 @@
                                                             allblocks)
 
     def teardown(self):
+        self.invalidate_positions = None
         self.pending_guard_tokens = None
         self.mc = None
         self.looppos = -1
@@ -435,15 +438,24 @@
         # tok.faildescr._x86_adr_jump_offset to contain the raw address of
         # the 4-byte target field in the JMP/Jcond instruction, and patch
         # the field in question to point (initially) to the recovery stub
+        inv_counter = 0
+        clt = self.current_clt
         for tok in self.pending_guard_tokens:
             addr = rawstart + tok.pos_jump_offset
             tok.faildescr._x86_adr_jump_offset = addr
             relative_target = tok.pos_recovery_stub - (tok.pos_jump_offset + 4)
             assert rx86.fits_in_32bits(relative_target)
             #
-            mc = codebuf.MachineCodeBlockWrapper()
-            mc.writeimm32(relative_target)
-            mc.copy_to_raw_memory(addr)
+            if tok.has_jump:
+                mc = codebuf.MachineCodeBlockWrapper()
+                mc.writeimm32(relative_target)
+                mc.copy_to_raw_memory(addr)
+            else:
+                # guard not invalidate, patch where it jumps
+                pos, _ = self.invalidate_positions[inv_counter]
+                clt.invalidate_positions.append((pos + rawstart,
+                                                 relative_target))
+                inv_counter += 1
 
     def get_asmmemmgr_blocks(self, looptoken):
         clt = looptoken.compiled_loop_token
@@ -1447,6 +1459,13 @@
         self.mc.CMP(heap(self.cpu.pos_exception()), imm0)
         self.implement_guard(guard_token, 'NZ')
 
+    def genop_guard_guard_not_invalidated(self, ign_1, guard_op, guard_token,
+                                     locs, ign_2):
+        pos = self.mc.get_relative_pos() + 1 # after potential jmp
+        guard_token.pos_jump_offset = pos
+        self.invalidate_positions.append((pos, 0))
+        self.pending_guard_tokens.append(guard_token)
+
     def genop_guard_guard_exception(self, ign_1, guard_op, guard_token,
                                     locs, resloc):
         loc = locs[0]
@@ -1545,7 +1564,8 @@
         exc = (guard_opnum == rop.GUARD_EXCEPTION or
                guard_opnum == rop.GUARD_NO_EXCEPTION or
                guard_opnum == rop.GUARD_NOT_FORCED)
-        return GuardToken(faildescr, failargs, fail_locs, exc)
+        return GuardToken(faildescr, failargs, fail_locs, exc, has_jump=
+                          guard_opnum != rop.GUARD_NOT_INVALIDATED)
 
     def generate_quick_failure(self, guardtok):
         """Generate the initial code for handling a failure.  We try to


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