diff --git a/lib/cretonne/meta/base/formats.py b/lib/cretonne/meta/base/formats.py
index 1746217958..7fd93de1f9 100644
--- a/lib/cretonne/meta/base/formats.py
+++ b/lib/cretonne/meta/base/formats.py
@@ -13,8 +13,6 @@ from .immediates import boolean, intcc, floatcc, memflags, regunit, trapcode
 from . import entities
 from .entities import ebb, sig_ref, func_ref, stack_slot, heap
 
-Nullary = InstructionFormat()
-
 Unary = InstructionFormat(VALUE)
 UnaryImm = InstructionFormat(imm64)
 UnaryIeee32 = InstructionFormat(ieee32)
diff --git a/lib/cretonne/src/ir/dfg.rs b/lib/cretonne/src/ir/dfg.rs
index b81bbe96fc..47c53100cf 100644
--- a/lib/cretonne/src/ir/dfg.rs
+++ b/lib/cretonne/src/ir/dfg.rs
@@ -898,20 +898,26 @@ impl<'a> fmt::Display for DisplayInst<'a> {
 mod tests {
     use super::*;
     use ir::types;
-    use ir::{Function, Cursor, CursorBase, Opcode, InstructionData};
+    use ir::{Function, Cursor, CursorBase, Opcode, InstructionData, TrapCode};
 
     #[test]
     fn make_inst() {
         let mut dfg = DataFlowGraph::new();
 
-        let idata = InstructionData::Nullary { opcode: Opcode::Iconst };
+        let idata = InstructionData::UnaryImm {
+            opcode: Opcode::Iconst,
+            imm: 0.into(),
+        };
         let next = dfg.next_inst();
         let inst = dfg.make_inst(idata);
         assert_eq!(next, inst);
 
         dfg.make_inst_results(inst, types::I32);
         assert_eq!(inst.to_string(), "inst0");
-        assert_eq!(dfg.display_inst(inst, None).to_string(), "v0 = iconst.i32");
+        assert_eq!(
+            dfg.display_inst(inst, None).to_string(),
+            "v0 = iconst.i32 0"
+        );
 
         // Immutable reference resolution.
         {
@@ -941,9 +947,12 @@ mod tests {
     fn no_results() {
         let mut dfg = DataFlowGraph::new();
 
-        let idata = InstructionData::Nullary { opcode: Opcode::Trap };
+        let idata = InstructionData::Trap {
+            opcode: Opcode::Trap,
+            code: TrapCode::User(0),
+        };
         let inst = dfg.make_inst(idata);
-        assert_eq!(dfg.display_inst(inst, None).to_string(), "trap");
+        assert_eq!(dfg.display_inst(inst, None).to_string(), "trap user0");
 
         // Result slice should be empty.
         assert_eq!(dfg.inst_results(inst), &[]);
diff --git a/lib/cretonne/src/ir/instructions.rs b/lib/cretonne/src/ir/instructions.rs
index 0b3c671cc6..14bd3de726 100644
--- a/lib/cretonne/src/ir/instructions.rs
+++ b/lib/cretonne/src/ir/instructions.rs
@@ -103,7 +103,6 @@ impl FromStr for Opcode {
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
 #[allow(missing_docs)]
 pub enum InstructionData {
-    Nullary { opcode: Opcode },
     Unary { opcode: Opcode, arg: Value },
     UnaryImm { opcode: Opcode, imm: Imm64 },
     UnaryIeee32 { opcode: Opcode, imm: Ieee32 },
diff --git a/lib/cretonne/src/verifier/mod.rs b/lib/cretonne/src/verifier/mod.rs
index 3fd6685133..82ba075f39 100644
--- a/lib/cretonne/src/verifier/mod.rs
+++ b/lib/cretonne/src/verifier/mod.rs
@@ -321,7 +321,6 @@ impl<'a> Verifier<'a> {
             }
 
             // Exhaustive list so we can't forget to add new formats
-            Nullary { .. } |
             Unary { .. } |
             UnaryImm { .. } |
             UnaryIeee32 { .. } |
@@ -1048,9 +1047,10 @@ mod tests {
         let mut func = Function::new();
         let ebb0 = func.dfg.make_ebb();
         func.layout.append_ebb(ebb0);
-        let nullary_with_bad_opcode = func.dfg.make_inst(
-            InstructionData::Nullary { opcode: Opcode::Jump },
-        );
+        let nullary_with_bad_opcode = func.dfg.make_inst(InstructionData::UnaryImm {
+            opcode: Opcode::Jump,
+            imm: 0.into(),
+        });
         func.layout.append_inst(nullary_with_bad_opcode, ebb0);
         let flags = &settings::Flags::new(&settings::builder());
         let verifier = Verifier::new(&func, flags.into());
diff --git a/lib/cretonne/src/write.rs b/lib/cretonne/src/write.rs
index 6dd6f333df..2cbc8766ab 100644
--- a/lib/cretonne/src/write.rs
+++ b/lib/cretonne/src/write.rs
@@ -269,7 +269,6 @@ pub fn write_operands(
     let pool = &dfg.value_lists;
     use ir::instructions::InstructionData::*;
     match dfg[inst] {
-        Nullary { .. } => write!(w, ""),
         Unary { arg, .. } => write!(w, " {}", arg),
         UnaryImm { imm, .. } => write!(w, " {}", imm),
         UnaryIeee32 { imm, .. } => write!(w, " {}", imm),
diff --git a/lib/reader/src/parser.rs b/lib/reader/src/parser.rs
index 8ce55d4445..331ca5eb92 100644
--- a/lib/reader/src/parser.rs
+++ b/lib/reader/src/parser.rs
@@ -1828,7 +1828,6 @@ impl<'a> Parser<'a> {
         opcode: Opcode,
     ) -> Result<InstructionData> {
         let idata = match opcode.format() {
-            InstructionFormat::Nullary => InstructionData::Nullary { opcode },
             InstructionFormat::Unary => {
                 InstructionData::Unary {
                     opcode,