diff --git a/lib/cretonne/meta/gen_legalizer.py b/lib/cretonne/meta/gen_legalizer.py
index 10c5bcf3ac..bee51a928a 100644
--- a/lib/cretonne/meta/gen_legalizer.py
+++ b/lib/cretonne/meta/gen_legalizer.py
@@ -9,6 +9,7 @@ the input instruction.
 """
 from __future__ import absolute_import
 from srcgen import Formatter
+from collections import defaultdict
 from base import instructions
 from cdsl.ast import Var
 from cdsl.ti import ti_rtl, TypeEnv, get_type_env, TypesEqual,\
@@ -18,7 +19,7 @@ from gen_instr import gen_typesets_table
 from cdsl.typevar import TypeVar
 
 try:
-    from typing import Sequence, List, Dict, Set # noqa
+    from typing import Sequence, List, Dict, Set, DefaultDict # noqa
     from cdsl.isa import TargetISA  # noqa
     from cdsl.ast import Def  # noqa
     from cdsl.xform import XForm, XFormGroup  # noqa
@@ -78,6 +79,11 @@ def emit_runtime_typecheck(check, fmt, type_sets):
     # type: (TypeConstraint, Formatter, UniqueTable) -> None
     """
     Emit rust code for the given check.
+
+    The emitted code is a statement redefining the `predicate` variable like
+    this:
+
+        let predicate = predicate && ...
     """
     def build_derived_expr(tv):
         # type: (TypeVar) -> str
@@ -116,33 +122,26 @@ def emit_runtime_typecheck(check, fmt, type_sets):
         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;')
+        fmt.format(
+                'let predicate = predicate && TYPE_SETS[{}].contains({});',
+                ts, tv)
     elif (isinstance(check, TypesEqual)):
-        with fmt.indented('{', '};'):
-            fmt.line('let a = {};'.format(build_derived_expr(check.tv1)))
-            fmt.line('let b = {};'.format(build_derived_expr(check.tv2)))
-
-            fmt.comment('On overflow constraint doesn\'t appply')
-            with fmt.indented('if a.is_none() || b.is_none() {', '};'):
-                fmt.line('return false;')
-
-            with fmt.indented('if a != b {', '};'):
-                fmt.line('return false;')
+        with fmt.indented(
+            'let predicate = predicate && match ({}, {}) {{'
+            .format(build_derived_expr(check.tv1),
+                    build_derived_expr(check.tv2)), '};'):
+            fmt.line('(Some(a), Some(b)) => a == b,')
+            fmt.comment('On overflow, constraint doesn\'t appply')
+            fmt.line('_ => false,')
     elif (isinstance(check, WiderOrEq)):
-        with fmt.indented('{', '};'):
-            fmt.line('let a = {};'.format(build_derived_expr(check.tv1)))
-            fmt.line('let b = {};'.format(build_derived_expr(check.tv2)))
-
-            fmt.comment('On overflow constraint doesn\'t appply')
-            with fmt.indented('if a.is_none() || b.is_none() {', '};'):
-                fmt.line('return false;')
-
-            with fmt.indented('if !a.wider_or_equal(b) {', '};'):
-                fmt.line('return false;')
+        with fmt.indented(
+            'let predicate = predicate && match ({}, {}) {{'
+            .format(build_derived_expr(check.tv1),
+                    build_derived_expr(check.tv2)), '};'):
+            fmt.line('(Some(a), Some(b)) => a.wider_or_equal(b),')
+            fmt.comment('On overflow, constraint doesn\'t appply')
+            fmt.line('_ => false,')
     else:
         assert False, "Unknown check {}".format(check)
 
@@ -216,14 +215,12 @@ def unwrap_inst(iref, node, fmt):
             replace_inst = True
         else:
             # Boring case: Detach the result values, capture them in locals.
-            fmt.comment('Detaching results.')
             for d in node.defs:
                 fmt.line('let {};'.format(d))
             with fmt.indented('{', '}'):
                 fmt.line('let r = dfg.inst_results(inst);')
                 for i in range(len(node.defs)):
                     fmt.line('{} = r[{}];'.format(node.defs[i], i))
-            fmt.line('dfg.clear_results(inst);')
             for d in node.defs:
                 if d.has_free_typevar():
                     fmt.line(
@@ -312,7 +309,7 @@ def emit_dst_inst(node, fmt):
 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
+    Emit code for `xform`, assuming that the opcode of xform's root instruction
     has already been matched.
 
     `inst: Inst` is the variable to be replaced. It is pointed to by `pos:
@@ -323,24 +320,33 @@ def gen_xform(xform, fmt, type_sets):
     # variables.
     replace_inst = 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?
+    # Check instruction predicate and emit type checks.
     instp = xform.src.rtl[0].expr.inst_predicate()
-    assert instp is None, "Instruction predicates not supported in legalizer"
+    # TODO: The instruction predicate should be evaluated with all the inst
+    # immediate fields available. Probably by unwrap_inst().
+    fmt.format('let predicate = {};',
+               instp.rust_predicate(0) if instp else 'true')
 
     # 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)
+    # Guard the actual expansion by `predicate`.
+    with fmt.indented('if predicate {', '}'):
+        # If we're going to delete `inst`, we need to detach its results first
+        # so they can be reattached during pattern expansion.
+        if not replace_inst:
+            fmt.line('dfg.clear_results(inst);')
 
-    # Delete the original instruction if we didn't have an opportunity to
-    # replace it.
-    if not replace_inst:
-        fmt.line('assert_eq!(pos.remove_inst(), inst);')
+        # Emit the destination pattern.
+        for dst in xform.dst.rtl:
+            emit_dst_inst(dst, fmt)
+
+        # Delete the original instruction if we didn't have an opportunity to
+        # replace it.
+        if not replace_inst:
+            fmt.line('assert_eq!(pos.remove_inst(), inst);')
+        fmt.line('return true;')
 
 
 def gen_xform_group(xgrp, fmt, type_sets):
@@ -358,19 +364,27 @@ def gen_xform_group(xgrp, fmt, type_sets):
         # pointing at an instruction.
         fmt.line('let inst = pos.current_inst().expect("need instruction");')
 
+        # Group the xforms by opcode so we can generate a big switch.
+        # Preserve ordering.
+        xforms = defaultdict(list)  # type: DefaultDict[str, List[XForm]]
+        for xform in xgrp.xforms:
+            inst = xform.src.rtl[0].expr.inst
+            xforms[inst.camel_name].append(xform)
+
         with fmt.indented('match dfg[inst].opcode() {', '}'):
-            for xform in xgrp.xforms:
-                inst = xform.src.rtl[0].expr.inst
+            for camel_name in sorted(xforms.keys()):
                 with fmt.indented(
-                        'ir::Opcode::{} => {{'.format(inst.camel_name), '}'):
-                    gen_xform(xform, fmt, type_sets)
+                        'ir::Opcode::{} => {{'.format(camel_name), '}'):
+                    for xform in xforms[camel_name]:
+                        gen_xform(xform, fmt, type_sets)
             # We'll assume there are uncovered opcodes.
-            if xgrp.chain:
-                fmt.format('_ => return {}(dfg, cfg, pos),',
-                           xgrp.chain.rust_name())
-            else:
-                fmt.line('_ => return false,')
-        fmt.line('true')
+            fmt.line('_ => {},')
+
+        # If we fall through, nothing was expanded. Call the chain if any.
+        if xgrp.chain:
+            fmt.format('{}(dfg, cfg, pos)', xgrp.chain.rust_name())
+        else:
+            fmt.line('false')
 
 
 def gen_isa(isa, fmt, shared_groups):
diff --git a/lib/cretonne/meta/test_gen_legalizer.py b/lib/cretonne/meta/test_gen_legalizer.py
index 6a317d887b..793555a42c 100644
--- a/lib/cretonne/meta/test_gen_legalizer.py
+++ b/lib/cretonne/meta/test_gen_legalizer.py
@@ -49,39 +49,27 @@ 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)
+        'let predicate = predicate && TYPE_SETS[{}].contains(typeof_{});\n',
+        ts, v)
 
 
 def equiv_check(tv1, tv2):
-    # type: (TypeVar, TypeVar) -> CheckProducer
+    # type: (str, str) -> CheckProducer
     return lambda typesets: format_check(
         typesets,
-        '{{\n' +
-        '    let a = {};\n' +
-        '    let b = {};\n' +
-        '    if a.is_none() || b.is_none() {{\n' +
-        '        return false;\n' +
-        '    }};\n' +
-        '    if a != b {{\n' +
-        '        return false;\n' +
-        '    }};\n' +
+        'let predicate = predicate && match ({}, {}) {{\n'
+        '    (Some(a), Some(b)) => a == b,\n'
+        '    _ => false,\n'
         '}};\n', tv1, tv2)
 
 
 def wider_check(tv1, tv2):
-    # type: (TypeVar, TypeVar) -> CheckProducer
+    # type: (str, str) -> CheckProducer
     return lambda typesets: format_check(
         typesets,
-        '{{\n' +
-        '    let a = {};\n' +
-        '    let b = {};\n' +
-        '    if a.is_none() || b.is_none() {{\n' +
-        '        return false;\n' +
-        '    }};\n' +
-        '    if !a.wider_or_equal(b) {{\n' +
-        '        return false;\n' +
-        '    }};\n' +
+        'let predicate = predicate && match ({}, {}) {{\n'
+        '    (Some(a), Some(b)) => a.wider_or_equal(b),\n'
+        '    _ => false,\n'
         '}};\n', tv1, tv2)