diff --git a/lib/cretonne/meta/cdsl/isa.py b/lib/cretonne/meta/cdsl/isa.py
index f1125f59da..9239bf2f70 100644
--- a/lib/cretonne/meta/cdsl/isa.py
+++ b/lib/cretonne/meta/cdsl/isa.py
@@ -42,6 +42,7 @@ class TargetISA(object):
         self.instruction_groups = instruction_groups
         self.cpumodes = list()  # type: List[CPUMode]
         self.regbanks = list()  # type: List[RegBank]
+        self.regclasses = list()  # type: List[RegClass]
 
     def finish(self):
         # type: () -> TargetISA
@@ -109,11 +110,23 @@ class TargetISA(object):
 
         Every register class needs a unique index, and the classes need to be
         topologically ordered.
+
+        We also want all the top-level register classes to be first.
         """
-        rc_index = 0
+        # Compute subclasses and top-level classes in each bank.
+        # Collect the top-level classes so they get numbered consecutively.
         for bank in self.regbanks:
-            bank.finish_regclasses(rc_index)
-            rc_index += len(bank.classes)
+            bank.finish_regclasses()
+            self.regclasses.extend(bank.toprcs)
+
+        # Collect all of the non-top-level register classes.
+        # They are numbered strictly after the top-level classes.
+        for bank in self.regbanks:
+            self.regclasses.extend(
+                    rc for rc in bank.classes if not rc.is_toprc())
+
+        for idx, rc in enumerate(self.regclasses):
+            rc.index = idx
 
 
 class CPUMode(object):
diff --git a/lib/cretonne/meta/cdsl/registers.py b/lib/cretonne/meta/cdsl/registers.py
index c16e760a5c..caaacd20e4 100644
--- a/lib/cretonne/meta/cdsl/registers.py
+++ b/lib/cretonne/meta/cdsl/registers.py
@@ -75,6 +75,8 @@ class RegBank(object):
         self.prefix = prefix
         self.names = names
         self.classes = list()  # type: List[RegClass]
+        self.toprcs = list()  # type: List[RegClass]
+        self.first_toprc_index = None  # type: int
 
         assert len(names) <= units
 
@@ -95,11 +97,10 @@ class RegBank(object):
         return ('RegBank({}, units={}, first_unit={})'
                 .format(self.name, self.units, self.first_unit))
 
-    def finish_regclasses(self, first_index):
-        # type: (int) -> None
+    def finish_regclasses(self):
+        # type: () -> None
         """
-        Assign indexes to the register classes in this bank, starting from
-        `first_index`.
+        Compute subclasses and the top-level register class.
 
         Verify that the set of register classes satisfies:
 
@@ -115,14 +116,10 @@ class RegBank(object):
         """
         cmap = dict()  # type: Dict[RCTup, RegClass]
 
-        for idx, rc in enumerate(self.classes):
+        for rc in self.classes:
             # All register classes must be given a name.
             assert rc.name, "Anonymous register class found"
 
-            # Assign a unique index.
-            assert rc.index is None
-            rc.index = idx + first_index
-
             # Check for duplicates.
             tup = rc.rctup()
             if tup in cmap:
@@ -133,6 +130,7 @@ class RegBank(object):
 
         # Check intersections and topological order.
         for idx, rc1 in enumerate(self.classes):
+            rc1.toprc = rc1
             for rc2 in self.classes[0:idx]:
                 itup = rc1.intersect(rc2)
                 if itup is None:
@@ -151,6 +149,10 @@ class RegBank(object):
                     # The intersection of rc1 and rc2 is rc1, so it must be a
                     # sub-class.
                     rc2.subclasses.append(rc1)
+                    rc1.toprc = rc2.toprc
+
+            if rc1.is_toprc():
+                self.toprcs.append(rc1)
 
     def unit_by_name(self, name):
         # type: (str) -> int
@@ -192,6 +194,7 @@ class RegClass(object):
 
         # This is computed later in `finish_regclasses()`.
         self.subclasses = list()  # type: List[RegClass]
+        self.toprc = None  # type: RegClass
 
         assert width > 0
         assert start >= 0 and start < bank.units
@@ -206,6 +209,16 @@ class RegClass(object):
         # type: () -> str
         return self.name
 
+    def is_toprc(self):
+        # type: () -> bool
+        """
+        Is this a top-level register class?
+
+        A top-level register class has no sub-classes. This can only be
+        answered aster running `finish_regclasses()`.
+        """
+        return self.toprc is self
+
     def rctup(self):
         # type: () -> RCTup
         """
diff --git a/lib/cretonne/meta/gen_registers.py b/lib/cretonne/meta/gen_registers.py
index 4199dce268..463364709a 100644
--- a/lib/cretonne/meta/gen_registers.py
+++ b/lib/cretonne/meta/gen_registers.py
@@ -19,13 +19,15 @@ def gen_regbank(regbank, fmt):
     Emit a static data definition for regbank.
     """
     with fmt.indented('RegBank {', '},'):
-        fmt.line('name: "{}",'.format(regbank.name))
-        fmt.line('first_unit: {},'.format(regbank.first_unit))
-        fmt.line('units: {},'.format(regbank.units))
-        fmt.line(
-                'names: &[{}],'
-                .format(', '.join('"{}"'.format(n) for n in regbank.names)))
-        fmt.line('prefix: "{}",'.format(regbank.prefix))
+        fmt.format('name: "{}",', regbank.name)
+        fmt.format('first_unit: {},', regbank.first_unit)
+        fmt.format('units: {},', regbank.units)
+        fmt.format(
+                'names: &[{}],',
+                ', '.join('"{}"'.format(n) for n in regbank.names))
+        fmt.format('prefix: "{}",', regbank.prefix)
+        fmt.format('first_toprc: {},', regbank.toprcs[0].index)
+        fmt.format('num_toprcs: {},', len(regbank.toprcs))
 
 
 def gen_regclass(rc, fmt):
@@ -38,6 +40,7 @@ def gen_regclass(rc, fmt):
         fmt.format('index: {},', rc.index)
         fmt.format('width: {},', rc.width)
         fmt.format('bank: {},', rc.bank.index)
+        fmt.format('toprc: {},', rc.toprc.index)
         fmt.format('first: {},', rc.bank.first_unit + rc.start)
         fmt.format('subclasses: 0x{:x},', rc.subclass_mask())
         mask = ', '.join('0x{:08x}'.format(x) for x in rc.mask())
@@ -52,24 +55,23 @@ def gen_isa(isa, fmt):
     if not isa.regbanks:
         print('cargo:warning={} has no register banks'.format(isa.name))
 
-    rcs = list()  # type: List[RegClass]
     with fmt.indented('pub static INFO: RegInfo = RegInfo {', '};'):
         # Bank descriptors.
         with fmt.indented('banks: &[', '],'):
             for regbank in isa.regbanks:
                 gen_regbank(regbank, fmt)
-                rcs += regbank.classes
         fmt.line('classes: &CLASSES,')
 
     # Register class descriptors.
     with fmt.indented(
-            'const CLASSES: [RegClassData; {}] = ['.format(len(rcs)), '];'):
-        for idx, rc in enumerate(rcs):
+            'const CLASSES: [RegClassData; {}] = ['
+            .format(len(isa.regclasses)), '];'):
+        for idx, rc in enumerate(isa.regclasses):
             assert idx == rc.index
             gen_regclass(rc, fmt)
 
     # Emit constants referencing the register classes.
-    for rc in rcs:
+    for rc in isa.regclasses:
         fmt.line('#[allow(dead_code)]')
         fmt.line(
                 'pub const {}: RegClass = &CLASSES[{}];'
diff --git a/lib/cretonne/src/isa/registers.rs b/lib/cretonne/src/isa/registers.rs
index 461309a9e9..a4527733c5 100644
--- a/lib/cretonne/src/isa/registers.rs
+++ b/lib/cretonne/src/isa/registers.rs
@@ -42,6 +42,15 @@ pub struct RegBank {
     /// The remaining register units will be named this prefix followed by their decimal offset in
     /// the bank. So with a prefix `r`, registers will be named `r8`, `r9`, ...
     pub prefix: &'static str,
+
+    /// Index of the first top-level register class in this bank.
+    pub first_toprc: usize,
+
+    /// Number of top-level register classes in this bank.
+    ///
+    /// The top-level register classes in a bank are guaranteed to be numbered sequentially from
+    /// `first_toprc`, and all top-level register classes across banks come before any sub-classes.
+    pub num_toprcs: usize,
 }
 
 impl RegBank {
@@ -111,6 +120,9 @@ pub struct RegClassData {
     /// Index of the register bank this class belongs to.
     pub bank: u8,
 
+    /// Index of the top-level register class contains this one.
+    pub toprc: u8,
+
     /// The first register unit in this class.
     pub first: RegUnit,
 
diff --git a/lib/cretonne/src/regalloc/allocatable_set.rs b/lib/cretonne/src/regalloc/allocatable_set.rs
index fa3e7a9153..756ad1d349 100644
--- a/lib/cretonne/src/regalloc/allocatable_set.rs
+++ b/lib/cretonne/src/regalloc/allocatable_set.rs
@@ -141,6 +141,7 @@ mod tests {
         index: 0,
         width: 1,
         bank: 0,
+        toprc: 0,
         first: 28,
         subclasses: 0,
         mask: [0xf0000000, 0x0000000f, 0],
@@ -150,6 +151,7 @@ mod tests {
         index: 0,
         width: 2,
         bank: 0,
+        toprc: 0,
         first: 28,
         subclasses: 0,
         mask: [0x50000000, 0x0000000a, 0],