Add a Control Flow Graph representation.

The CFG must be instantiated against an existing function but may be
modified after creation

cranelift/src/libcretonne/cfg.rs

@@ -0,0 +1,270 @@
+//! A control flow graph represented as mappings of extended basic blocks to their predecessors.
+//! Predecessors are denoted by tuples of EBB and branch/jump instructions. Each predecessor
+//! tuple corresponds to the end of a basic block.
+//!
+//!```c
+//!     Ebb0:
+//!         ...          ; beginning of basic block
+//!
+//!         ...
+//!
+//!         brz vx, Ebb1 ; end of basic block
+//!
+//!         ...          ; beginning of basic block
+//!
+//!         ...
+//!
+//!         jmp Ebb2     ; end of basic block
+//!```
+//!
+//! Here Ebb1 and Ebb2 would each have a single predecessor denoted as (Ebb0, `brz vx, Ebb1`)
+//! and (Ebb0, `jmp Ebb2`) respectively.
+
+use repr::Function;
+use entities::{Inst, Ebb};
+use instructions::InstructionData;
+use std::collections::{BTreeSet, BTreeMap, btree_map};
+
+/// A basic block denoted by its enclosing Ebb and last instruction.
+pub type Predecessor = (Ebb, Inst);
+
+/// Storing predecessors in a BTreeSet ensures that their ordering is
+/// stable with no duplicates.
+pub type PredecessorSet = BTreeSet<Predecessor>;
+
+/// The Control Flow Graph maintains a mapping of ebbs to their predecessors
+/// where predecessors are basic blocks.
+#[derive(Debug)]
+pub struct ControlFlowGraph {
+    data: BTreeMap<Ebb, PredecessorSet>,
+}
+
+impl ControlFlowGraph {
+
+    /// During initialization mappings will be generated for any existing
+    /// blocks within the CFG's associated function. Basic sanity checks will
+    /// also be performed to ensure that the blocks are well formed.
+    pub fn new(func: &Function) -> Result<ControlFlowGraph, String> {
+        let mut cfg = ControlFlowGraph{data: BTreeMap::new()};
+
+        // Even ebbs without predecessors should show up in the CFG, albeit
+        // with no entires.
+        for ebb in func.ebbs_numerically() {
+            try!(cfg.init_ebb(ebb));
+        }
+
+        for ebb in func.ebbs_numerically() {
+            // Flips to true when a terminating instruction is seen. So that if additional
+            // instructions occur an error may be returned.
+            let mut terminated = false;
+            for inst in func.ebb_insts(ebb) {
+                if terminated {
+                    return Err(format!("{} contains unreachable instructions.", ebb));
+                }
+
+                match func[inst] {
+                    InstructionData::Branch { ty: _, opcode: _, ref data } => {
+                        try!(cfg.add_predecessor(data.destination, (ebb, inst)));
+                    }
+                    InstructionData::Jump { ty: _, opcode: _, ref data } => {
+                        try!(cfg.add_predecessor(data.destination, (ebb, inst)));
+                        terminated = true;
+                    }
+                    InstructionData::Return { ty: _, opcode: _, data: _ } => {
+                        terminated = true;
+                    }
+                    InstructionData::Nullary { ty: _, opcode: _ } => {
+                        terminated = true;
+                    }
+                    _ => ()
+                }
+            }
+        }
+        Ok(cfg)
+    }
+
+    /// Initializes a predecessor set for some ebb. If an ebb already has an
+    /// entry it will be clobbered.
+    pub fn init_ebb(&mut self, ebb: Ebb) -> Result<&mut PredecessorSet, &'static str> {
+        self.data.insert(ebb, BTreeSet::new());
+        match self.data.get_mut(&ebb) {
+            Some(predecessors) => Ok(predecessors),
+            None => Err("Ebb initialization failed.")
+        }
+    }
+
+    /// Attempts to add a predecessor for some ebb, attempting to initialize
+    /// any ebb which has no entry.
+    pub fn add_predecessor(&mut self, ebb: Ebb, predecessor: Predecessor) -> Result<(), &'static str> {
+        let success = match self.data.get_mut(&ebb) {
+            Some(predecessors) => predecessors.insert(predecessor),
+            None => false
+        };
+
+        if success {
+            Ok(())
+        } else {
+            let mut predecessors = try!(self.init_ebb(ebb));
+            if predecessors.insert(predecessor) {
+                return Ok(());
+            }
+            Err("Predecessor insertion failed.")
+        }
+
+    }
+
+    /// Returns all of the predecessors for some ebb, if it has an entry.
+    pub fn get_predecessors(&self, ebb: Ebb) -> Option<&PredecessorSet> {
+        self.data.get(&ebb)
+    }
+
+    /// An iterator over all of the ebb to predecessor mappings in the CFG.
+    pub fn iter<'a>(&'a self) -> btree_map::Iter<'a, Ebb, PredecessorSet> {
+       self.data.iter()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use instructions::*;
+    use entities::{Ebb, Inst, NO_VALUE};
+    use repr::Function;
+    use super::*;
+    use types;
+
+    // Some instructions will be re-used in several tests.
+
+    fn nullary(func: &mut Function) -> Inst {
+        func.make_inst(InstructionData::Nullary {
+            opcode: Opcode::Iconst,
+            ty: types::I32,
+        })
+    }
+
+    fn jump(func: &mut Function, dest: Ebb) -> Inst {
+        func.make_inst(InstructionData::Jump {
+            opcode: Opcode::Jump,
+            ty: types::VOID,
+            data: Box::new(JumpData {
+                destination: dest,
+                arguments: VariableArgs::new(),
+            }),
+        })
+    }
+
+    fn branch(func: &mut Function, dest: Ebb) -> Inst {
+        func.make_inst(InstructionData::Branch {
+            opcode: Opcode::Brz,
+            ty: types::VOID,
+            data: Box::new(BranchData {
+                arg: NO_VALUE,
+                destination: dest,
+                arguments: VariableArgs::new(),
+            }),
+        })
+    }
+
+    #[test]
+    fn empty() {
+        let func = Function::new();
+        let cfg = ControlFlowGraph::new(&func).unwrap();
+        assert_eq!(None, cfg.iter().next());
+    }
+
+    #[test]
+    fn no_predecessors() {
+        let mut func = Function::new();
+        func.make_ebb();
+        func.make_ebb();
+        func.make_ebb();
+        let cfg = ControlFlowGraph::new(&func).unwrap();
+        let nodes = cfg.iter().collect::<Vec<_>>();
+        assert_eq!(nodes.len(), 3);
+
+        let mut fun_ebbs = func.ebbs_numerically();
+        for (ebb, predecessors) in nodes {
+            assert_eq!(ebb.index(), fun_ebbs.next().unwrap().index());
+            assert_eq!(predecessors.len(), 0);
+        }
+    }
+
+    #[test]
+    #[should_panic(expected = "instructions")]
+    fn nullable_before_branch() {
+        // Ensure that branching after a nullary, within an ebb, triggers an error.
+        let mut func = Function::new();
+        let ebb0 = func.make_ebb();
+        let ebb1_malformed = func.make_ebb();
+        let ebb2 = func.make_ebb();
+
+        let nullary_inst = nullary(&mut func);
+        func.append_inst(ebb1_malformed, nullary_inst);
+
+        // This jump should not be recorded since a nullary takes place
+        // before it appears.
+        let jmp_ebb1_ebb0 = jump(&mut func, ebb0);
+        func.append_inst(ebb1_malformed, jmp_ebb1_ebb0);
+
+        let jmp_ebb0_ebb2 = jump(&mut func, ebb2);
+        func.append_inst(ebb0, jmp_ebb0_ebb2);
+
+        ControlFlowGraph::new(&func).unwrap();
+    }
+
+    #[test]
+    #[should_panic(expected = "instructions")]
+    fn jump_before_branch() {
+        // Ensure that branching after a jump, within an ebb, triggers an error.
+        let mut func = Function::new();
+        let ebb0 = func.make_ebb();
+        let ebb1_malformed = func.make_ebb();
+        let ebb2 = func.make_ebb();
+
+        let jmp_ebb0_ebb1 = jump(&mut func, ebb2);
+        func.append_inst(ebb0, jmp_ebb0_ebb1);
+
+        let jmp_ebb1_ebb2 = jump(&mut func, ebb2);
+        func.append_inst(ebb1_malformed, jmp_ebb1_ebb2);
+
+        // This branch should not be recorded since a jump takes place
+        // before it appears.
+        let br_ebb1_ebb0 = branch(&mut func, ebb0);
+        func.append_inst(ebb1_malformed, br_ebb1_ebb0);
+
+        ControlFlowGraph::new(&func).unwrap();
+    }
+
+    #[test]
+    fn branches_and_jumps() {
+        let mut func = Function::new();
+        let ebb0 = func.make_ebb();
+        let ebb1 = func.make_ebb();
+        let ebb2 = func.make_ebb();
+
+        let br_ebb0_ebb2 = branch(&mut func, ebb2);
+        func.append_inst(ebb0, br_ebb0_ebb2);
+
+        let jmp_ebb0_ebb1 = jump(&mut func, ebb1);
+        func.append_inst(ebb0, jmp_ebb0_ebb1);
+
+        let br_ebb1_ebb1 = branch(&mut func, ebb1);
+        func.append_inst(ebb1, br_ebb1_ebb1);
+
+        let jmp_ebb1_ebb2 = jump(&mut func, ebb2);
+        func.append_inst(ebb1, jmp_ebb1_ebb2);
+
+        let cfg = ControlFlowGraph::new(&func).unwrap();
+        let ebb0_predecessors = cfg.get_predecessors(ebb0).unwrap();
+        let ebb1_predecessors = cfg.get_predecessors(ebb1).unwrap();
+        let ebb2_predecessors = cfg.get_predecessors(ebb2).unwrap();
+        assert_eq!(ebb0_predecessors.len(), 0);
+        assert_eq!(ebb1_predecessors.len(), 2);
+        assert_eq!(ebb2_predecessors.len(), 2);
+
+        assert_eq!(ebb1_predecessors.contains(&(ebb0, jmp_ebb0_ebb1)), true);
+        assert_eq!(ebb1_predecessors.contains(&(ebb1, br_ebb1_ebb1)), true);
+        assert_eq!(ebb2_predecessors.contains(&(ebb0, br_ebb0_ebb2)), true);
+        assert_eq!(ebb2_predecessors.contains(&(ebb1, jmp_ebb1_ebb2)), true);
+    }
+
+}

cranelift/src/libcretonne/entities.rs

@@ -24,7 +24,7 @@ use std::fmt::{self, Display, Formatter};
 use std::u32;
 
 /// An opaque reference to an extended basic block in a function.
-#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, PartialOrd, Ord)]
 pub struct Ebb(u32);
 
 impl Ebb {
@@ -64,7 +64,7 @@ impl Default for Ebb {
 }
 
 /// An opaque reference to an instruction in a function.
-#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, PartialOrd, Ord)]
 pub struct Inst(u32);
 
 impl Inst {

cranelift/src/libcretonne/lib.rs

@@ -14,3 +14,4 @@ pub mod entities;
 pub mod instructions;
 pub mod repr;
 pub mod write;
+pub mod cfg;