LICM pass (#87)

* LICM pass

* Uses loop analysis to detect loop tree
* For each loop (starting with the inner ones), create a pre-header and move there loop-invariant instructions
* An instruction is loop invariant if it does not use as argument a value defined earlier in the loop
* File tests to check LICM's correctness
* Optimized pre-header creation
If the loop already has a natural pre-header, we use it instead of creating a new one.
The natural pre-header of a loop is the only predecessor of the header it doesn't dominate.

src/filetest/licm.rs

@@ -0,0 +1,51 @@
+//! Test command for testing the LICM pass.
+//!
+//! The `licm` test command runs each function through the LICM pass after ensuring
+//! that all instructions are legal for the target.
+//!
+//! The resulting function is sent to `filecheck`.
+
+use cretonne::ir::Function;
+use cretonne;
+use cton_reader::TestCommand;
+use filetest::subtest::{SubTest, Context, Result, run_filecheck};
+use std::borrow::Cow;
+use std::fmt::Write;
+use utils::pretty_error;
+
+struct TestLICM;
+
+pub fn subtest(parsed: &TestCommand) -> Result<Box<SubTest>> {
+    assert_eq!(parsed.command, "licm");
+    if !parsed.options.is_empty() {
+        Err(format!("No options allowed on {}", parsed))
+    } else {
+        Ok(Box::new(TestLICM))
+    }
+}
+
+impl SubTest for TestLICM {
+    fn name(&self) -> Cow<str> {
+        Cow::from("licm")
+    }
+
+    fn is_mutating(&self) -> bool {
+        true
+    }
+
+    fn run(&self, func: Cow<Function>, context: &Context) -> Result<()> {
+        // Create a compilation context, and drop in the function.
+        let mut comp_ctx = cretonne::Context::new();
+        comp_ctx.func = func.into_owned();
+
+        comp_ctx.flowgraph();
+        comp_ctx
+            .licm()
+            .map_err(|e| pretty_error(&comp_ctx.func, e))?;
+
+        let mut text = String::new();
+        write!(&mut text, "{}", &comp_ctx.func)
+            .map_err(|e| e.to_string())?;
+        run_filecheck(&text, context)
+    }
+}