Generate a constant hash table for recognizing opcodes.

Use a simple quadratically probed, open addressed hash table. We could use a
parfect hash function, but it would take longer to compute in Python, and this
is not in the critical path performancewise.

src/libcretonne/immediates.rs

@@ -8,8 +8,12 @@
 use std::fmt::{self, Display, Formatter};
 use std::mem;
 
-// The `Opcode` enum and the `opcode_name` function are generated from the meta instruction
-// descriptions.
+// Include code generated by `meta/gen_instr.py`. This file contains:
+//
+// - The `pub enum Opcode` definition with all known opcodes,
+// - The private `fn opcode_name(Opcode) -> &'static str` function, and
+// - The hash table `const OPCODE_HASH_TABLE: [Opcode; N]`.
+//
 include!(concat!(env!("OUT_DIR"), "/opcodes.rs"));
 
 impl Display for Opcode {
@@ -18,6 +22,46 @@ impl Display for Opcode {
     }
 }
 
+// A primitive hash function for matching opcodes.
+// Must match `meta/constant_hash.py`.
+fn simple_hash(s: &str) -> u32 {
+    let mut h: u32 = 5381;
+    for c in s.chars() {
+        h = (h ^ c as u32).wrapping_add(h.rotate_right(6));
+    }
+    h
+}
+
+impl Opcode {
+    /// Parse an Opcode name from a string.
+    pub fn from_str(s: &str) -> Option<Opcode> {
+        let tlen = OPCODE_HASH_TABLE.len();
+        assert!(tlen.is_power_of_two());
+        let mut idx = simple_hash(s) as usize;
+        let mut step: usize = 0;
+        loop {
+            idx = idx % tlen;
+            let entry = OPCODE_HASH_TABLE[idx];
+
+            if entry == Opcode::NotAnOpcode {
+                return None;
+            }
+
+            if *opcode_name(entry) == *s {
+                return Some(entry);
+            }
+
+            // Quadratic probing.
+            step += 1;
+            // When `tlen` is a power of two, it can be proven that idx will visit all entries.
+            // This means that this loop will always terminate if the hash table has even one
+            // unused entry.
+            assert!(step < tlen);
+            idx += step;
+        }
+    }
+}
+
 /// 64-bit immediate integer operand.
 ///
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
@@ -178,6 +222,13 @@ mod tests {
 
         assert_eq!(format!("{:?}", Opcode::IaddImm), "IaddImm");
         assert_eq!(format!("{}", Opcode::IaddImm), "iadd_imm");
+
+        // Check the matcher.
+        assert_eq!(Opcode::from_str("iadd"), Some(Opcode::Iadd));
+        assert_eq!(Opcode::from_str("iadd_imm"), Some(Opcode::IaddImm));
+        assert_eq!(Opcode::from_str("iadd\0"), None);
+        assert_eq!(Opcode::from_str(""), None);
+        assert_eq!(Opcode::from_str("\0"), None);
     }
 
     #[test]