diff --git a/cranelift/src/libcretonne/ir/immediates.rs b/cranelift/src/libcretonne/ir/immediates.rs
index 99276bfd66..e8d4a20773 100644
--- a/cranelift/src/libcretonne/ir/immediates.rs
+++ b/cranelift/src/libcretonne/ir/immediates.rs
@@ -123,6 +123,11 @@ impl FromStr for Imm64 {
     }
 }
 
+/// 8-bit unsigned integer immediate operand.
+///
+/// This is used to indicate lane indexes typically.
+pub type Uimm8 = u8;
+
 /// An IEEE binary32 immediate floating point value.
 ///
 /// All bit patterns are allowed.
@@ -420,6 +425,12 @@ impl FromStr for Ieee64 {
     }
 }
 
+/// Arbitrary vector immediate.
+///
+/// This kind of immediate can represent any kind of SIMD vector constant.
+/// The representation is simply the sequence of bytes that would be used to store the vector.
+pub type ImmVector = Vec<u8>;
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/cranelift/src/libcretonne/ir/instructions.rs b/cranelift/src/libcretonne/ir/instructions.rs
index 56ce5ed5fd..b971a05ffa 100644
--- a/cranelift/src/libcretonne/ir/instructions.rs
+++ b/cranelift/src/libcretonne/ir/instructions.rs
@@ -11,7 +11,7 @@ use std::str::FromStr;
 use std::ops::{Deref, DerefMut};
 
 use ir::{Value, Type, Ebb, JumpTable, FuncRef};
-use ir::immediates::{Imm64, Ieee32, Ieee64};
+use ir::immediates::{Imm64, Uimm8, Ieee32, Ieee64, ImmVector};
 use ir::condcodes::*;
 use ir::types;
 
@@ -117,7 +117,8 @@ pub enum InstructionData {
     },
     UnaryImmVector {
         opcode: Opcode,
-        ty: Type, // TBD: imm: Box<ImmVectorData>
+        ty: Type,
+        data: Box<UnaryImmVectorData>,
     },
     UnarySplit {
         opcode: Opcode,
@@ -162,13 +163,13 @@ pub enum InstructionData {
     InsertLane {
         opcode: Opcode,
         ty: Type,
-        lane: u8,
+        lane: Uimm8,
         args: [Value; 2],
     },
     ExtractLane {
         opcode: Opcode,
         ty: Type,
-        lane: u8,
+        lane: Uimm8,
         arg: Value,
     },
     IntCompare {
@@ -264,6 +265,22 @@ impl Default for VariableArgs {
     }
 }
 
+/// Payload data for `vconst`.
+#[derive(Clone, Debug)]
+pub struct UnaryImmVectorData {
+    pub imm: ImmVector,
+}
+
+impl Display for UnaryImmVectorData {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        try!(write!(f, "#"));
+        for b in &self.imm {
+            try!(write!(f, "{:02x}", b));
+        }
+        Ok(())
+    }
+}
+
 /// Payload data for ternary instructions with multiple results, such as `iadd_carry`.
 #[derive(Clone, Debug)]
 pub struct TernaryOverflowData {
diff --git a/cranelift/src/libcretonne/write.rs b/cranelift/src/libcretonne/write.rs
index e49c8382bf..3ef72d7178 100644
--- a/cranelift/src/libcretonne/write.rs
+++ b/cranelift/src/libcretonne/write.rs
@@ -184,7 +184,7 @@ fn write_instruction(w: &mut Write,
         UnaryImm { imm, .. } => writeln!(w, " {}", imm),
         UnaryIeee32 { imm, .. } => writeln!(w, " {}", imm),
         UnaryIeee64 { imm, .. } => writeln!(w, " {}", imm),
-        UnaryImmVector { .. } => writeln!(w, " [...]"),
+        UnaryImmVector { ref data, .. } => writeln!(w, " {}", data),
         UnarySplit { arg, .. } => writeln!(w, " {}", arg),
         Binary { args, .. } => writeln!(w, " {}, {}", args[0], args[1]),
         BinaryImm { arg, imm, .. } => writeln!(w, " {}, {}", arg, imm),
diff --git a/meta/cretonne/formats.py b/meta/cretonne/formats.py
index 11a1c5ef3b..8e5d4e25cb 100644
--- a/meta/cretonne/formats.py
+++ b/meta/cretonne/formats.py
@@ -16,7 +16,7 @@ Unary = InstructionFormat(value)
 UnaryImm = InstructionFormat(imm64)
 UnaryIeee32 = InstructionFormat(ieee32)
 UnaryIeee64 = InstructionFormat(ieee64)
-UnaryImmVector = InstructionFormat(immvector)
+UnaryImmVector = InstructionFormat(immvector, boxed_storage=True)
 UnarySplit = InstructionFormat(value, multiple_results=True)
 
 Binary = InstructionFormat(value, value)