diff --git a/lib/cretonne/meta/cdsl/ast.py b/lib/cretonne/meta/cdsl/ast.py
index 6efc492cf5..0d87bf8914 100644
--- a/lib/cretonne/meta/cdsl/ast.py
+++ b/lib/cretonne/meta/cdsl/ast.py
@@ -10,7 +10,7 @@ from .typevar import TypeVar
 from .predicates import IsEqual, And
 
 try:
-    from typing import Union, Tuple, Sequence, TYPE_CHECKING  # noqa
+    from typing import Union, Tuple, Sequence, TYPE_CHECKING, Dict, List  # noqa
     if TYPE_CHECKING:
         from .operands import ImmediateKind  # noqa
         from .predicates import PredNode  # noqa
@@ -18,6 +18,19 @@ except ImportError:
     pass
 
 
+def replace_var(arg, m):
+    # type: (Expr, Dict[Var, Var]) -> Expr
+    """
+    Given a var v return either m[v] or a new variable v' (and remember
+    m[v]=v'). Otherwise return the argument unchanged
+    """
+    if isinstance(arg, Var):
+        new_arg = m.get(arg, Var(arg.name))  # type: Var
+        m[arg] = new_arg
+        return new_arg
+    return arg
+
+
 class Def(object):
     """
     An AST definition associates a set of variables with the values produced by
@@ -60,6 +73,21 @@ class Def(object):
             return "({}) << {!s}".format(
                     ', '.join(map(str, self.defs)), self.expr)
 
+    def copy(self, m):
+        # type: (Dict[Var, Var]) -> Def
+        """
+        Return a copy of this Def with vars replaced with fresh variables,
+        in accordance with the map m. Update m as neccessary.
+        """
+        new_expr = self.expr.copy(m)
+        new_defs = []  # type: List[Var]
+        for v in self.defs:
+            new_v = replace_var(v, m)
+            assert(isinstance(new_v, Var))
+            new_defs.append(new_v)
+
+        return Def(tuple(new_defs), new_expr)
+
 
 class Expr(object):
     """
@@ -303,6 +331,15 @@ class Apply(Expr):
 
         return pred
 
+    def copy(self, m):
+        # type: (Dict[Var, Var]) -> Apply
+        """
+        Return a copy of this Expr with vars replaced with fresh variables,
+        in accordance with the map m. Update m as neccessary.
+        """
+        return Apply(self.inst, tuple(map(lambda e: replace_var(e, m),
+                                          self.args)))
+
 
 class Enumerator(Expr):
     """
diff --git a/lib/cretonne/meta/cdsl/test_ti.py b/lib/cretonne/meta/cdsl/test_ti.py
index 396d5ef844..71f439864e 100644
--- a/lib/cretonne/meta/cdsl/test_ti.py
+++ b/lib/cretonne/meta/cdsl/test_ti.py
@@ -6,30 +6,17 @@ from base.immediates import intcc
 from .typevar import TypeVar
 from .ast import Var, Def
 from .xform import Rtl, XForm
-from .ti import ti_rtl, subst, TypeEnv, get_type_env
+from .ti import ti_rtl, subst, TypeEnv, get_type_env, ConstrainTVsEqual
 from unittest import TestCase
 from functools import reduce
 
 try:
     from .ti import TypeMap, ConstraintList, VarMap, TypingOrError # noqa
-    from .ti import Constraint
     from typing import List, Dict, Tuple, TYPE_CHECKING, cast # noqa
 except ImportError:
     TYPE_CHECKING = False
 
 
-def sort_constr(c):
-    # type: (Constraint) -> Constraint
-    """
-    Sort the 2 typevars in a constraint by name for comparison
-    """
-    r = tuple(sorted(c, key=lambda y: y.name))
-    if TYPE_CHECKING:
-        return cast(Constraint, r)
-    else:
-        return r
-
-
 def agree(me, other):
     # type: (TypeEnv, TypeEnv) -> bool
     """
@@ -63,13 +50,10 @@ def agree(me, other):
                 return False
 
     # Translate our constraints using m, and sort
-    me_equiv_constr = [(subst(a, m), subst(b, m)) for (a, b) in me.constraints]
-    me_equiv_constr = sorted([sort_constr(x) for x in me_equiv_constr])
-
+    me_equiv_constr = sorted([constr.translate(m)
+                              for constr in me.constraints])
     # Sort other's constraints
-    other_equiv_constr = sorted([sort_constr(x) for x in other.constraints],
-                                key=lambda y:   y[0].name)
-
+    other_equiv_constr = sorted(other.constraints)
     return me_equiv_constr == other_equiv_constr
 
 
@@ -224,7 +208,7 @@ class TestRTL(TypeCheckingBaseTest):
             self.v3: txn,
             self.v4: txn,
             self.v5: txn,
-        }, [(ixn.as_bool(), txn.as_bool())]))
+        }, [ConstrainTVsEqual(ixn.as_bool(), txn.as_bool())]))
 
     def test_vselect_vsplits(self):
         # type: () -> None
diff --git a/lib/cretonne/meta/cdsl/ti.py b/lib/cretonne/meta/cdsl/ti.py
index ce2f05c750..fe602e1342 100644
--- a/lib/cretonne/meta/cdsl/ti.py
+++ b/lib/cretonne/meta/cdsl/ti.py
@@ -8,13 +8,12 @@ from itertools import product
 
 try:
     from typing import Dict, TYPE_CHECKING, Union, Tuple, Optional, Set # noqa
-    from typing import Iterable # noqa
-    from typing import cast, List
+    from typing import Iterable, List # noqa
+    from typing import cast
     from .xform import Rtl, XForm # noqa
     from .ast import Expr # noqa
+    from .typevar import TypeSet # noqa
     if TYPE_CHECKING:
-        Constraint = Tuple[TypeVar, TypeVar]
-        ConstraintList = List[Constraint]
         TypeMap = Dict[TypeVar, TypeVar]
         VarMap = Dict[Var, TypeVar]
 except ImportError:
@@ -22,6 +21,122 @@ except ImportError:
     pass
 
 
+class TypeConstraint(object):
+    """
+    Base class for all runtime-emittable type constraints.
+    """
+
+
+class ConstrainTVsEqual(TypeConstraint):
+    """
+    Constraint specifying that two derived type vars must have the same runtime
+    type.
+    """
+    def __init__(self, tv1, tv2):
+        # type: (TypeVar, TypeVar) -> None
+        assert tv1.is_derived and tv2.is_derived
+        (self.tv1, self.tv2) = sorted([tv1, tv2], key=repr)
+
+    def is_trivial(self):
+        # type: () -> bool
+        """
+        Return true if this constrain is statically decidable.
+        """
+        return self.tv1 == self.tv2 or \
+            (self.tv1.singleton_type() is not None and
+             self.tv2.singleton_type() is not None)
+
+    def translate(self, m):
+        # type: (Union[TypeEnv, TypeMap]) -> ConstrainTVsEqual
+        """
+        Translate any TypeVars in the constraint according to the map m
+        """
+        if isinstance(m, TypeEnv):
+            return ConstrainTVsEqual(m[self.tv1], m[self.tv2])
+        else:
+            return ConstrainTVsEqual(subst(self.tv1, m), subst(self.tv2, m))
+
+    def __eq__(self, other):
+        # type: (object) -> bool
+        if (not isinstance(other, ConstrainTVsEqual)):
+            return False
+
+        return (self.tv1, self.tv2) == (other.tv1, other.tv2)
+
+    def __hash__(self):
+        # type: () -> int
+        return hash((self.tv1, self.tv2))
+
+    def eval(self):
+        # type: () -> bool
+        """
+        Evaluate this constraint. Should only be called when the constraint has
+        been translated to concrete types.
+        """
+        assert self.tv1.singleton_type() is not None and \
+            self.tv2.singleton_type() is not None
+        return self.tv1.singleton_type() == self.tv2.singleton_type()
+
+
+class ConstrainTVInTypeset(TypeConstraint):
+    """
+    Constraint specifying that a type var must belong to some typeset.
+    """
+    def __init__(self, tv, ts):
+        # type: (TypeVar, TypeSet) -> None
+        assert not tv.is_derived and tv.name.startswith("typeof_")
+        self.tv = tv
+        self.ts = ts
+
+    def is_trivial(self):
+        # type: () -> bool
+        """
+        Return true if this constrain is statically decidable.
+        """
+        tv_ts = self.tv.get_typeset().copy()
+
+        # Trivially True
+        if (tv_ts.issubset(self.ts)):
+            return True
+
+        # Trivially false
+        tv_ts &= self.ts
+        if (tv_ts.size() == 0):
+            return True
+
+        return False
+
+    def translate(self, m):
+        # type: (Union[TypeEnv, TypeMap]) -> ConstrainTVInTypeset
+        """
+        Translate any TypeVars in the constraint according to the map m
+        """
+        if isinstance(m, TypeEnv):
+            return ConstrainTVInTypeset(m[self.tv], self.ts)
+        else:
+            return ConstrainTVInTypeset(subst(self.tv, m), self.ts)
+
+    def __eq__(self, other):
+        # type: (object) -> bool
+        if (not isinstance(other, ConstrainTVInTypeset)):
+            return False
+
+        return (self.tv, self.ts) == (other.tv, other.ts)
+
+    def __hash__(self):
+        # type: () -> int
+        return hash((self.tv, self.ts))
+
+    def eval(self):
+        # type: () -> bool
+        """
+        Evaluate this constraint. Should only be called when the constraint has
+        been translated to concrete types.
+        """
+        assert self.tv.singleton_type() is not None
+        return self.tv.get_typeset().issubset(self.ts)
+
+
 class TypeEnv(object):
     """
     Class encapsulating the neccessary book keeping for type inference.
@@ -43,13 +158,13 @@ class TypeEnv(object):
     RANK_INTERNAL = 0
 
     def __init__(self, arg=None):
-        # type: (Optional[Tuple[TypeMap, ConstraintList]]) -> None
+        # type: (Optional[Tuple[TypeMap, List[TypeConstraint]]]) -> None
         self.ranks = {}  # type: Dict[TypeVar, int]
         self.vars = set()  # type: Set[Var]
 
         if arg is None:
             self.type_map = {}  # type: TypeMap
-            self.constraints = []  # type: ConstraintList
+            self.constraints = []  # type: List[TypeConstraint]
         else:
             self.type_map, self.constraints = arg
 
@@ -94,7 +209,9 @@ class TypeEnv(object):
         """
         Add a new equivalence constraint between tv1 and tv2
         """
-        self.constraints.append((tv1, tv2))
+        constr = ConstrainTVsEqual(tv1, tv2)
+        if (constr not in self.constraints):
+            self.constraints.append(constr)
 
     def get_uid(self):
         # type: () -> str
@@ -206,15 +323,24 @@ class TypeEnv(object):
         """
         vars_tvs = set([v.get_typevar() for v in self.vars])
         new_type_map = {tv: self[tv] for tv in vars_tvs if tv != self[tv]}
-        new_constraints = [(self[tv1], self[tv2])
-                           for (tv1, tv2) in self.constraints]
 
-        # Sanity: new constraints and the new type_map should only contain
-        # tvs associated with real vars
-        for (a, b) in new_constraints:
-            assert a.free_typevar() in vars_tvs and\
-                   b.free_typevar() in vars_tvs
+        new_constraints = []  # type: List[TypeConstraint]
+        for constr in self.constraints:
+            # Currently typeinference only generates ConstrainTVsEqual
+            # constraints
+            assert isinstance(constr, ConstrainTVsEqual)
+            constr = constr.translate(self)
 
+            if constr.is_trivial() or constr in new_constraints:
+                continue
+
+            # Sanity: translated constraints should refer to only real vars
+            assert constr.tv1.free_typevar() in vars_tvs and\
+                constr.tv2.free_typevar() in vars_tvs
+
+            new_constraints.append(constr)
+
+        # Sanity: translated typemap should refer to only real vars
         for (k, v) in new_type_map.items():
             assert k in vars_tvs
             assert v.free_typevar() is None or v.free_typevar() in vars_tvs
@@ -245,13 +371,13 @@ class TypeEnv(object):
 
             # Check if constraints are satisfied for this typing
             failed = None
-            for (tv1, tv2) in self.constraints:
-                tv1 = subst(tv1, m)
-                tv2 = subst(tv2, m)
-                assert tv1.get_typeset().size() == 1 and\
-                    tv2.get_typeset().size() == 1
-                if (tv1.get_typeset() != tv2.get_typeset()):
-                    failed = (tv1, tv2)
+            for constr in self.constraints:
+                # Currently typeinference only generates ConstrainTVsEqual
+                # constraints
+                assert isinstance(constr, ConstrainTVsEqual)
+                concrete_constr = constr.translate(m)
+                if not concrete_constr.eval():
+                    failed = concrete_constr
                     break
 
             if (failed is not None):
@@ -287,9 +413,10 @@ class TypeEnv(object):
                 edges.add((v, v.base, "solid", v.derived_func))
                 v = v.base
 
-        for (a, b) in self.constraints:
-            assert a in nodes and b in nodes
-            edges.add((a, b, "dashed", None))
+        for constr in self.constraints:
+            assert isinstance(constr, ConstrainTVsEqual)
+            assert constr.tv1 in nodes and constr.tv2 in nodes
+            edges.add((constr.tv1, constr.tv2, "dashed", None))
 
         root_nodes = set([x for x in nodes
                           if x not in self.type_map and not x.is_derived])
diff --git a/lib/cretonne/meta/cdsl/xform.py b/lib/cretonne/meta/cdsl/xform.py
index fd3c1dff1f..f809a91ce6 100644
--- a/lib/cretonne/meta/cdsl/xform.py
+++ b/lib/cretonne/meta/cdsl/xform.py
@@ -41,6 +41,14 @@ class Rtl(object):
         # type: (*DefApply) -> None
         self.rtl = tuple(map(canonicalize_defapply, args))
 
+    def copy(self, m):
+        # type: (Dict[Var, Var]) -> Rtl
+        """
+        Return a copy of this rtl with all Vars substituted with copies or
+        according to m. Update m as neccessary.
+        """
+        return Rtl(*[d.copy(m) for d in self.rtl])
+
 
 class XForm(object):
     """
diff --git a/lib/cretonne/meta/gen_instr.py b/lib/cretonne/meta/gen_instr.py
index 2d67311ae2..242e5152d3 100644
--- a/lib/cretonne/meta/gen_instr.py
+++ b/lib/cretonne/meta/gen_instr.py
@@ -336,6 +336,25 @@ def get_constraint(op, ctrl_typevar, type_sets):
     return 'Same'
 
 
+# TypeSet indexes are encoded in 8 bits, with `0xff` reserved.
+typeset_limit = 0xff
+
+
+def gen_typesets_table(fmt, type_sets):
+    # type: (srcgen.Formatter, UniqueTable) -> None
+    """
+    Generate the table of ValueTypeSets described by type_sets.
+    """
+    fmt.comment('Table of value type sets.')
+    assert len(type_sets.table) <= typeset_limit, "Too many type sets"
+    with fmt.indented(
+            'const TYPE_SETS : [ValueTypeSet; {}] = ['
+            .format(len(type_sets.table)), '];'):
+        for ts in type_sets.table:
+            with fmt.indented('ValueTypeSet {', '},'):
+                ts.emit_fields(fmt)
+
+
 def gen_type_constraints(fmt, instrs):
     # type: (srcgen.Formatter, Sequence[Instruction]) -> None
     """
@@ -360,9 +379,6 @@ def gen_type_constraints(fmt, instrs):
     # Preload table with constraints for typical binops.
     operand_seqs.add(['Same'] * 3)
 
-    # TypeSet indexes are encoded in 8 bits, with `0xff` reserved.
-    typeset_limit = 0xff
-
     fmt.comment('Table of opcode constraints.')
     with fmt.indented(
             'const OPCODE_CONSTRAINTS : [OpcodeConstraints; {}] = ['
@@ -418,14 +434,7 @@ def gen_type_constraints(fmt, instrs):
                 fmt.line('typeset_offset: {},'.format(ctrl_typeset))
                 fmt.line('constraint_offset: {},'.format(offset))
 
-    fmt.comment('Table of value type sets.')
-    assert len(type_sets.table) <= typeset_limit, "Too many type sets"
-    with fmt.indented(
-            'const TYPE_SETS : [ValueTypeSet; {}] = ['
-            .format(len(type_sets.table)), '];'):
-        for ts in type_sets.table:
-            with fmt.indented('ValueTypeSet {', '},'):
-                ts.emit_fields(fmt)
+    gen_typesets_table(fmt, type_sets)
 
     fmt.comment('Table of operand constraint sequences.')
     with fmt.indented(
diff --git a/lib/cretonne/meta/gen_legalizer.py b/lib/cretonne/meta/gen_legalizer.py
index 1376e3bc21..ce2f559ef2 100644
--- a/lib/cretonne/meta/gen_legalizer.py
+++ b/lib/cretonne/meta/gen_legalizer.py
@@ -11,16 +11,116 @@ from __future__ import absolute_import
 from srcgen import Formatter
 from base import legalize, instructions
 from cdsl.ast import Var
+from cdsl.ti import ti_rtl, TypeEnv, get_type_env, ConstrainTVsEqual,\
+    ConstrainTVInTypeset
+from unique_table import UniqueTable
+from gen_instr import gen_typesets_table
+from cdsl.typevar import TypeVar
 
 try:
-    from typing import Sequence  # noqa
+    from typing import Sequence, List, Dict # noqa
     from cdsl.isa import TargetISA  # noqa
     from cdsl.ast import Def  # noqa
     from cdsl.xform import XForm, XFormGroup  # noqa
+    from cdsl.typevar import TypeSet # noqa
+    from cdsl.ti import TypeConstraint # noqa
 except ImportError:
     pass
 
 
+def get_runtime_typechecks(xform):
+    # type: (XForm) -> List[TypeConstraint]
+    """
+    Given a XForm build a list of runtime type checks neccessary to determine
+    if it applies. We have 2 types of runtime checks:
+        1) typevar tv belongs to typeset T - needed for free tvs whose
+               typeset is constrainted by their use in the dst pattern
+
+        2) tv1 == tv2 where tv1 and tv2 are derived TVs - caused by unification
+                of non-bijective functions
+    """
+    check_l = []  # type: List[TypeConstraint]
+
+    # 1) Perform ti only on the source RTL. Accumulate any free tvs that have a
+    #    different inferred type in src, compared to the type inferred for both
+    #    src and dst.
+    symtab = {}  # type: Dict[Var, Var]
+    src_copy = xform.src.copy(symtab)
+    src_typenv = get_type_env(ti_rtl(src_copy, TypeEnv()))
+
+    for v in xform.ti.vars:
+        if not v.has_free_typevar():
+            continue
+
+        # In rust the local variable containing a free TV associated with var v
+        # has name typeof_v. We rely on the python TVs having the same name.
+        assert "typeof_{}".format(v) == xform.ti[v].name
+
+        if v not in symtab:
+            # We can have singleton vars defined only on dst. Ignore them
+            assert v.get_typevar().singleton_type() is not None
+            continue
+
+        src_ts = src_typenv[symtab[v]].get_typeset()
+        xform_ts = xform.ti[v].get_typeset()
+
+        assert xform_ts.issubset(src_ts)
+        if src_ts != xform_ts:
+            check_l.append(ConstrainTVInTypeset(xform.ti[v], xform_ts))
+
+    # 2,3) Add any constraints that appear in xform.ti
+    check_l.extend(xform.ti.constraints)
+
+    return check_l
+
+
+def emit_runtime_typecheck(check, fmt, type_sets):
+    # type: (TypeConstraint, Formatter, UniqueTable) -> None
+    """
+    Emit rust code for the given check.
+    """
+    def build_derived_expr(tv):
+        # type: (TypeVar) -> str
+        if not tv.is_derived:
+            assert tv.name.startswith('typeof_')
+            return "Some({})".format(tv.name)
+
+        base_exp = build_derived_expr(tv.base)
+        if (tv.derived_func == TypeVar.LANEOF):
+            return "{}.map(|t: Type| -> t.lane_type())".format(base_exp)
+        elif (tv.derived_func == TypeVar.ASBOOL):
+            return "{}.map(|t: Type| -> t.as_bool())".format(base_exp)
+        elif (tv.derived_func == TypeVar.HALFWIDTH):
+            return "{}.and_then(|t: Type| -> t.half_width())".format(base_exp)
+        elif (tv.derived_func == TypeVar.DOUBLEWIDTH):
+            return "{}.and_then(|t: Type| -> t.double_width())"\
+                .format(base_exp)
+        elif (tv.derived_func == TypeVar.HALFVECTOR):
+            return "{}.and_then(|t: Type| -> t.half_vector())".format(base_exp)
+        elif (tv.derived_func == TypeVar.DOUBLEVECTOR):
+            return "{}.and_then(|t: Type| -> t.by(2))".format(base_exp)
+        else:
+            assert False, "Unknown derived function {}".format(tv.derived_func)
+
+    if (isinstance(check, ConstrainTVInTypeset)):
+        tv = check.tv.name
+        if check.ts not in type_sets.index:
+            type_sets.add(check.ts)
+        ts = type_sets.index[check.ts]
+
+        fmt.comment("{} must belong to {}".format(tv, check.ts))
+        with fmt.indented('if !TYPE_SETS[{}].contains({}) {{'.format(ts, tv),
+                          '};'):
+            fmt.line('return false;')
+    elif (isinstance(check, ConstrainTVsEqual)):
+        tv1 = build_derived_expr(check.tv1)
+        tv2 = build_derived_expr(check.tv2)
+        with fmt.indented('if {} != {} {{'.format(tv1, tv2), '};'):
+            fmt.line('return false;')
+    else:
+        assert False, "Unknown check {}".format(check)
+
+
 def unwrap_inst(iref, node, fmt):
     # type: (str, Def, Formatter) -> bool
     """
@@ -183,8 +283,8 @@ def emit_dst_inst(node, fmt):
             fmt.line('pos.next_inst();')
 
 
-def gen_xform(xform, fmt):
-    # type: (XForm, Formatter) -> None
+def gen_xform(xform, fmt, type_sets):
+    # type: (XForm, Formatter, UniqueTable) -> None
     """
     Emit code for `xform`, assuming the the opcode of xform's root instruction
     has already been matched.
@@ -203,6 +303,10 @@ def gen_xform(xform, fmt):
     instp = xform.src.rtl[0].expr.inst_predicate()
     assert instp is None, "Instruction predicates not supported in legalizer"
 
+    # Emit any runtime checks.
+    for check in get_runtime_typechecks(xform):
+        emit_runtime_typecheck(check, fmt, type_sets)
+
     # Emit the destination pattern.
     for dst in xform.dst.rtl:
         emit_dst_inst(dst, fmt)
@@ -213,8 +317,8 @@ def gen_xform(xform, fmt):
         fmt.line('assert_eq!(pos.remove_inst(), inst);')
 
 
-def gen_xform_group(xgrp, fmt):
-    # type: (XFormGroup, Formatter) -> None
+def gen_xform_group(xgrp, fmt, type_sets):
+    # type: (XFormGroup, Formatter, UniqueTable) -> None
     fmt.doc_comment("Legalize the instruction pointed to by `pos`.")
     fmt.line('#[allow(unused_variables,unused_assignments)]')
     with fmt.indented(
@@ -231,7 +335,7 @@ def gen_xform_group(xgrp, fmt):
                 inst = xform.src.rtl[0].expr.inst
                 with fmt.indented(
                         'Opcode::{} => {{'.format(inst.camel_name), '}'):
-                    gen_xform(xform, fmt)
+                    gen_xform(xform, fmt, type_sets)
             # We'll assume there are uncovered opcodes.
             fmt.line('_ => return false,')
         fmt.line('true')
@@ -240,6 +344,11 @@ def gen_xform_group(xgrp, fmt):
 def generate(isas, out_dir):
     # type: (Sequence[TargetISA], str) -> None
     fmt = Formatter()
-    gen_xform_group(legalize.narrow, fmt)
-    gen_xform_group(legalize.expand, fmt)
+    # Table of TypeSet instances
+    type_sets = UniqueTable()
+
+    gen_xform_group(legalize.narrow, fmt, type_sets)
+    gen_xform_group(legalize.expand, fmt, type_sets)
+
+    gen_typesets_table(fmt, type_sets)
     fmt.update_file('legalizer.rs', out_dir)
diff --git a/lib/cretonne/meta/test_gen_legalizer.py b/lib/cretonne/meta/test_gen_legalizer.py
new file mode 100644
index 0000000000..38f26959f4
--- /dev/null
+++ b/lib/cretonne/meta/test_gen_legalizer.py
@@ -0,0 +1,145 @@
+import doctest
+import gen_legalizer
+from unittest import TestCase
+from srcgen import Formatter
+from gen_legalizer import get_runtime_typechecks, emit_runtime_typecheck
+from base.instructions import vselect, vsplit, isplit, iconcat, vconcat, \
+    iconst, b1, icmp, copy # noqa
+from base.legalize import narrow, expand # noqa
+from base.immediates import intcc # noqa
+from cdsl.typevar import TypeVar, TypeSet
+from cdsl.ast import Var, Def # noqa
+from cdsl.xform import Rtl, XForm # noqa
+from cdsl.ti import ti_rtl, subst, TypeEnv, get_type_env # noqa
+from unique_table import UniqueTable
+from functools import reduce
+
+try:
+    from typing import Callable, TYPE_CHECKING, Iterable, Any # noqa
+    if TYPE_CHECKING:
+        CheckProducer = Callable[[UniqueTable], str]
+except ImportError:
+    TYPE_CHECKING = False
+
+
+def load_tests(loader, tests, ignore):
+    # type: (Any, Any, Any) -> Any
+    tests.addTests(doctest.DocTestSuite(gen_legalizer))
+    return tests
+
+
+def format_check(typesets, s, *args):
+    # type: (...) -> str
+    def transform(x):
+        # type: (Any) -> str
+        if isinstance(x, TypeSet):
+            return str(typesets.index[x])
+        elif isinstance(x, TypeVar):
+            assert not x.is_derived
+            return x.name
+        else:
+            return str(x)
+
+    dummy_s = s  # type: str
+    args = tuple(map(lambda x:  transform(x), args))
+    return dummy_s.format(*args)
+
+
+def typeset_check(v, ts):
+    # type: (Var, TypeSet) -> CheckProducer
+    return lambda typesets: format_check(
+        typesets,
+        'if !TYPE_SETS[{}].contains(typeof_{}) ' +
+        '{{\n    return false;\n}};\n', ts, v)
+
+
+def equiv_check(tv1, tv2):
+    # type: (TypeVar, TypeVar) -> CheckProducer
+    return lambda typesets: format_check(
+        typesets,
+        'if Some({}).map(|t: Type| -> t.as_bool()) != ' +
+        'Some({}).map(|t: Type| -> t.as_bool()) ' +
+        '{{\n    return false;\n}};\n', tv1, tv2)
+
+
+def sequence(*args):
+    # type: (...) -> CheckProducer
+    dummy = args  # type: Iterable[CheckProducer]
+
+    def sequenceF(typesets):
+        # type: (UniqueTable) -> str
+        def strconcat(acc, el):
+            # type: (str, CheckProducer) -> str
+            return acc + el(typesets)
+
+        return reduce(strconcat, dummy, "")
+    return sequenceF
+
+
+class TestRuntimeChecks(TestCase):
+
+    def setUp(self):
+        # type: () -> None
+        self.v0 = Var("v0")
+        self.v1 = Var("v1")
+        self.v2 = Var("v2")
+        self.v3 = Var("v3")
+        self.v4 = Var("v4")
+        self.v5 = Var("v5")
+        self.v6 = Var("v6")
+        self.v7 = Var("v7")
+        self.v8 = Var("v8")
+        self.v9 = Var("v9")
+        self.imm0 = Var("imm0")
+        self.IxN_nonscalar = TypeVar("IxN_nonscalar", "", ints=True,
+                                     scalars=False, simd=True)
+        self.TxN = TypeVar("TxN", "", ints=True, bools=True, floats=True,
+                           scalars=False, simd=True)
+        self.b1 = TypeVar.singleton(b1)
+
+    def check_yo_check(self, xform, expected_f):
+        # type: (XForm, CheckProducer) -> None
+        fmt = Formatter()
+        type_sets = UniqueTable()
+        for check in get_runtime_typechecks(xform):
+            emit_runtime_typecheck(check, fmt, type_sets)
+
+        # Remove comments
+        got = "".join([l for l in fmt.lines if not l.strip().startswith("//")])
+        expected = expected_f(type_sets)
+        self.assertEqual(got, expected)
+
+    def test_width_check(self):
+        # type: () -> None
+        x = XForm(Rtl(self.v0 << copy(self.v1)),
+                  Rtl((self.v2, self.v3) << isplit(self.v1),
+                      self.v0 << iconcat(self.v2, self.v3)))
+
+        WideInt = TypeSet(lanes=(1, 256), ints=(16, 64))
+        self.check_yo_check(x, typeset_check(self.v1, WideInt))
+
+    def test_lanes_check(self):
+        # type: () -> None
+        x = XForm(Rtl(self.v0 << copy(self.v1)),
+                  Rtl((self.v2, self.v3) << vsplit(self.v1),
+                      self.v0 << vconcat(self.v2, self.v3)))
+
+        WideVec = TypeSet(lanes=(2, 256), ints=(8, 64), floats=(32, 64),
+                          bools=(1, 64))
+        self.check_yo_check(x, typeset_check(self.v1, WideVec))
+
+    def test_vselect_imm(self):
+        # type: () -> None
+        ts = TypeSet(lanes=(2, 256), ints=(8, 64),
+                     floats=(32, 64), bools=(8, 64))
+        r = Rtl(
+                self.v0 << iconst(self.imm0),
+                self.v1 << icmp(intcc.eq, self.v2, self.v0),
+                self.v5 << vselect(self.v1, self.v3, self.v4),
+        )
+        x = XForm(r, r)
+
+        self.check_yo_check(
+            x, sequence(typeset_check(self.v3, ts),
+                        equiv_check(self.v2.get_typevar(),
+                                    self.v3.get_typevar())))
diff --git a/lib/cretonne/src/ir/instructions.rs b/lib/cretonne/src/ir/instructions.rs
index 35a200de08..cf914b853a 100644
--- a/lib/cretonne/src/ir/instructions.rs
+++ b/lib/cretonne/src/ir/instructions.rs
@@ -506,10 +506,14 @@ type BitSet16 = BitSet<u16>;
 /// A value type set describes the permitted set of types for a type variable.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct ValueTypeSet {
-    lanes: BitSet16,
-    ints: BitSet8,
-    floats: BitSet8,
-    bools: BitSet8,
+    /// Allowed lane sizes
+    pub lanes: BitSet16,
+    /// Allowed int widths
+    pub ints: BitSet8,
+    /// Allowed float widths
+    pub floats: BitSet8,
+    /// Allowed bool widths
+    pub bools: BitSet8,
 }
 
 impl ValueTypeSet {
diff --git a/lib/cretonne/src/legalizer/mod.rs b/lib/cretonne/src/legalizer/mod.rs
index 803f5808c2..f21f7e1d91 100644
--- a/lib/cretonne/src/legalizer/mod.rs
+++ b/lib/cretonne/src/legalizer/mod.rs
@@ -18,6 +18,8 @@ use flowgraph::ControlFlowGraph;
 use ir::{Function, Cursor, DataFlowGraph, InstructionData, Opcode, InstBuilder};
 use ir::condcodes::IntCC;
 use isa::{TargetIsa, Legalize};
+use bitset::BitSet;
+use ir::instructions::ValueTypeSet;
 
 mod boundary;
 mod split;