diff --git a/meta/cretonne/__init__.py b/meta/cretonne/__init__.py
index 36f96ef6f5..4b8267d2d1 100644
--- a/meta/cretonne/__init__.py
+++ b/meta/cretonne/__init__.py
@@ -335,6 +335,9 @@ class InstructionFormat(object):
     # Map (multiple_results, kind, kind, ...) -> InstructionFormat
     _registry = dict()
 
+    # All existing formats.
+    all_formats = list()
+
     def __init__(self, *kinds, **kwargs):
         self.name = kwargs.get('name', None)
         self.kinds = kinds
@@ -346,6 +349,7 @@ class InstructionFormat(object):
                 "Format '{}' has the same signature as existing format '{}'"
                 .format(self.name, InstructionFormat._registry[sig]))
         InstructionFormat._registry[sig] = self
+        InstructionFormat.all_formats.append(self)
 
     @staticmethod
     def lookup(ins, outs):
diff --git a/meta/gen_instr.py b/meta/gen_instr.py
index 70ae0ee8e7..8c5ad8a9ce 100644
--- a/meta/gen_instr.py
+++ b/meta/gen_instr.py
@@ -4,6 +4,22 @@ Generate sources with instruction info.
 
 import srcgen
 import constant_hash
+import cretonne
+
+
+def gen_formats(fmt):
+    """Generate an instruction format enumeration"""
+
+    fmt.doc_comment('An instruction format')
+    fmt.doc_comment('')
+    fmt.doc_comment('Every opcode has a corresponding instruction format')
+    fmt.doc_comment('which is represented by both the `InstructionFormat`')
+    fmt.doc_comment('and the `InstructionData` enums.')
+    fmt.line('#[derive(Copy, Clone, PartialEq, Eq, Debug)]')
+    with fmt.indented('pub enum InstructionFormat {', '}'):
+        for f in cretonne.InstructionFormat.all_formats:
+            fmt.line(f.name + ',')
+    fmt.line()
 
 
 def collect_instr_groups(targets):
@@ -17,9 +33,8 @@ def collect_instr_groups(targets):
     return groups
 
 
-def gen_opcodes(groups, out_dir):
+def gen_opcodes(groups, fmt):
     """Generate opcode enumerations."""
-    fmt = srcgen.Formatter()
 
     fmt.doc_comment('An instruction opcode.')
     fmt.doc_comment('')
@@ -41,6 +56,18 @@ def gen_opcodes(groups, out_dir):
                         .format(prefix, i.name, suffix, i.format.name))
                 # Enum variant itself.
                 fmt.line(i.camel_name + ',')
+    fmt.line()
+
+    # Generate a private opcode_format table.
+    with fmt.indented(
+            'const OPCODE_FORMAT: [InstructionFormat; {}] = ['
+            .format(len(instrs)),
+            '];'):
+        for i in instrs:
+            fmt.format(
+                    'InstructionFormat::{}, // {}',
+                    i.format.name, i.name)
+    fmt.line()
 
     # Generate a private opcode_name function.
     with fmt.indented('fn opcode_name(opc: Opcode) -> &\'static str {', '}'):
@@ -48,6 +75,7 @@ def gen_opcodes(groups, out_dir):
             fmt.line('Opcode::NotAnOpcode => "<not an opcode>",')
             for i in instrs:
                 fmt.format('Opcode::{} => "{}",', i.camel_name, i.name)
+    fmt.line()
 
     # Generate an opcode hash table for looking up opcodes by name.
     hash_table = constant_hash.compute_quadratic(
@@ -61,10 +89,13 @@ def gen_opcodes(groups, out_dir):
                 fmt.line('Opcode::NotAnOpcode,')
             else:
                 fmt.format('Opcode::{},', i.camel_name)
-
-    fmt.update_file('opcodes.rs', out_dir)
+    fmt.line()
 
 
 def generate(targets, out_dir):
     groups = collect_instr_groups(targets)
-    gen_opcodes(groups, out_dir)
+    # opcodes.rs
+    fmt = srcgen.Formatter()
+    gen_formats(fmt)
+    gen_opcodes(groups, fmt)
+    fmt.update_file('opcodes.rs', out_dir)
diff --git a/meta/srcgen.py b/meta/srcgen.py
index 7895b0ea38..1ebd745286 100644
--- a/meta/srcgen.py
+++ b/meta/srcgen.py
@@ -49,9 +49,12 @@ class Formatter(object):
         assert self.indent != '', 'Already at top level indentation'
         self.indent = self.indent[0:-self.shiftwidth]
 
-    def line(self, s):
+    def line(self, s=None):
         """And an indented line."""
-        self.lines.append('{}{}\n'.format(self.indent, s))
+        if s:
+            self.lines.append('{}{}\n'.format(self.indent, s))
+        else:
+            self.lines.append('\n')
 
     def writelines(self, f=None):
         """Write all lines to `f`."""
diff --git a/src/libcretonne/immediates.rs b/src/libcretonne/immediates.rs
index 619d790bfe..135d3c6b65 100644
--- a/src/libcretonne/immediates.rs
+++ b/src/libcretonne/immediates.rs
@@ -23,6 +23,17 @@ impl Display for Opcode {
     }
 }
 
+impl Opcode {
+    /// Get the instruction format for this opcode.
+    pub fn format(self) -> Option<InstructionFormat> {
+        if self == Opcode::NotAnOpcode {
+            None
+        } else {
+            Some(OPCODE_FORMAT[self as usize - 1])
+        }
+    }
+}
+
 // A primitive hash function for matching opcodes.
 // Must match `meta/constant_hash.py`.
 fn simple_hash(s: &str) -> u32 {
@@ -491,6 +502,7 @@ mod tests {
         assert!(x != y);
         y = Opcode::Iadd;
         assert_eq!(x, y);
+        assert_eq!(x.format(), Some(InstructionFormat::Binary));
 
         assert_eq!(format!("{:?}", Opcode::IaddImm), "IaddImm");
         assert_eq!(Opcode::IaddImm.to_string(), "iadd_imm");