Add a generic implementation of quadratic hash table probing.

We have multiple pre-computed constant hash tables that all use the same
quadratic probing algorithm.

Add a constant_hash Rust module to match the meta/constant_hash.py module.

Move the simple_hash() function into constant_hash. Its Python equivalent is in
the constant_hash.py module.

cranelift/src/libcretonne/constant_hash.rs

@@ -0,0 +1,74 @@
+//! Runtime support for precomputed constant hash tables.
+//!
+//! The `meta/constant_hash.py` Python module can generate constant hash tables using open
+//! addressing and quadratic probing. The hash tables are arrays that are guaranteed to:
+//!
+//! - Have a power-of-two size.
+//! - Contain at least one empty slot.
+//!
+//! This module provides runtime support for lookups in these tables.
+
+/// Trait that must be implemented by the entries in a constant hash table.
+pub trait Table<K: Copy + Eq> {
+    /// Get the number of entries in this table which must be a power of two.
+    fn len(&self) -> usize;
+
+    /// Get the key corresponding to the entry at `idx`, or `None` if the entry is empty.
+    /// The `idx` must be in range.
+    fn key(&self, idx: usize) -> Option<K>;
+}
+
+
+/// Look for `key` in `table`.
+///
+/// The provided `hash` value must have been computed from `key` using the same hash function that
+/// was used to construct the table.
+///
+/// Returns the table index containing the found entry, or `None` if no entry could be found.
+pub fn probe<K: Copy + Eq, T: Table<K> + ?Sized>(table: &T, key: K, hash: usize) -> Option<usize> {
+    debug_assert!(table.len().is_power_of_two());
+    let mask = table.len() - 1;
+
+    let mut idx = hash;
+    let mut step = 0;
+
+    loop {
+        idx &= mask;
+
+        match table.key(idx) {
+            None => return None,
+            Some(k) if k == key => return Some(idx),
+            _ => {}
+        }
+
+        // Quadratic probing.
+        step += 1;
+        // When `table.len()` 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.
+        debug_assert!(step < table.len());
+        idx += step;
+    }
+}
+
+/// A primitive hash function for matching opcodes.
+/// Must match `meta/constant_hash.py`.
+pub fn simple_hash(s: &str) -> usize {
+    let mut h: u32 = 5381;
+    for c in s.chars() {
+        h = (h ^ c as u32).wrapping_add(h.rotate_right(6));
+    }
+    h as usize
+}
+
+#[cfg(test)]
+mod tests {
+    use super::simple_hash;
+
+    #[test]
+    fn basic() {
+        // c.f. meta/constant_hash.py tests.
+        assert_eq!(simple_hash("Hello"), 0x2fa70c01);
+        assert_eq!(simple_hash("world"), 0x5b0c31d5);
+    }
+}

cranelift/src/libcretonne/ir/instructions.rs

@@ -64,30 +64,25 @@ impl FromStr for Opcode {
 
     /// Parse an Opcode name from a string.
     fn from_str(s: &str) -> Result<Opcode, &'static str> {
-        use simple_hash::simple_hash;
-        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];
+        use constant_hash::{Table, simple_hash, probe};
 
-            if entry == Opcode::NotAnOpcode {
-                return Err("Unknown opcode");
+        impl<'a> Table<&'a str> for [Opcode] {
+            fn len(&self) -> usize {
+                self.len()
             }
 
-            if *opcode_name(entry) == *s {
-                return Ok(entry);
+            fn key(&self, idx: usize) -> Option<&'a str> {
+                if self[idx] == Opcode::NotAnOpcode {
+                    None
+                } else {
+                    Some(opcode_name(self[idx]))
+                }
             }
+        }
 
-            // 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;
+        match probe::<&str, [Opcode]>(&OPCODE_HASH_TABLE, s, simple_hash(s)) {
+            None => Err("Unknown opcode"),
+            Some(i) => Ok(OPCODE_HASH_TABLE[i]),
         }
     }
 }

cranelift/src/libcretonne/lib.rs

@@ -15,6 +15,6 @@ pub mod dominator_tree;
 pub mod entity_map;
 pub mod settings;
 
-mod simple_hash;
+mod constant_hash;
 
 #[cfg(test)]pub mod test_utils;

cranelift/src/libcretonne/settings.rs

@@ -23,7 +23,7 @@
 use std::fmt;
 use std::result;
 
-use simple_hash::simple_hash;
+use constant_hash::simple_hash;
 
 /// A string-based configurator for settings groups.
 ///

cranelift/src/libcretonne/simple_hash.rs

@@ -1,21 +0,0 @@
-/// A primitive hash function for matching opcodes.
-/// Must match `meta/constant_hash.py`.
-pub 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
-}
-
-#[cfg(test)]
-mod tests {
-    use super::simple_hash;
-
-    #[test]
-    fn basic() {
-        // c.f. meta/constant_hash.py tests.
-        assert_eq!(simple_hash("Hello"), 0x2fa70c01);
-        assert_eq!(simple_hash("world"), 0x5b0c31d5);
-    }
-}