Implement Tarjan's strongly-connected components algorithm

This commit implements [Tarjan's algorithm] for finding strongly-connected
components.

This algorithm takes `O(V+E)` time and uses `O(V+E)` space.

Tarjan's algorithm is usually presented as a recursive algorithm, but we do not
trust the input and cannot recurse over it for fear of blowing the
stack. Therefore, this implementation is iterative.

This will be used to do bottom-up inlining in Wasmtime's compilation
orchestration.

[Tarjan's algorithm]: https://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm

Author: fitzgen

cranelift/entity/src/set.rs

@@ -1,7 +1,7 @@
 //! Densely numbered entity references as set keys.
 
-use crate::EntityRef;
 use crate::keys::Keys;
+use crate::EntityRef;
 use core::fmt;
 use core::marker::PhantomData;
 use cranelift_bitset::CompoundBitSet;
@@ -148,6 +148,14 @@ where
         self.bitset.insert(index)
     }
 
+    /// Remove `k` from this bitset.
+    ///
+    /// Returns whether `k` was previously in this set or not.
+    pub fn remove(&mut self, k: K) -> bool {
+        let index = k.index();
+        self.bitset.remove(index)
+    }
+
     /// Removes and returns the entity from the set if it exists.
     pub fn pop(&mut self) -> Option<K> {
         let index = self.bitset.pop()?;

crates/wasmtime/src/compile.rs

@@ -43,6 +43,8 @@ use wasmtime_environ::{
     StaticModuleIndex,
 };
 
+mod scc;
+
 mod code_builder;
 pub use self::code_builder::{CodeBuilder, CodeHint, HashedEngineCompileEnv};