diff --git a/cranelift/src/libcretonne/ir/immediates.rs b/cranelift/src/libcretonne/ir/immediates.rs
index 077ea4205f..99276bfd66 100644
--- a/cranelift/src/libcretonne/ir/immediates.rs
+++ b/cranelift/src/libcretonne/ir/immediates.rs
@@ -11,16 +11,20 @@ use std::str::FromStr;
 
 /// 64-bit immediate integer operand.
 ///
+/// An `Imm64` operand can also be used to represent immediate values of smaller integer types by
+/// sign-extending to `i64`.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub struct Imm64(i64);
 
 impl Imm64 {
-    pub fn from_bits(x: u64) -> Imm64 {
-        Imm64(x as i64)
+    pub fn new(x: i64) -> Imm64 {
+        Imm64(x)
     }
+}
 
-    pub fn to_bits(&self) -> u64 {
-        self.0 as u64
+impl Into<i64> for Imm64 {
+    fn into(self) -> i64 {
+        self.0
     }
 }
 
@@ -115,7 +119,7 @@ impl FromStr for Imm64 {
                 return Err("Negative number too small for Imm64");
             }
         }
-        Ok(Imm64::from_bits(value))
+        Ok(Imm64::new(value as i64))
     }
 }
 
diff --git a/cranelift/src/libreader/parser.rs b/cranelift/src/libreader/parser.rs
index 87c336906a..fa6a445158 100644
--- a/cranelift/src/libreader/parser.rs
+++ b/cranelift/src/libreader/parser.rs
@@ -596,8 +596,11 @@ impl<'a> Parser<'a> {
         try!(self.match_identifier("stack_slot", "expected 'stack_slot'"));
 
         // stack-slot-decl ::= StackSlot(ss) "=" "stack_slot" * Bytes {"," stack-slot-flag}
-        let bytes = try!(self.match_imm64("expected byte-size in stack_slot decl")).to_bits();
-        if bytes > u32::MAX as u64 {
+        let bytes: i64 = try!(self.match_imm64("expected byte-size in stack_slot decl")).into();
+        if bytes < 0 {
+            return err!(self.loc, "negative stack slot size");
+        }
+        if bytes > u32::MAX as i64 {
             return err!(self.loc, "stack slot too large");
         }
         let data = StackSlotData::new(bytes as u32);