/rpython/jit/metainterp/optimizeopt/rewrite.py

from rpython.jit.codewriter.effectinfo import EffectInfo
from rpython.jit.codewriter import longlong
from rpython.jit.metainterp import compile
from rpython.jit.metainterp.history import (Const, ConstInt, make_hashable_int,
                                            ConstFloat)
from rpython.jit.metainterp.optimize import InvalidLoop
from rpython.jit.metainterp.optimizeopt.intutils import IntBound
from rpython.jit.metainterp.optimizeopt.optimizer import (Optimization, REMOVED,
    CONST_0, CONST_1)
from rpython.jit.metainterp.optimizeopt.info import INFO_NONNULL, INFO_NULL
from rpython.jit.metainterp.optimizeopt.util import _findall, make_dispatcher_method
from rpython.jit.metainterp.resoperation import rop, ResOperation, opclasses,\
     OpHelpers
from rpython.rlib.rarithmetic import highest_bit
from rpython.rtyper.lltypesystem import llmemory
from rpython.rtyper import rclass
import math

class OptRewrite(Optimization):
    """Rewrite operations into equivalent, cheaper operations.
       This includes already executed operations and constants.
    """
    def __init__(self):
        self.loop_invariant_results = {}
        self.loop_invariant_producer = {}

    def setup(self):
        self.optimizer.optrewrite = self

    def produce_potential_short_preamble_ops(self, sb):
        for op in self.loop_invariant_producer.values():
            sb.add_loopinvariant_op(op)

    def propagate_forward(self, op):
        if opclasses[op.opnum].boolinverse != -1 or opclasses[op.opnum].boolreflex != -1:
            if self.find_rewritable_bool(op):
                return

        dispatch_opt(self, op)

    def try_boolinvers(self, op, targs):
        oldop = self.get_pure_result(targs)
        if oldop is not None:
            b = self.getintbound(oldop)
            if b.equal(1):
                self.make_constant(op, CONST_0)
                return True
            elif b.equal(0):
                self.make_constant(op, CONST_1)
                return True
        return False


    def find_rewritable_bool(self, op):
        oldopnum = op.boolinverse
        arg0 = op.getarg(0)
        arg1 = op.getarg(1)
        if oldopnum != -1:
            top = ResOperation(oldopnum, [arg0, arg1])
            if self.try_boolinvers(op, top):
                return True

        oldopnum = op.boolreflex # FIXME: add INT_ADD, INT_MUL
        if oldopnum != -1:
            top = ResOperation(oldopnum, [arg1, arg0])
            oldop = self.get_pure_result(top)
            if oldop is not None:
                self.optimizer.make_equal_to(op, oldop)
                return True

        if op.boolreflex == -1:
            return False
        oldopnum = opclasses[op.boolreflex].boolinverse
        if oldopnum != -1:
            top = ResOperation(oldopnum, [arg1, arg0])
            if self.try_boolinvers(op, top):
                return True

        return False

    def optimize_INT_AND(self, op):
        b1 = self.getintbound(op.getarg(0))
        b2 = self.getintbound(op.getarg(1))
        if b1.equal(0) or b2.equal(0):
            self.make_constant_int(op, 0)
            return
        elif b2.is_constant():
            val = b2.lower
            if val == -1 or b1.lower >= 0 \
                and b1.upper <= val & ~(val + 1):
                self.make_equal_to(op, op.getarg(0))
                return
        elif b1.is_constant():
            val = b1.lower
            if val == -1 or b2.lower >= 0 \
                and b2.upper <= val & ~(val + 1):
                self.make_equal_to(op, op.getarg(1))
                return

        self.emit_operation(op)

    def optimize_INT_OR(self, op):
        b1 = self.getintbound(op.getarg(0))
        b2 = self.getintbound(op.getarg(1))
        if b1.equal(0):
            self.make_equal_to(op, op.getarg(1))
        elif b2.equal(0):
            self.make_equal_to(op, op.getarg(0))
        else:
            self.emit_operation(op)

    def optimize_INT_SUB(self, op):
        arg1 = self.get_box_replacement(op.getarg(0))
        arg2 = self.get_box_replacement(op.getarg(1))
        b1 = self.getintbound(arg1)
        b2 = self.getintbound(arg2)
        if b2.equal(0):
            self.make_equal_to(op, arg1)
        elif b1.equal(0):
            op = self.replace_op_with(op, rop.INT_NEG, args=[arg2])
            self.emit_operation(op)
        elif arg1 == arg2:
            self.make_constant_int(op, 0)
        else:
            self.emit_operation(op)
            self.optimizer.pure_reverse(op)

    def optimize_INT_ADD(self, op):
        if self.is_raw_ptr(op.getarg(0)) or self.is_raw_ptr(op.getarg(1)):
            self.emit_operation(op)
            return
        arg1 = self.get_box_replacement(op.getarg(0))
        b1 = self.getintbound(arg1)
        arg2 = self.get_box_replacement(op.getarg(1))
        b2 = self.getintbound(arg2)

        # If one side of the op is 0 the result is the other side.
        if b1.equal(0):
            self.make_equal_to(op, arg2)
        elif b2.equal(0):
            self.make_equal_to(op, arg1)
        else:
            self.emit_operation(op)
            self.optimizer.pure_reverse(op)

    def optimize_INT_MUL(self, op):
        arg1 = self.get_box_replacement(op.getarg(0))
        b1 = self.getintbound(arg1)
        arg2 = self.get_box_replacement(op.getarg(1))
        b2 = self.getintbound(arg2)

        # If one side of the op is 1 the result is the other side.
        if b1.equal(1):
            self.make_equal_to(op, arg2)
        elif b2.equal(1):
            self.make_equal_to(op, arg1)
        elif b1.equal(0) or b2.equal(0):
            self.make_constant_int(op, 0)
        else:
            for lhs, rhs in [(arg1, arg2), (arg2, arg1)]:
                lh_info = self.getintbound(lhs)
                if lh_info.is_constant():
                    x = lh_info.getint()
                    # x & (x - 1) == 0 is a quick test for power of 2
                    if x & (x - 1) == 0:
                        new_rhs = ConstInt(highest_bit(lh_info.getint()))
                        op = self.replace_op_with(op, rop.INT_LSHIFT, args=[rhs, new_rhs])
                        break
            self.emit_operation(op)

    def _optimize_CALL_INT_UDIV(self, op):
        b2 = self.getintbound(op.getarg(2))
        if b2.is_constant() and b2.getint() == 1:
            self.make_equal_to(op, op.getarg(1))
            self.last_emitted_operation = REMOVED
            return True
        return False

    def optimize_INT_LSHIFT(self, op):
        b1 = self.getintbound(op.getarg(0))
        b2 = self.getintbound(op.getarg(1))

        if b2.is_constant() and b2.getint() == 0:
            self.make_equal_to(op, op.getarg(0))
        elif b1.is_constant() and b1.getint() == 0:
            self.make_constant_int(op, 0)
        else:
            self.emit_operation(op)

    def optimize_INT_RSHIFT(self, op):
        b1 = self.getintbound(op.getarg(0))
        b2 = self.getintbound(op.getarg(1))

        if b2.is_constant() and b2.getint() == 0:
            self.make_equal_to(op, op.getarg(0))
        elif b1.is_constant() and b1.getint() == 0:
            self.make_constant_int(op, 0)
        else:
            self.emit_operation(op)

    def optimize_INT_XOR(self, op):
        b1 = self.getintbound(op.getarg(0))
        b2 = self.getintbound(op.getarg(1))

        if b1.equal(0):
            self.make_equal_to(op, op.getarg(1))
        elif b2.equal(0):
            self.make_equal_to(op, op.getarg(0))
        else:
            self.emit_operation(op)

    def optimize_FLOAT_MUL(self, op):
        arg1 = op.getarg(0)
        arg2 = op.getarg(1)

        # Constant fold f0 * 1.0 and turn f0 * -1.0 into a FLOAT_NEG, these
        # work in all cases, including NaN and inf
        for lhs, rhs in [(arg1, arg2), (arg2, arg1)]:
            v1 = self.get_box_replacement(lhs)
            v2 = self.get_box_replacement(rhs)

            if v1.is_constant():
                if v1.getfloat() == 1.0:
                    self.make_equal_to(op, v2)
                    return
                elif v1.getfloat() == -1.0:
                    newop = self.replace_op_with(op, rop.FLOAT_NEG, args=[rhs])
                    self.emit_operation(newop)
                    return
        self.emit_operation(op)
        self.optimizer.pure_reverse(op)

    def optimize_FLOAT_TRUEDIV(self, op):
        arg1 = op.getarg(0)
        arg2 = op.getarg(1)
        v2 = self.get_box_replacement(arg2)

        # replace "x / const" by "x * (1/const)" if possible
        newop = op
        if v2.is_constant():
            divisor = v2.getfloat()
            fraction = math.frexp(divisor)[0]
            # This optimization is valid for powers of two
            # but not for zeroes, some denormals and NaN:
            if fraction == 0.5 or fraction == -0.5:
                reciprocal = 1.0 / divisor
                rfraction = math.frexp(reciprocal)[0]
                if rfraction == 0.5 or rfraction == -0.5:
                    c = ConstFloat(longlong.getfloatstorage(reciprocal))
                    newop = self.replace_op_with(op, rop.FLOAT_MUL,
                                                 args=[arg1, c])
        self.emit_operation(newop)

    def optimize_FLOAT_NEG(self, op):
        self.emit_operation(op)
        self.optimizer.pure_reverse(op)

    def optimize_guard(self, op, constbox, emit_operation=True):
        box = op.getarg(0)
        if box.type == 'i':
            intbound = self.getintbound(box)
            if intbound.is_constant():
                if not intbound.getint() == constbox.getint():
                    r = self.optimizer.metainterp_sd.logger_ops.repr_of_resop(
                        op)
                    raise InvalidLoop('A GUARD_{VALUE,TRUE,FALSE} (%s) '
                                      'was proven to always fail' % r)
                return
        elif box.type == 'r':
            box = self.get_box_replacement(box)
            if box.is_constant():
                if not box.same_constant(constbox):
                    r = self.optimizer.metainterp_sd.logger_ops.repr_of_resop(
                        op)
                    raise InvalidLoop('A GUARD_VALUE (%s) '
                                      'was proven to always fail' % r)
                return
                    
        if emit_operation:
            self.emit_operation(op)
        self.make_constant(box, constbox)
        #if self.optimizer.optheap:  XXX
        #    self.optimizer.optheap.value_updated(value, self.getvalue(constbox))

    def optimize_GUARD_ISNULL(self, op):
        info = self.getptrinfo(op.getarg(0))
        if info is not None:
            if info.is_null():
                return
            elif info.is_nonnull():
                r = self.optimizer.metainterp_sd.logger_ops.repr_of_resop(op)
                raise InvalidLoop('A GUARD_ISNULL (%s) was proven to always '
                                  'fail' % r)
        self.emit_operation(op)
        self.make_constant(op.getarg(0), self.optimizer.cpu.ts.CONST_NULL)

    def optimize_GUARD_IS_OBJECT(self, op):
        info = self.getptrinfo(op.getarg(0))
        if info and info.is_constant():
            if info.is_null():
                raise InvalidLoop("A GUARD_IS_OBJECT(NULL) found")
            c = self.get_box_replacement(op.getarg(0))
            if self.optimizer.cpu.check_is_object(c.getref_base()):
                return
            raise InvalidLoop("A GUARD_IS_OBJECT(not-an-object) found")
        if info is not None:
            if info.is_about_object():
                return
            if info.is_precise():
                raise InvalidLoop()
        self.emit_operation(op)

    def optimize_GUARD_GC_TYPE(self, op):
        info = self.getptrinfo(op.getarg(0))
        if info and info.is_constant():
            c = self.get_box_replacement(op.getarg(0))
            tid = self.optimizer.cpu.get_actual_typeid(c.getref_base())
            if tid != op.getarg(1).getint():
                raise InvalidLoop("wrong GC type ID found on a constant")
            return
        if info is not None and info.get_descr() is not None:
            if info.get_descr().get_type_id() != op.getarg(1).getint():
                raise InvalidLoop("wrong GC types passed around!")
            return
        self.emit_operation(op)

    def _check_subclass(self, vtable1, vtable2):
        # checks that vtable1 is a subclass of vtable2
        known_class = llmemory.cast_adr_to_ptr(
            llmemory.cast_int_to_adr(vtable1),
            rclass.CLASSTYPE)
        expected_class = llmemory.cast_adr_to_ptr(
            llmemory.cast_int_to_adr(vtable2),
            rclass.CLASSTYPE)
        if (expected_class.subclassrange_min
                <= known_class.subclassrange_min
                <= expected_class.subclassrange_max):
            return True
        return False

    def optimize_GUARD_SUBCLASS(self, op):
        info = self.getptrinfo(op.getarg(0))
        if info and info.is_constant():
            c = self.get_box_replacement(op.getarg(0))
            vtable = self.optimizer.cpu.ts.cls_of_box(c).getint()
            if self._check_subclass(vtable, op.getarg(1).getint()):
                return
            raise InvalidLoop("GUARD_SUBCLASS(const) proven to always fail")
        if info is not None and info.is_about_object():
            known_class = info.get_known_class(self.optimizer.cpu)
            if known_class:
                if self._check_subclass(known_class.getint(),
                                        op.getarg(1).getint()):
                    return
            elif info.get_descr() is not None:
                if self._check_subclass(info.get_descr().get_vtable(),
                                        op.getarg(1).getint()):
                    return
        self.emit_operation(op)

    def optimize_GUARD_NONNULL(self, op):
        opinfo = self.getptrinfo(op.getarg(0))
        if opinfo is not None:
            if opinfo.is_nonnull():
                return
            elif opinfo.is_null():
                r = self.optimizer.metainterp_sd.logger_ops.repr_of_resop(op)
                raise InvalidLoop('A GUARD_NONNULL (%s) was proven to always '
                                  'fail' % r)
        self.emit_operation(op)
        self.make_nonnull(op.getarg(0))
        self.getptrinfo(op.getarg(0)).mark_last_guard(self.optimizer)

    def optimize_GUARD_VALUE(self, op):
        arg0 = op.getarg(0)
        if arg0.type == 'r':
            info = self.getptrinfo(arg0)
            if info:
                if info.is_virtual():
                    raise InvalidLoop("promote of a virtual")
                old_guard_op = info.get_last_guard(self.optimizer)
                if old_guard_op is not None:
                    op = self.replace_old_guard_with_guard_value(op, info,
                                                              old_guard_op)
        elif arg0.type == 'f':
            arg0 = self.get_box_replacement(arg0)
            if arg0.is_constant():
                return
        constbox = op.getarg(1)
        assert isinstance(constbox, Const)
        self.optimize_guard(op, constbox)

    def replace_old_guard_with_guard_value(self, op, info, old_guard_op):
        # there already has been a guard_nonnull or guard_class or
        # guard_nonnull_class on this value, which is rather silly.
        # This function replaces the original guard with a
        # guard_value.  Must be careful: doing so is unsafe if the
        # original guard checks for something inconsistent,
        # i.e. different than what it would give if the guard_value
        # passed (this is a rare case, but possible).  If we get
        # inconsistent results in this way, then we must not do the
        # replacement, otherwise we'd put guard_value up there but all
        # intermediate ops might be executed by assuming something
        # different, from the old guard that is now removed...

        c_value = op.getarg(1)
        if not c_value.nonnull():
            raise InvalidLoop('A GUARD_VALUE(..., NULL) follows some other '
                              'guard that it is not NULL')
        previous_classbox = info.get_known_class(self.optimizer.cpu)
        if previous_classbox is not None:
            expected_classbox = self.optimizer.cpu.ts.cls_of_box(c_value)
            assert expected_classbox is not None
            if not previous_classbox.same_constant(
                    expected_classbox):
                r = self.optimizer.metainterp_sd.logger_ops.repr_of_resop(op)
                raise InvalidLoop('A GUARD_VALUE (%s) was proven to '
                                  'always fail' % r)
        descr = compile.ResumeGuardDescr()
        op = old_guard_op.copy_and_change(rop.GUARD_VALUE,
                         args = [old_guard_op.getarg(0), op.getarg(1)],
                         descr = descr)
        # Note: we give explicitly a new descr for 'op'; this is why the
        # old descr must not be ResumeAtPositionDescr (checked above).
        # Better-safe-than-sorry but it should never occur: we should
        # not put in short preambles guard_xxx and guard_value
        # on the same box.
        self.optimizer.replace_guard(op, info)
        # to be safe
        info.reset_last_guard_pos()
        return op

    def optimize_GUARD_TRUE(self, op):
        self.optimize_guard(op, CONST_1)

    def optimize_GUARD_FALSE(self, op):
        self.optimize_guard(op, CONST_0)

    def optimize_RECORD_EXACT_CLASS(self, op):
        opinfo = self.getptrinfo(op.getarg(0))
        expectedclassbox = op.getarg(1)
        assert isinstance(expectedclassbox, Const)
        if opinfo is not None:
            realclassbox = opinfo.get_known_class(self.optimizer.cpu)
            if realclassbox is not None:
                assert realclassbox.same_constant(expectedclassbox)
                return
        self.make_constant_class(op.getarg(0), expectedclassbox,
                                 update_last_guard=False)

    def optimize_GUARD_CLASS(self, op):
        expectedclassbox = op.getarg(1)
        info = self.ensure_ptr_info_arg0(op)
        assert isinstance(expectedclassbox, Const)
        realclassbox = info.get_known_class(self.optimizer.cpu)
        if realclassbox is not None:
            if realclassbox.same_constant(expectedclassbox):
                return
            r = self.optimizer.metainterp_sd.logger_ops.repr_of_resop(op)
            raise InvalidLoop('A GUARD_CLASS (%s) was proven to always fail'
                              % r)
        old_guard_op = info.get_last_guard(self.optimizer)
        if old_guard_op and not isinstance(old_guard_op.getdescr(),
                                           compile.ResumeAtPositionDescr):
            # there already has been a guard_nonnull or guard_class or
            # guard_nonnull_class on this value.
            if old_guard_op.getopnum() == rop.GUARD_NONNULL:
                # it was a guard_nonnull, which we replace with a
                # guard_nonnull_class.
                descr = compile.ResumeGuardDescr()
                op = old_guard_op.copy_and_change (rop.GUARD_NONNULL_CLASS,
                            args = [old_guard_op.getarg(0), op.getarg(1)],
                            descr=descr)
                # Note: we give explicitly a new descr for 'op'; this is why the
                # old descr must not be ResumeAtPositionDescr (checked above).
                # Better-safe-than-sorry but it should never occur: we should
                # not put in short preambles guard_nonnull and guard_class
                # on the same box.
                self.optimizer.replace_guard(op, info)
                self.emit_operation(op)
                self.make_constant_class(op.getarg(0), expectedclassbox, False)
                return
        self.emit_operation(op)
        self.make_constant_class(op.getarg(0), expectedclassbox)

    def optimize_GUARD_NONNULL_CLASS(self, op):
        info = self.getptrinfo(op.getarg(0))
        if info and info.is_null():
            r = self.optimizer.metainterp_sd.logger_ops.repr_of_resop(op)
            raise InvalidLoop('A GUARD_NONNULL_CLASS (%s) was proven to '
                              'always fail' % r)
        self.optimize_GUARD_CLASS(op)

    def optimize_CALL_LOOPINVARIANT_I(self, op):
        arg = op.getarg(0)
        # 'arg' must be a Const, because residual_call in codewriter
        # expects a compile-time constant
        assert isinstance(arg, Const)
        key = make_hashable_int(arg.getint())

        resvalue = self.loop_invariant_results.get(key, None)
        if resvalue is not None:
            resvalue = self.optimizer.force_op_from_preamble(resvalue)
            self.loop_invariant_results[key] = resvalue
            self.make_equal_to(op, resvalue)
            self.last_emitted_operation = REMOVED
            return
        # change the op to be a normal call, from the backend's point of view
        # there is no reason to have a separate operation for this
        newop = self.replace_op_with(op,
                                     OpHelpers.call_for_descr(op.getdescr()))
        self.emit_operation(newop)
        self.loop_invariant_producer[key] = self.optimizer.getlastop()
        self.loop_invariant_results[key] = op
    optimize_CALL_LOOPINVARIANT_R = optimize_CALL_LOOPINVARIANT_I
    optimize_CALL_LOOPINVARIANT_F = optimize_CALL_LOOPINVARIANT_I
    optimize_CALL_LOOPINVARIANT_N = optimize_CALL_LOOPINVARIANT_I

    def optimize_COND_CALL(self, op):
        arg = op.getarg(0)
        b = self.getintbound(arg)
        if b.is_constant():
            if b.getint() == 0:
                self.last_emitted_operation = REMOVED
                return
            opnum = OpHelpers.call_for_type(op.type)
            op = op.copy_and_change(opnum, args=op.getarglist()[1:])
        self.emit_operation(op)

    def _optimize_nullness(self, op, box, expect_nonnull):
        info = self.getnullness(box)
        if info == INFO_NONNULL:
            self.make_constant_int(op, expect_nonnull)
        elif info == INFO_NULL:
            self.make_constant_int(op, not expect_nonnull)
        else:
            self.emit_operation(op)

    def optimize_INT_IS_TRUE(self, op):
        if (not self.is_raw_ptr(op.getarg(0)) and
            self.getintbound(op.getarg(0)).is_bool()):
            self.make_equal_to(op, op.getarg(0))
            return
        self._optimize_nullness(op, op.getarg(0), True)

    def optimize_INT_IS_ZERO(self, op):
        self._optimize_nullness(op, op.getarg(0), False)

    def _optimize_oois_ooisnot(self, op, expect_isnot, instance):
        arg0 = self.get_box_replacement(op.getarg(0))
        arg1 = self.get_box_replacement(op.getarg(1))
        info0 = self.getptrinfo(arg0)
        info1 = self.getptrinfo(arg1)
        if info0 and info0.is_virtual():
            if info1 and info1.is_virtual():
                intres = (info0 is info1) ^ expect_isnot
                self.make_constant_int(op, intres)
            else:
                self.make_constant_int(op, expect_isnot)
        elif info1 and info1.is_virtual():
            self.make_constant_int(op, expect_isnot)
        elif info1 and info1.is_null():
            self._optimize_nullness(op, op.getarg(0), expect_isnot)
        elif info0 and info0.is_null():
            self._optimize_nullness(op, op.getarg(1), expect_isnot)
        elif arg0 is arg1:
            self.make_constant_int(op, not expect_isnot)
        else:
            if instance:
                if info0 is None:
                    cls0 = None
                else:
                    cls0 = info0.get_known_class(self.optimizer.cpu)
                if cls0 is not None:
                    if info1 is None:
                        cls1 = None
                    else:
                        cls1 = info1.get_known_class(self.optimizer.cpu)
                    if cls1 is not None and not cls0.same_constant(cls1):
                        # cannot be the same object, as we know that their
                        # class is different
                        self.make_constant_int(op, expect_isnot)
                        return
            self.emit_operation(op)

    def optimize_PTR_EQ(self, op):
        self._optimize_oois_ooisnot(op, False, False)

    def optimize_PTR_NE(self, op):
        self._optimize_oois_ooisnot(op, True, False)

    def optimize_INSTANCE_PTR_EQ(self, op):
        self._optimize_oois_ooisnot(op, False, True)

    def optimize_INSTANCE_PTR_NE(self, op):
        self._optimize_oois_ooisnot(op, True, True)

    def optimize_CALL_N(self, op):
        # dispatch based on 'oopspecindex' to a method that handles
        # specifically the given oopspec call.  For non-oopspec calls,
        # oopspecindex is just zero.
        effectinfo = op.getdescr().get_extra_info()
        oopspecindex = effectinfo.oopspecindex
        if oopspecindex == EffectInfo.OS_ARRAYCOPY:
            if self._optimize_CALL_ARRAYCOPY(op):
                return
        self.emit_operation(op)

    def _optimize_CALL_ARRAYCOPY(self, op):
        length = self.get_constant_box(op.getarg(5))
        if length and length.getint() == 0:
            return True # 0-length arraycopy

        source_info = self.getptrinfo(op.getarg(1))
        dest_info = self.getptrinfo(op.getarg(2))
        source_start_box = self.get_constant_box(op.getarg(3))
        dest_start_box = self.get_constant_box(op.getarg(4))
        extrainfo = op.getdescr().get_extra_info()
        if (source_start_box and dest_start_box
            and length and ((dest_info and dest_info.is_virtual()) or
                            length.getint() <= 8) and
            ((source_info and source_info.is_virtual()) or length.getint() <= 8)
            and extrainfo.single_write_descr_array is not None): #<-sanity check
            source_start = source_start_box.getint()
            dest_start = dest_start_box.getint()
            arraydescr = extrainfo.single_write_descr_array
            if arraydescr.is_array_of_structs():
                return False       # not supported right now

            # XXX fish fish fish
            for index in range(length.getint()):
                if source_info and source_info.is_virtual():
                    val = source_info.getitem(arraydescr, index + source_start)
                else:
                    opnum = OpHelpers.getarrayitem_for_descr(arraydescr)
                    newop = ResOperation(opnum,
                                      [op.getarg(1),
                                       ConstInt(index + source_start)],
                                       descr=arraydescr)
                    self.optimizer.send_extra_operation(newop)
                    val = newop
                if val is None:
                    continue
                if dest_info and dest_info.is_virtual():
                    dest_info.setitem(arraydescr, index + dest_start,
                                      self.get_box_replacement(op.getarg(2)),
                                      val)
                else:
                    newop = ResOperation(rop.SETARRAYITEM_GC,
                                         [op.getarg(2),
                                          ConstInt(index + dest_start),
                                          val],
                                         descr=arraydescr)
                    self.emit_operation(newop)
            return True
        return False

    def optimize_CALL_PURE_I(self, op):
        # this removes a CALL_PURE with all constant arguments.
        # Note that it's also done in pure.py.  For now we need both...
        result = self._can_optimize_call_pure(op)
        if result is not None:
            self.make_constant(op, result)
            self.last_emitted_operation = REMOVED
            return
        # dispatch based on 'oopspecindex' to a method that handles
        # specifically the given oopspec call.
        effectinfo = op.getdescr().get_extra_info()
        oopspecindex = effectinfo.oopspecindex
        if oopspecindex == EffectInfo.OS_INT_UDIV:
            if self._optimize_CALL_INT_UDIV(op):
                return
        elif oopspecindex == EffectInfo.OS_INT_PY_DIV:
            if self._optimize_CALL_INT_PY_DIV(op):
                return
        elif oopspecindex == EffectInfo.OS_INT_PY_MOD:
            if self._optimize_CALL_INT_PY_MOD(op):
                return
        self.emit_operation(op)
    optimize_CALL_PURE_R = optimize_CALL_PURE_I
    optimize_CALL_PURE_F = optimize_CALL_PURE_I
    optimize_CALL_PURE_N = optimize_CALL_PURE_I

    def optimize_GUARD_NO_EXCEPTION(self, op):
        if self.last_emitted_operation is REMOVED:
            # it was a CALL_PURE or a CALL_LOOPINVARIANT that was killed;
            # so we also kill the following GUARD_NO_EXCEPTION
            return
        self.emit_operation(op)

    def optimize_GUARD_FUTURE_CONDITION(self, op):
        self.optimizer.notice_guard_future_condition(op)

    def _optimize_CALL_INT_PY_DIV(self, op):
        arg1 = op.getarg(1)
        b1 = self.getintbound(arg1)
        arg2 = op.getarg(2)
        b2 = self.getintbound(arg2)

        if b1.is_constant() and b1.getint() == 0:
            self.make_constant_int(op, 0)
            self.last_emitted_operation = REMOVED
            return True
        # This is Python's integer division: 'x // (2**shift)' can always
        # be replaced with 'x >> shift', even for negative values of x
        if not b2.is_constant():
            return False
        val = b2.getint()
        if val <= 0:
            return False
        if val == 1:
            self.make_equal_to(op, arg1)
            self.last_emitted_operation = REMOVED
            return True
        elif val & (val - 1) == 0:   # val == 2**shift
            from rpython.jit.metainterp.history import DONT_CHANGE
            op = self.replace_op_with(op, rop.INT_RSHIFT,
                        args=[arg1, ConstInt(highest_bit(val))],
                        descr=DONT_CHANGE)  # <- xxx rename? means "kill"
            self.optimizer.send_extra_operation(op)
            return True
        else:
            from rpython.jit.metainterp.optimizeopt import intdiv
            known_nonneg = b1.known_ge(IntBound(0, 0))
            operations = intdiv.division_operations(arg1, val, known_nonneg)
            newop = None
            for newop in operations:
                self.optimizer.send_extra_operation(newop)
            self.make_equal_to(op, newop)
            return True

    def _optimize_CALL_INT_PY_MOD(self, op):
        arg1 = op.getarg(1)
        b1 = self.getintbound(arg1)
        arg2 = op.getarg(2)
        b2 = self.getintbound(arg2)

        if b1.is_constant() and b1.getint() == 0:
            self.make_constant_int(op, 0)
            self.last_emitted_operation = REMOVED
            return True
        # This is Python's integer division: 'x // (2**shift)' can always
        # be replaced with 'x >> shift', even for negative values of x
        if not b2.is_constant():
            return False
        val = b2.getint()
        if val <= 0:
            return False
        if val == 1:
            self.make_constant_int(op, 0)
            self.last_emitted_operation = REMOVED
            return True
        elif val & (val - 1) == 0:   # val == 2**shift
            from rpython.jit.metainterp.history import DONT_CHANGE
            # x % power-of-two ==> x & (power-of-two - 1)
            # with Python's modulo, this is valid even if 'x' is negative.
            op = self.replace_op_with(op, rop.INT_AND,
                        args=[arg1, ConstInt(val - 1)],
                        descr=DONT_CHANGE)  # <- xxx rename? means "kill"
            self.optimizer.send_extra_operation(op)
            return True
        else:
            from rpython.jit.metainterp.optimizeopt import intdiv
            known_nonneg = b1.known_ge(IntBound(0, 0))
            operations = intdiv.modulo_operations(arg1, val, known_nonneg)
            newop = None
            for newop in operations:
                self.optimizer.send_extra_operation(newop)
            self.make_equal_to(op, newop)
            return True

    def optimize_CAST_PTR_TO_INT(self, op):
        self.optimizer.pure_reverse(op)
        self.emit_operation(op)

    def optimize_CAST_INT_TO_PTR(self, op):
        self.optimizer.pure_reverse(op)
        self.emit_operation(op)

    def optimize_SAME_AS_I(self, op):
        self.make_equal_to(op, op.getarg(0))
    optimize_SAME_AS_R = optimize_SAME_AS_I
    optimize_SAME_AS_F = optimize_SAME_AS_I

dispatch_opt = make_dispatcher_method(OptRewrite, 'optimize_',
        default=OptRewrite.emit_operation)
optimize_guards = _findall(OptRewrite, 'optimize_', 'GUARD')