diff --git a/lib/cretonne/meta/cdsl/ast.py b/lib/cretonne/meta/cdsl/ast.py
index d2296e8fdb..2b671fc46e 100644
--- a/lib/cretonne/meta/cdsl/ast.py
+++ b/lib/cretonne/meta/cdsl/ast.py
@@ -7,11 +7,13 @@ for patern matching an rewriting of cretonne instructions.
 from __future__ import absolute_import
 from . import instructions
 from .typevar import TypeVar
+from .predicates import IsEqual, And
 
 try:
     from typing import Union, Tuple, Sequence, TYPE_CHECKING  # noqa
     if TYPE_CHECKING:
         from .operands import ImmediateKind  # noqa
+        from .predicates import PredNode  # noqa
 except ImportError:
     pass
 
@@ -412,6 +414,32 @@ class Apply(Expr):
         method = self.inst.snake_name()
         return '{}({})'.format(method, args)
 
+    def inst_predicate(self):
+        # type: () -> PredNode
+        """
+        Construct an instruction predicate that verifies the immediate operands
+        on this instruction.
+
+        Immediate operands in a source pattern can be either free variables or
+        constants like `Enumerator`. We don't currently support constraints on
+        free variables, but we may in the future.
+        """
+        pred = None  # type: PredNode
+        iform = self.inst.format
+
+        # Examine all of the immediate operands.
+        for ffield, opnum in zip(iform.imm_fields, self.inst.imm_opnums):
+            arg = self.args[opnum]
+
+            # Ignore free variables for now. We may add variable predicates
+            # later.
+            if isinstance(arg, Var):
+                continue
+
+            pred = And.combine(pred, IsEqual(ffield, arg))
+
+        return pred
+
 
 class Enumerator(Expr):
     """
diff --git a/lib/cretonne/meta/cdsl/predicates.py b/lib/cretonne/meta/cdsl/predicates.py
index 273f8be9a1..8a1cc147f9 100644
--- a/lib/cretonne/meta/cdsl/predicates.py
+++ b/lib/cretonne/meta/cdsl/predicates.py
@@ -216,7 +216,8 @@ class FieldPredicate(object):
         This predicate can be evaluated in the context of an instruction
         format.
         """
-        return self.field.format
+        iform = self.field.format  # type: InstructionFormat
+        return iform
 
     def predicate_leafs(self, leafs):
         # type: (Set[PredLeaf]) -> None
diff --git a/lib/cretonne/meta/gen_legalizer.py b/lib/cretonne/meta/gen_legalizer.py
index 7beda84771..41cd9bab56 100644
--- a/lib/cretonne/meta/gen_legalizer.py
+++ b/lib/cretonne/meta/gen_legalizer.py
@@ -211,6 +211,12 @@ def gen_xform(xform, fmt):
     # variables.
     unwrap_inst('inst', xform.src.rtl[0], fmt)
 
+    # We could support instruction predicates, but not yet. Should we just
+    # return false if it fails? What about multiple patterns with different
+    # predicates for the same opcode?
+    instp = xform.src.rtl[0].expr.inst_predicate()
+    assert instp is None, "Instruction predicates not supported in legalizer"
+
     # Emit the destination pattern.
     for dst in xform.dst.rtl:
         emit_dst_inst(dst, fmt)