Add a compilation context struct.

This will provide main entry points for compiling functions, and it
serves as a place for keeping data structures that should be preserved
between function compilations to reduce allocator thrashing.

So far, Context is just basic scaffolding. More to be added.

lib/cretonne/src/context.rs

@@ -0,0 +1,23 @@
+//! Cretonne compilation context and main entry point.
+//!
+//! When compiling many small functions, it is important to avoid repeatedly allocating and
+//! deallocating the data structures needed for compilation. The `Context` struct is used to hold
+//! on to memory allocations between function compilations.
+
+use ir::Function;
+
+/// Persistent data structures and compilation pipeline.
+pub struct Context {
+    /// The function we're compiling.
+    pub func: Function,
+}
+
+impl Context {
+    /// Allocate a new compilation context.
+    ///
+    /// The returned instance should be reused for compiling multiple functions in order to avoid
+    /// needless allocator thrashing.
+    pub fn new() -> Context {
+        Context { func: Function::new() }
+    }
+}

lib/cretonne/src/lib.rs

@@ -2,28 +2,30 @@
 
 #![deny(missing_docs)]
 
+pub use context::Context;
+pub use legalizer::legalize_function;
 pub use verifier::verify_function;
 pub use write::write_function;
-pub use legalizer::legalize_function;
 
 /// Version number of the cretonne crate.
 pub const VERSION: &'static str = env!("CARGO_PKG_VERSION");
 
-pub mod ir;
-pub mod isa;
 pub mod cfg;
 pub mod dominator_tree;
-pub mod entity_map;
 pub mod entity_list;
-pub mod sparse_map;
+pub mod entity_map;
-pub mod settings;
+pub mod ir;
-pub mod verifier;
+pub mod isa;
 pub mod regalloc;
+pub mod settings;
+pub mod sparse_map;
+pub mod verifier;
 
-mod write;
 mod constant_hash;
-mod predicates;
+mod context;
 mod legalizer;
-mod ref_slice;
-mod partition_slice;
 mod packed_option;
+mod partition_slice;
+mod predicates;
+mod ref_slice;
+mod write;