Removed the Opcode::NotAnOpcode variant, replaced its uses with Option<Opcode>, and used the NonZero optimization to maintain the small 1-byte size of an optional Opcode.

lib/cretonne/src/ir/instructions.rs

@@ -43,12 +43,8 @@ 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])
-        }
+    pub fn format(self) -> InstructionFormat {
+        OPCODE_FORMAT[self as usize - 1]
     }
 
     /// Get the constraint descriptor for this opcode.
@@ -69,23 +65,21 @@ impl FromStr for Opcode {
     fn from_str(s: &str) -> Result<Opcode, &'static str> {
         use constant_hash::{Table, simple_hash, probe};
 
-        impl<'a> Table<&'a str> for [Opcode] {
+        impl<'a> Table<&'a str> for [Option<Opcode>] {
             fn len(&self) -> usize {
                 self.len()
             }
 
             fn key(&self, idx: usize) -> Option<&'a str> {
-                if self[idx] == Opcode::NotAnOpcode {
-                    None
-                } else {
-                    Some(opcode_name(self[idx]))
-                }
+                self[idx].map(opcode_name)
             }
         }
 
-        match probe::<&str, [Opcode]>(&OPCODE_HASH_TABLE, s, simple_hash(s)) {
+        match probe::<&str, [Option<Opcode>]>(&OPCODE_HASH_TABLE, s, simple_hash(s)) {
             None => Err("Unknown opcode"),
-            Some(i) => Ok(OPCODE_HASH_TABLE[i]),
+            // We unwrap here because probe() should have ensured that the entry
+            // at this index is not None.
+            Some(i) => Ok(OPCODE_HASH_TABLE[i].unwrap()),
         }
     }
 }

lib/cretonne/src/isa/enc_tables.rs

@@ -53,7 +53,7 @@ impl<OffT: Into<u32> + Copy> Table<Type> for [Level1Entry<OffT>] {
 ///
 /// Empty entries are encoded with a `NotAnOpcode` `opcode` field.
 pub struct Level2Entry<OffT: Into<u32> + Copy> {
-    pub opcode: Opcode,
+    pub opcode: Option<Opcode>,
     pub offset: OffT,
 }
 
@@ -63,12 +63,7 @@ impl<OffT: Into<u32> + Copy> Table<Opcode> for [Level2Entry<OffT>] {
     }
 
     fn key(&self, idx: usize) -> Option<Opcode> {
-        let opc = self[idx].opcode;
-        if opc != Opcode::NotAnOpcode {
-            Some(opc)
-        } else {
-            None
-        }
+        self[idx].opcode
     }
 }
 

lib/cretonne/src/verifier.rs

@@ -149,7 +149,7 @@ impl<'a> Verifier<'a> {
         let inst_data = &self.func.dfg[inst];
 
         // The instruction format matches the opcode
-        if inst_data.opcode().format() != Some(InstructionFormat::from(inst_data)) {
+        if inst_data.opcode().format() != InstructionFormat::from(inst_data) {
             return err!(inst, "instruction opcode doesn't match instruction format");
         }