wasmtime/lib/frontend/src/ssa.rs

//! A SSA-building API that handles incomplete CFGs.
//!
//! The algorithm is based upon Braun M., Buchwald S., Hack S., Leißa R., Mallon C.,
//! Zwinkau A. (2013) Simple and Efficient Construction of Static Single Assignment Form.
//! In: Jhala R., De Bosschere K. (eds) Compiler Construction. CC 2013.
//! Lecture Notes in Computer Science, vol 7791. Springer, Berlin, Heidelberg

use cretonne::ir::{Ebb, Value, Inst, Type, DataFlowGraph, JumpTables, Layout, Cursor, InstBuilder};
use cretonne::ir::instructions::BranchInfo;
use std::hash::Hash;
use cretonne::entity_map::{EntityMap, PrimaryEntityData};
use cretonne::entity_ref::EntityRef;
use cretonne::packed_option::PackedOption;
use cretonne::packed_option::ReservedValue;
use std::u32;
use cretonne::ir::types::{F32, F64};
use cretonne::ir::immediates::{Ieee32, Ieee64};
use std::collections::HashMap;

/// Structure containing the data relevant the construction of SSA for a given function.
///
/// The parameter struct `Variable` corresponds to the way variables are represented in the
/// non-SSA language you're translating from.
///
/// The SSA building relies on information about the variables used and defined, as well as
/// their position relative to basic blocks which are stricter than extended basic blocks since
/// they don't allow branching in the middle of them.
///
/// This SSA building module allows you to def and use variables on the fly while you are
/// constructing the CFG, no need for a separate SSA pass after the CFG is completed.
///
/// A basic block is said _filled_ if all the instruction that it contains have been translated,
/// and it is said _sealed_ if all of its predecessors have been declared. Only filled predecessors
/// can be declared.
pub struct SSABuilder<Variable>
    where Variable: EntityRef + Default
{
    // Records for every variable and for every revelant block, the last definition of
    // the variable in the block.
    variables: EntityMap<Variable, HashMap<Block, Value>>,
    // Records the position of the basic blocks and the list of values used but not defined in the
    // block.
    blocks: EntityMap<Block, BlockData<Variable>>,
    // Records the basic blocks at the beginning of the `Ebb`s.
    ebb_headers: EntityMap<Ebb, PackedOption<Block>>,
}

/// Side effects of a `use_var` or a `seal_ebb_header_block` method call.
pub struct SideEffects {
    /// When we want to append jump arguments to a `br_table` instruction, the critical edge is
    /// splitted and the newly created `Ebb`s are signaled here.
    pub split_ebbs_created: Vec<Ebb>,
    /// When a variable is used but has never been defined before (this happens in the case of
    /// unreachable code), a placeholder `iconst` or `fconst` value is added to the right `Ebb`.
    /// This field signals if it is the case and return the `Ebb` to which the initialization has
    /// been added.
    pub instructions_added_to_ebbs: Vec<Ebb>,
}

// Describes the current position of a basic block in the control flow graph.
enum BlockData<Variable> {
    // A block at the top of an `Ebb`.
    EbbHeader(EbbHeaderBlockData<Variable>),
    // A block inside an `Ebb` with an unique other block as its predecessor.
    // The block is implicitely sealed at creation.
    EbbBody { predecessor: Block },
}
impl<Variable> PrimaryEntityData for BlockData<Variable> {}

impl<Variable> BlockData<Variable> {
    fn add_predecessor(&mut self, pred: Block, inst: Inst) {
        match self {
            &mut BlockData::EbbBody { .. } => panic!("you can't add a predecessor to a body block"),
            &mut BlockData::EbbHeader(ref mut data) => {
                data.predecessors.insert(pred, inst);
                ()
            }
        }
    }
    fn remove_predecessor(&mut self, inst: Inst) -> Block {
        match self {
            &mut BlockData::EbbBody { .. } => panic!("should not happen"),
            &mut BlockData::EbbHeader(ref mut data) => {
                // This a linear complexity operation but the number of predecessors is low
                // in all non-pathological cases
                let pred: Block = match data.predecessors
                          .iter()
                          .find(|&(_, &jump_inst)| jump_inst == inst) {
                    None => panic!("the predecessor you are trying to remove is not declared"),
                    Some((&b, _)) => b.clone(),
                };
                data.predecessors.remove(&pred);
                pred
            }
        }
    }
}

struct EbbHeaderBlockData<Variable> {
    // The predecessors of the Ebb header block, with the block and branch instruction.
    predecessors: HashMap<Block, Inst>,
    // A ebb header block is sealed if all of its predecessors have been declared.
    sealed: bool,
    // The ebb which this block is part of.
    ebb: Ebb,
    // List of current Ebb arguments for which a earlier def has not been found yet.
    undef_variables: Vec<(Variable, Value)>,
}

/// A opaque reference to a basic block.
#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
pub struct Block(u32);
impl EntityRef for Block {
    fn new(index: usize) -> Self {
        assert!(index < (u32::MAX as usize));
        Block(index as u32)
    }

    fn index(self) -> usize {
        self.0 as usize
    }
}

impl ReservedValue for Block {
    fn reserved_value() -> Block {
        Block(u32::MAX)
    }
}

impl<Variable> SSABuilder<Variable>
    where Variable: EntityRef + Default
{
    /// Allocate a new blank SSA builder struct. Use the API function to interact with the struct.
    pub fn new() -> SSABuilder<Variable> {
        SSABuilder {
            variables: EntityMap::new(),
            blocks: EntityMap::new(),
            ebb_headers: EntityMap::new(),
        }
    }

    /// Clears a `SSABuilder` from all its data, letting it in a pristine state without
    /// deallocating memory.
    pub fn clear(&mut self) {
        self.variables.clear();
        self.blocks.clear();
        self.ebb_headers.clear();
    }
}

// Small enum used for clarity in some functions.
#[derive(Debug)]
enum ZeroOneOrMore<T> {
    Zero(),
    One(T),
    More(),
}

/// TODO: use entity list instead of vec
#[derive(Debug)]
enum UseVarCases {
    Unsealed(Value),
    SealedOnePredecessor(Block),
    SealedMultiplePredecessors(Vec<(Block, Inst)>, Value, Ebb),
}

/// The following methods are the API of the SSA builder. Here is how it should be used when
/// translating to Cretonne IL:
///
/// - for each sequence of contiguous instructions (with no branches), create a corresponding
///   basic block with `declare_ebb_body_block` or `declare_ebb_header_block` depending on the
///   position of the basic block;
///
/// - while traversing a basic block and translating instruction, use `def_var` and `use_var`
///   to record definitions and uses of variables, these methods will give you the corresponding
///   SSA values;
///
/// - when all the instructions in a basic block have translated, the block is said _filled_ and
///   only then you can add it as a predecessor to other blocks with `declare_ebb_predecessor`;
///
/// - when you have constructed all the predecessor to a basic block at the beginning of an `Ebb`,
///   call `seal_ebb_header_block` on it with the `Function` that you are building.
///
/// This API will give you the correct SSA values to use as arguments of your instructions,
/// as well as modify the jump instruction and `Ebb` headers arguments to account for the SSA
/// Phi functions.
///
impl<Variable> SSABuilder<Variable>
    where Variable: EntityRef + Hash + Default
{
    /// Declares a new definition of a variable in a given basic block.
    /// The SSA value is passed as an argument because it should be created with
    /// `ir::DataFlowGraph::append_result`.
    pub fn def_var(&mut self, var: Variable, val: Value, block: Block) {
        self.variables.ensure(var).insert(block, val);
    }

    /// Declares a use of a variable in a given basic block. Returns the SSA value corresponding
    /// to the current SSA definition of this variable and a list of newly created Ebbs that
    /// are the results of critical edge splitting for `br_table` with arguments.
    ///
    /// If the variable has never been defined in this blocks or recursively in its predecessors,
    /// this method will silently create an initializer with `iconst` or `fconst`. You are
    /// responsible for making sure that you initialize your variables.
    pub fn use_var(&mut self,
                   dfg: &mut DataFlowGraph,
                   layout: &mut Layout,
                   jts: &mut JumpTables,
                   var: Variable,
                   ty: Type,
                   block: Block)
                   -> (Value, SideEffects) {
        // First we lookup for the current definition of the variable in this block
        if let Some(var_defs) = self.variables.get(var) {
            if let Some(val) = var_defs.get(&block) {
                return (*val,
                        SideEffects {
                            split_ebbs_created: Vec::new(),
                            instructions_added_to_ebbs: Vec::new(),
                        });
            }
        };
        // At this point if we haven't returned it means that we have to search in the
        // predecessors.
        let case = match self.blocks[block] {
            BlockData::EbbHeader(ref mut data) => {
                // The block has multiple predecessors so we append an Ebb argument that
                // will serve as a value.
                if data.sealed {
                    if data.predecessors.len() == 1 {
                        // Only one predecessor, straightforward case
                        UseVarCases::SealedOnePredecessor(*data.predecessors.keys().next().unwrap())
                    } else {
                        let val = dfg.append_ebb_arg(data.ebb, ty);
                        let preds = data.predecessors
                            .iter()
                            .map(|(&pred, &inst)| (pred, inst))
                            .collect();
                        UseVarCases::SealedMultiplePredecessors(preds, val, data.ebb)
                    }
                } else {
                    let val = dfg.append_ebb_arg(data.ebb, ty);
                    data.undef_variables.push((var, val));
                    UseVarCases::Unsealed(val)
                }
            }
            BlockData::EbbBody { predecessor: pred, .. } => UseVarCases::SealedOnePredecessor(pred),
        };
        // TODO: avoid recursion for the calls to use_var and predecessors_lookup.
        match case {
            // The block has a single predecessor or multiple predecessor with
            // the same value, we look into it.
            UseVarCases::SealedOnePredecessor(pred) => {
                let (val, mids) = self.use_var(dfg, layout, jts, var, ty, pred);
                self.def_var(var, val, block);
                return (val, mids);
            }
            // The block has multiple predecessors, we register the ebb argument as the current
            // definition for the variable.
            UseVarCases::Unsealed(val) => {
                self.def_var(var, val, block);
                return (val,
                        SideEffects {
                            split_ebbs_created: Vec::new(),
                            instructions_added_to_ebbs: Vec::new(),
                        });
            }
            UseVarCases::SealedMultiplePredecessors(preds, val, ebb) => {
                // If multiple predecessor we look up a use_var in each of them:
                // if they all yield the same value no need for an Ebb argument
                self.def_var(var, val, block);
                return self.predecessors_lookup(dfg, layout, jts, val, var, ebb, &preds);
            }
        };

    }

    /// Declares a new basic block belonging to the body of a certain `Ebb` and having `pred`
    /// as a predecessor. `pred` is the only predecessor of the block and the block is sealed
    /// at creation.
    ///
    /// To declare a `Ebb` header block, see `declare_ebb_header_block`.
    pub fn declare_ebb_body_block(&mut self, pred: Block) -> Block {
        self.blocks.push(BlockData::EbbBody { predecessor: pred })
    }

    /// Declares a new basic block at the beginning of an `Ebb`. No predecessors are declared
    /// here and the block is not sealed.
    /// Predecessors have to be added with `declare_ebb_predecessor`.
    pub fn declare_ebb_header_block(&mut self, ebb: Ebb) -> Block {
        let block = self.blocks
            .push(BlockData::EbbHeader(EbbHeaderBlockData {
                                           predecessors: HashMap::new(),
                                           sealed: false,
                                           ebb: ebb,
                                           undef_variables: Vec::new(),
                                       }));
        *self.ebb_headers.ensure(ebb) = block.into();
        block
    }
    /// Gets the header block corresponding to an Ebb, panics if the Ebb or the header block
    /// isn't declared.
    pub fn header_block(&self, ebb: Ebb) -> Block {
        match self.ebb_headers.get(ebb) {
            Some(&header) => {
                match header.expand() {
                    Some(header) => header,
                    None => panic!("the header block has not been defined"),
                }
            }
            None => panic!("the ebb has not been declared"),
        }
    }

    /// Declares a new predecessor for an `Ebb` header block and record the branch instruction
    /// of the predecessor that leads to it.
    ///
    /// Note that the predecessor is a `Block` and not an `Ebb`. This `Block` must be filled
    /// before added as predecessor. Note that you must provide no jump arguments to the branch
    /// instruction when you create it since `SSABuilder` will fill them for you.
    pub fn declare_ebb_predecessor(&mut self, ebb: Ebb, pred: Block, inst: Inst) {
        let header_block = match self.blocks[self.header_block(ebb)] {
            BlockData::EbbBody { .. } => panic!("you can't add predecessors to an Ebb body block"),
            BlockData::EbbHeader(ref data) => {
                assert!(!data.sealed);
                self.header_block(ebb)
            }
        };
        self.blocks[header_block].add_predecessor(pred, inst)
    }

    /// Remove a previously declared Ebb predecessor by giving a reference to the jump
    /// instruction. Returns the basic block containing the instruction.
    ///
    /// Note: use only when you know what you are doing, this might break the SSA bbuilding problem
    pub fn remove_ebb_predecessor(&mut self, ebb: Ebb, inst: Inst) -> Block {
        let header_block = match self.blocks[self.header_block(ebb)] {
            BlockData::EbbBody { .. } => panic!("you can't add predecessors to an Ebb body block"),
            BlockData::EbbHeader(ref data) => {
                assert!(!data.sealed);
                self.header_block(ebb)
            }
        };
        self.blocks[header_block].remove_predecessor(inst)
    }

    /// Completes the global value numbering for an `Ebb`, all of its predecessors having been
    /// already sealed.
    ///
    /// This method modifies the function's `Layout` by adding arguments to the `Ebb`s to
    /// take into account the Phi function placed by the SSA algorithm.
    pub fn seal_ebb_header_block(&mut self,
                                 ebb: Ebb,
                                 dfg: &mut DataFlowGraph,
                                 layout: &mut Layout,
                                 jts: &mut JumpTables)
                                 -> SideEffects {
        let block = self.header_block(ebb);

        // Sanity check
        match self.blocks[block] {
            BlockData::EbbBody { .. } => panic!("you can't seal an Ebb body block"),
            BlockData::EbbHeader(ref data) => {
                assert!(!data.sealed);
            }
        }

        // Recurse over the predecessors to find good definitions.
        let side_effects = self.resolve_undef_vars(block, dfg, layout, jts);

        // Then we mark the block as sealed.
        match self.blocks[block] {
            BlockData::EbbBody { .. } => panic!("this should not happen"),
            BlockData::EbbHeader(ref mut data) => data.sealed = true,
        };
        side_effects
    }

    // For each undef_var in an Ebb header block, lookup in the predecessors to append the right
    // jump argument to the branch instruction.
    // Panics if called with a non-header block.
    // Returns the list of newly created ebbs for critical edge splitting.
    fn resolve_undef_vars(&mut self,
                          block: Block,
                          dfg: &mut DataFlowGraph,
                          layout: &mut Layout,
                          jts: &mut JumpTables)
                          -> SideEffects {
        // TODO: find a way to not allocate vectors
        let (predecessors, undef_vars, ebb): (Vec<(Block, Inst)>,
                                              Vec<(Variable, Value)>,
                                              Ebb) = match self.blocks[block] {
            BlockData::EbbBody { .. } => panic!("this should not happen"),
            BlockData::EbbHeader(ref mut data) => {
                (data.predecessors.iter().map(|(&x, &y)| (x, y)).collect(),
                 data.undef_variables.clone(),
                 data.ebb)
            }
        };

        let mut side_effects = SideEffects {
            split_ebbs_created: Vec::new(),
            instructions_added_to_ebbs: Vec::new(),
        };
        // For each undef var we look up values in the predecessors and create an Ebb argument
        // only if necessary.
        for &(var, val) in undef_vars.iter() {
            let (_, mut local_side_effects) =
                self.predecessors_lookup(dfg, layout, jts, val, var, ebb, &predecessors);
            side_effects
                .split_ebbs_created
                .append(&mut local_side_effects.split_ebbs_created);
            side_effects
                .instructions_added_to_ebbs
                .append(&mut local_side_effects.instructions_added_to_ebbs);
        }

        // Then we clear the undef_vars and mark the block as sealed.
        match self.blocks[block] {
            BlockData::EbbBody { .. } => panic!("this should not happen"),
            BlockData::EbbHeader(ref mut data) => {
                data.undef_variables.clear();
            }
        };
        side_effects
    }

    /// Look up in the predecessors of an Ebb the def for a value an decides wether or not
    /// to keep the eeb arg, and act accordingly. Returns the chosen value and optionnaly a
    /// list of Ebb that are the middle of newly created critical edges splits.
    fn predecessors_lookup(&mut self,
                           dfg: &mut DataFlowGraph,
                           layout: &mut Layout,
                           jts: &mut JumpTables,
                           temp_arg_val: Value,
                           temp_arg_var: Variable,
                           dest_ebb: Ebb,
                           preds: &Vec<(Block, Inst)>)
                           -> (Value, SideEffects) {
        let mut pred_values: ZeroOneOrMore<Value> = ZeroOneOrMore::Zero();
        // TODO: find a way not not allocate a vector
        let mut jump_args_to_append: Vec<(Block, Inst, Value)> = Vec::new();
        let ty = dfg.value_type(temp_arg_val);
        let mut side_effects = SideEffects {
            split_ebbs_created: Vec::new(),
            instructions_added_to_ebbs: Vec::new(),
        };
        for &(pred, last_inst) in preds.iter() {
            // For undef value  and each predecessor we query what is the local SSA value
            // corresponding to var and we put it as an argument of the branch instruction.
            let (pred_val, mut local_side_effects) =
                self.use_var(dfg, layout, jts, temp_arg_var, ty, pred);
            pred_values = match pred_values {
                ZeroOneOrMore::Zero() => {
                    if pred_val == temp_arg_val {
                        ZeroOneOrMore::Zero()
                    } else {
                        ZeroOneOrMore::One(pred_val)
                    }
                }
                ZeroOneOrMore::One(old_val) => {
                    if pred_val == temp_arg_val || pred_val == old_val {
                        ZeroOneOrMore::One(old_val)
                    } else {
                        ZeroOneOrMore::More()
                    }
                }
                ZeroOneOrMore::More() => ZeroOneOrMore::More(),
            };
            jump_args_to_append.push((pred, last_inst, pred_val));
            side_effects
                .split_ebbs_created
                .append(&mut local_side_effects.split_ebbs_created);
            side_effects
                .instructions_added_to_ebbs
                .append(&mut local_side_effects.instructions_added_to_ebbs);
        }
        match pred_values {
            ZeroOneOrMore::Zero() => {
                // The variable is used but never defined before. This is an irregularity in the
                // code, but rather than throwing an error we silently initialize the variable to
                // 0. This will have no effect since this situation happens in unreachable code.
                if !layout.is_ebb_inserted(dest_ebb) {
                    layout.append_ebb(dest_ebb)
                };
                let mut cur = Cursor::new(layout);
                cur.goto_top(dest_ebb);
                cur.next_inst();
                let ty = dfg.value_type(temp_arg_val);
                let val = if ty.is_int() {
                    dfg.ins(&mut cur).iconst(ty, 0)
                } else if ty == F32 {
                    dfg.ins(&mut cur).f32const(Ieee32::new(0.0))
                } else if ty == F64 {
                    dfg.ins(&mut cur).f64const(Ieee64::new(0.0))
                } else {
                    panic!("value used but never declared and initialization not supported")
                };
                side_effects.instructions_added_to_ebbs.push(dest_ebb);
                (val, side_effects)
            }
            ZeroOneOrMore::One(pred_val) => {
                // Here all the predecessors use a single value to represent our variable
                // so we don't need to have it as an ebb argument.
                // We need to replace all the occurences of val with pred_val but since
                // we can't afford a re-writing pass right now we just declare an alias.
                dfg.remove_ebb_arg(temp_arg_val);
                dfg.change_to_alias(temp_arg_val, pred_val);
                (pred_val, side_effects)
            }
            ZeroOneOrMore::More() => {
                // There is disagreement in the predecessors on which value to use so we have
                // to keep the ebb argument.
                for (pred_block, last_inst, pred_val) in jump_args_to_append {
                    match self.append_jump_argument(dfg,
                                                    layout,
                                                    last_inst,
                                                    pred_block,
                                                    dest_ebb,
                                                    pred_val,
                                                    temp_arg_var,
                                                    jts) {
                        None => (),
                        Some(middle_ebb) => side_effects.split_ebbs_created.push(middle_ebb),
                    };
                }
                (temp_arg_val, side_effects)
            }
        }
    }

    /// Appends a jump argument to a jump instruction, returns ebb created in case of
    /// critical edge splitting.
    fn append_jump_argument(&mut self,
                            dfg: &mut DataFlowGraph,
                            layout: &mut Layout,
                            jump_inst: Inst,
                            jump_inst_block: Block,
                            dest_ebb: Ebb,
                            val: Value,
                            var: Variable,
                            jts: &mut JumpTables)
                            -> Option<Ebb> {
        match dfg[jump_inst].analyze_branch(&dfg.value_lists) {
            BranchInfo::NotABranch => {
                panic!("you have declared a non-branch instruction  as a predecessor to an ebb");
            }
            // For a single destination appending a jump argument to the instruction
            // is sufficient.
            BranchInfo::SingleDest(_, _) => {
                dfg.append_inst_arg(jump_inst, val);
                None
            }
            BranchInfo::Table(jt) => {
                // In the case of a jump table, the situation is tricky because br_table doesn't
                // support arguments.
                // We have to split the critical edge
                let indexes: Vec<usize> = jts[jt]
                    .entries()
                    .fold(Vec::new(), |mut acc, (index, dest)| if dest == dest_ebb {
                        acc.push(index);
                        acc
                    } else {
                        acc
                    });
                let middle_ebb = dfg.make_ebb();
                layout.append_ebb(middle_ebb);
                let block = self.declare_ebb_header_block(middle_ebb);
                self.blocks[block].add_predecessor(jump_inst_block, jump_inst);
                self.seal_ebb_header_block(middle_ebb, dfg, layout, jts);
                for index in indexes {
                    jts[jt].set_entry(index, middle_ebb)
                }
                let mut cur = Cursor::new(layout);
                cur.goto_bottom(middle_ebb);
                let middle_jump_inst = dfg.ins(&mut cur).jump(dest_ebb, &[val]);
                let dest_header_block = self.header_block(dest_ebb);
                self.blocks[dest_header_block].add_predecessor(block, middle_jump_inst);
                self.blocks[dest_header_block].remove_predecessor(jump_inst);
                self.def_var(var, val, block);
                Some(middle_ebb)
            }
        }
    }

    /// Returns the list of `Ebb`s that have been declared as predecessors of the argument.
    pub fn predecessors(&self, ebb: Ebb) -> &HashMap<Block, Inst> {
        let block = match self.ebb_headers[ebb].expand() {
            Some(block) => block,
            None => panic!("the ebb has not been declared yet"),
        };
        match self.blocks[block] {
            BlockData::EbbBody { .. } => panic!("should not happen"),
            BlockData::EbbHeader(ref data) => &data.predecessors,
        }
    }

    /// Returns `true` if and only if `seal_ebb_header_block` has been called on the argument.
    pub fn is_sealed(&self, ebb: Ebb) -> bool {
        match self.blocks[self.header_block(ebb)] {
            BlockData::EbbBody { .. } => panic!("should not happen"),
            BlockData::EbbHeader(ref data) => data.sealed,
        }
    }
}

#[cfg(test)]
mod tests {
    use cretonne::entity_ref::EntityRef;
    use cretonne::ir::{Function, InstBuilder, Cursor, Inst, JumpTableData};
    use cretonne::ir::types::*;
    use cretonne::verify_function;
    use cretonne::ir::instructions::BranchInfo;
    use ssa::SSABuilder;
    use std::u32;

    /// An opaque reference to variable.
    #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
    pub struct Variable(u32);
    impl EntityRef for Variable {
        fn new(index: usize) -> Self {
            assert!(index < (u32::MAX as usize));
            Variable(index as u32)
        }

        fn index(self) -> usize {
            self.0 as usize
        }
    }
    impl Default for Variable {
        fn default() -> Variable {
            Variable(u32::MAX)
        }
    }

    #[test]
    fn simple_block() {
        let mut func = Function::new();
        let mut ssa: SSABuilder<Variable> = SSABuilder::new();
        let ebb0 = func.dfg.make_ebb();
        // Here is the pseudo-program we want to translate:
        // x = 1;
        // y = 2;
        // z = x + y;
        // z = x + z;

        let block = ssa.declare_ebb_header_block(ebb0);
        let x_var = Variable(0);
        let x_ssa = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.insert_ebb(ebb0);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iconst(I32, 1)
        };
        ssa.def_var(x_var, x_ssa, block);
        let y_var = Variable(1);
        let y_ssa = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iconst(I32, 2)
        };
        ssa.def_var(y_var, y_ssa, block);

        assert_eq!(ssa.use_var(&mut func.dfg,
                               &mut func.layout,
                               &mut func.jump_tables,
                               x_var,
                               I32,
                               block)
                       .0,
                   x_ssa);
        assert_eq!(ssa.use_var(&mut func.dfg,
                               &mut func.layout,
                               &mut func.jump_tables,
                               y_var,
                               I32,
                               block)
                       .0,
                   y_ssa);
        let z_var = Variable(2);
        let x_use1 = ssa.use_var(&mut func.dfg,
                                 &mut func.layout,
                                 &mut func.jump_tables,
                                 x_var,
                                 I32,
                                 block)
            .0;
        let y_use1 = ssa.use_var(&mut func.dfg,
                                 &mut func.layout,
                                 &mut func.jump_tables,
                                 y_var,
                                 I32,
                                 block)
            .0;
        let z1_ssa = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iadd(x_use1, y_use1)
        };
        ssa.def_var(z_var, z1_ssa, block);
        assert_eq!(ssa.use_var(&mut func.dfg,
                               &mut func.layout,
                               &mut func.jump_tables,
                               z_var,
                               I32,
                               block)
                       .0,
                   z1_ssa);
        let x_use2 = ssa.use_var(&mut func.dfg,
                                 &mut func.layout,
                                 &mut func.jump_tables,
                                 x_var,
                                 I32,
                                 block)
            .0;
        let z_use1 = ssa.use_var(&mut func.dfg,
                                 &mut func.layout,
                                 &mut func.jump_tables,
                                 z_var,
                                 I32,
                                 block)
            .0;
        let z2_ssa = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iadd(x_use2, z_use1)
        };
        ssa.def_var(z_var, z2_ssa, block);
        assert_eq!(ssa.use_var(&mut func.dfg,
                               &mut func.layout,
                               &mut func.jump_tables,
                               z_var,
                               I32,
                               block)
                       .0,
                   z2_ssa);
    }

    #[test]
    fn sequence_of_blocks() {
        let mut func = Function::new();
        let mut ssa: SSABuilder<Variable> = SSABuilder::new();
        let ebb0 = func.dfg.make_ebb();
        let ebb1 = func.dfg.make_ebb();
        // Here is the pseudo-program we want to translate:
        // ebb0:
        //    x = 1;
        //    y = 2;
        //    z = x + y;
        //    brnz y, ebb1;
        //    z = x + z;
        // ebb1:
        //    y = x + y;

        let block0 = ssa.declare_ebb_header_block(ebb0);
        let x_var = Variable(0);
        let x_ssa = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.insert_ebb(ebb0);
            cur.insert_ebb(ebb1);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iconst(I32, 1)
        };
        ssa.def_var(x_var, x_ssa, block0);
        let y_var = Variable(1);
        let y_ssa = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iconst(I32, 2)
        };
        ssa.def_var(y_var, y_ssa, block0);
        assert_eq!(ssa.use_var(&mut func.dfg,
                               &mut func.layout,
                               &mut func.jump_tables,
                               x_var,
                               I32,
                               block0)
                       .0,
                   x_ssa);
        assert_eq!(ssa.use_var(&mut func.dfg,
                               &mut func.layout,
                               &mut func.jump_tables,
                               y_var,
                               I32,
                               block0)
                       .0,
                   y_ssa);
        let z_var = Variable(2);
        let x_use1 = ssa.use_var(&mut func.dfg,
                                 &mut func.layout,
                                 &mut func.jump_tables,
                                 x_var,
                                 I32,
                                 block0)
            .0;
        let y_use1 = ssa.use_var(&mut func.dfg,
                                 &mut func.layout,
                                 &mut func.jump_tables,
                                 y_var,
                                 I32,
                                 block0)
            .0;
        let z1_ssa = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iadd(x_use1, y_use1)
        };
        ssa.def_var(z_var, z1_ssa, block0);
        assert_eq!(ssa.use_var(&mut func.dfg,
                               &mut func.layout,
                               &mut func.jump_tables,
                               z_var,
                               I32,
                               block0)
                       .0,
                   z1_ssa);
        let y_use2 = ssa.use_var(&mut func.dfg,
                                 &mut func.layout,
                                 &mut func.jump_tables,
                                 y_var,
                                 I32,
                                 block0)
            .0;
        let jump_inst: Inst = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).brnz(y_use2, ebb1, &[])
        };
        let block1 = ssa.declare_ebb_body_block(block0);
        let x_use2 = ssa.use_var(&mut func.dfg,
                                 &mut func.layout,
                                 &mut func.jump_tables,
                                 x_var,
                                 I32,
                                 block1)
            .0;
        assert_eq!(x_use2, x_ssa);
        let z_use1 = ssa.use_var(&mut func.dfg,
                                 &mut func.layout,
                                 &mut func.jump_tables,
                                 z_var,
                                 I32,
                                 block1)
            .0;
        assert_eq!(z_use1, z1_ssa);
        let z2_ssa = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iadd(x_use2, z_use1)
        };
        ssa.def_var(z_var, z2_ssa, block1);
        assert_eq!(ssa.use_var(&mut func.dfg,
                               &mut func.layout,
                               &mut func.jump_tables,
                               z_var,
                               I32,
                               block1)
                       .0,
                   z2_ssa);
        ssa.seal_ebb_header_block(ebb0, &mut func.dfg, &mut func.layout, &mut func.jump_tables);
        let block2 = ssa.declare_ebb_header_block(ebb1);
        ssa.declare_ebb_predecessor(ebb1, block0, jump_inst);
        ssa.seal_ebb_header_block(ebb1, &mut func.dfg, &mut func.layout, &mut func.jump_tables);
        let x_use3 = ssa.use_var(&mut func.dfg,
                                 &mut func.layout,
                                 &mut func.jump_tables,
                                 x_var,
                                 I32,
                                 block2)
            .0;
        assert_eq!(x_ssa, x_use3);
        let y_use3 = ssa.use_var(&mut func.dfg,
                                 &mut func.layout,
                                 &mut func.jump_tables,
                                 y_var,
                                 I32,
                                 block2)
            .0;
        assert_eq!(y_ssa, y_use3);
        let y2_ssa = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb1);
            func.dfg.ins(cur).iadd(x_use3, y_use3)
        };
        ssa.def_var(y_var, y2_ssa, block2);
        match func.dfg[jump_inst].analyze_branch(&func.dfg.value_lists) {
            BranchInfo::SingleDest(dest, jump_args) => {
                assert_eq!(dest, ebb1);
                assert_eq!(jump_args.len(), 0);
            }
            _ => assert!(false),
        };
    }

    #[test]
    fn program_with_loop() {
        let mut func = Function::new();
        let mut ssa: SSABuilder<Variable> = SSABuilder::new();
        let ebb0 = func.dfg.make_ebb();
        let ebb1 = func.dfg.make_ebb();
        let ebb2 = func.dfg.make_ebb();
        // Here is the pseudo-program we want to translate:
        // ebb0:
        //    x = 1;
        //    y = 2;
        //    z = x + y;
        //    jump ebb1
        // ebb1:
        //    z = z + y;
        //    brnz y, ebb1;
        //    z = z - x;
        //    return y
        // ebb2:
        //    y = y - x
        //    jump ebb1

        let block0 = ssa.declare_ebb_header_block(ebb0);
        ssa.seal_ebb_header_block(ebb0, &mut func.dfg, &mut func.layout, &mut func.jump_tables);
        let x_var = Variable(0);
        let x1 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.insert_ebb(ebb0);
            cur.insert_ebb(ebb1);
            cur.insert_ebb(ebb2);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iconst(I32, 1)
        };
        ssa.def_var(x_var, x1, block0);
        assert_eq!(ssa.use_var(&mut func.dfg,
                               &mut func.layout,
                               &mut func.jump_tables,
                               x_var,
                               I32,
                               block0)
                       .0,
                   x1);
        let y_var = Variable(1);
        let y1 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iconst(I32, 2)
        };
        ssa.def_var(y_var, y1, block0);
        assert_eq!(ssa.use_var(&mut func.dfg,
                               &mut func.layout,
                               &mut func.jump_tables,
                               y_var,
                               I32,
                               block0)
                       .0,
                   y1);
        let z_var = Variable(2);
        let x2 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             x_var,
                             I32,
                             block0)
            .0;
        assert_eq!(x2, x1);
        let y2 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             y_var,
                             I32,
                             block0)
            .0;
        assert_eq!(y2, y1);
        let z1 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iadd(x2, y2)
        };
        ssa.def_var(z_var, z1, block0);
        let jump_ebb0_ebb1 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).jump(ebb1, &[])
        };
        let block1 = ssa.declare_ebb_header_block(ebb1);
        ssa.declare_ebb_predecessor(ebb1, block0, jump_ebb0_ebb1);
        let z2 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             z_var,
                             I32,
                             block1)
            .0;
        let y3 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             y_var,
                             I32,
                             block1)
            .0;
        let z3 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb1);
            func.dfg.ins(cur).iadd(z2, y3)
        };
        ssa.def_var(z_var, z3, block1);
        let y4 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             y_var,
                             I32,
                             block1)
            .0;
        assert_eq!(y4, y3);
        let jump_ebb1_ebb2 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb1);
            func.dfg.ins(cur).brnz(y4, ebb2, &[])
        };
        let block2 = ssa.declare_ebb_body_block(block1);
        let z4 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             z_var,
                             I32,
                             block2)
            .0;
        assert_eq!(z4, z3);
        let x3 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             x_var,
                             I32,
                             block2)
            .0;
        let z5 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb1);
            func.dfg.ins(cur).isub(z4, x3)
        };
        ssa.def_var(z_var, z5, block2);
        let y5 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             y_var,
                             I32,
                             block2)
            .0;
        assert_eq!(y5, y3);
        {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb1);
            func.dfg.ins(cur).return_(&[y5])
        };

        let block3 = ssa.declare_ebb_header_block(ebb2);
        ssa.declare_ebb_predecessor(ebb2, block1, jump_ebb1_ebb2);
        ssa.seal_ebb_header_block(ebb2, &mut func.dfg, &mut func.layout, &mut func.jump_tables);
        let y6 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             y_var,
                             I32,
                             block3)
            .0;
        assert_eq!(y6, y3);
        let x4 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             x_var,
                             I32,
                             block3)
            .0;
        assert_eq!(x4, x3);
        let y7 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb2);
            func.dfg.ins(cur).isub(y6, x4)
        };
        ssa.def_var(y_var, y7, block3);
        let jump_ebb2_ebb1 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb2);
            func.dfg.ins(cur).jump(ebb1, &[])
        };

        ssa.declare_ebb_predecessor(ebb1, block3, jump_ebb2_ebb1);
        ssa.seal_ebb_header_block(ebb1, &mut func.dfg, &mut func.layout, &mut func.jump_tables);
        assert_eq!(func.dfg.ebb_args(ebb1)[0], z2);
        assert_eq!(func.dfg.ebb_args(ebb1)[1], y3);
        assert_eq!(func.dfg.resolve_aliases(x3), x1);

    }

    #[test]
    fn br_table_with_args() {
        // This tests the on-demand splitting of critical edges for br_table with jump arguments
        let mut func = Function::new();
        let mut ssa: SSABuilder<Variable> = SSABuilder::new();
        let ebb0 = func.dfg.make_ebb();
        let ebb1 = func.dfg.make_ebb();
        // Here is the pseudo-program we want to translate:
        // ebb0:
        //    x = 0;
        //    br_table x ebb1
        //    x = 1
        //    jump ebb1
        // ebb1:
        //    x = x + 1
        //    return
        //
        let block0 = ssa.declare_ebb_header_block(ebb0);
        ssa.seal_ebb_header_block(ebb0, &mut func.dfg, &mut func.layout, &mut func.jump_tables);
        let x_var = Variable(0);
        let x1 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.insert_ebb(ebb0);
            cur.insert_ebb(ebb1);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iconst(I32, 1)
        };
        ssa.def_var(x_var, x1, block0);
        let mut jt_data = JumpTableData::new();
        jt_data.set_entry(0, ebb1);
        let jt = func.jump_tables.push(jt_data);
        ssa.use_var(&mut func.dfg,
                     &mut func.layout,
                     &mut func.jump_tables,
                     x_var,
                     I32,
                     block0)
            .0;
        let br_table = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).br_table(x1, jt)
        };
        let block1 = ssa.declare_ebb_body_block(block0);
        let x3 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iconst(I32, 2)
        };
        ssa.def_var(x_var, x3, block1);
        let jump_inst = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).jump(ebb1, &[])
        };
        let block2 = ssa.declare_ebb_header_block(ebb1);
        ssa.declare_ebb_predecessor(ebb1, block1, jump_inst);
        ssa.declare_ebb_predecessor(ebb1, block0, br_table);
        ssa.seal_ebb_header_block(ebb1, &mut func.dfg, &mut func.layout, &mut func.jump_tables);
        let x4 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             x_var,
                             I32,
                             block2)
            .0;
        {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb1);
            func.dfg.ins(cur).iadd_imm(x4, 1)
        };
        {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb1);
            func.dfg.ins(cur).return_(&[])
        };
        match verify_function(&func, None) {
            Ok(()) => {}
            Err(err) => panic!(err.message),
        }
    }

    #[test]
    fn undef_values_reordering() {
        let mut func = Function::new();
        let mut ssa: SSABuilder<Variable> = SSABuilder::new();
        let ebb0 = func.dfg.make_ebb();
        let ebb1 = func.dfg.make_ebb();
        // Here is the pseudo-program we want to translate:
        // ebb0:
        //    x = 0
        //    y = 1
        //    z = 2
        //    jump ebb1
        // ebb1:
        //    x = z + x
        //    y = y - x
        //    jump ebb1
        //
        let block0 = ssa.declare_ebb_header_block(ebb0);
        let x_var = Variable(0);
        let y_var = Variable(1);
        let z_var = Variable(2);
        ssa.seal_ebb_header_block(ebb0, &mut func.dfg, &mut func.layout, &mut func.jump_tables);
        let x1 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.insert_ebb(ebb0);
            cur.insert_ebb(ebb1);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iconst(I32, 0)
        };
        ssa.def_var(x_var, x1, block0);
        let y1 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iconst(I32, 1)
        };
        ssa.def_var(y_var, y1, block0);
        let z1 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).iconst(I32, 2)
        };
        ssa.def_var(z_var, z1, block0);
        let jump_inst = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb0);
            func.dfg.ins(cur).jump(ebb1, &[])
        };
        let block1 = ssa.declare_ebb_header_block(ebb1);
        ssa.declare_ebb_predecessor(ebb1, block0, jump_inst);
        let z2 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             z_var,
                             I32,
                             block1)
            .0;
        assert_eq!(func.dfg.ebb_args(ebb1)[0], z2);
        let x2 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             x_var,
                             I32,
                             block1)
            .0;
        assert_eq!(func.dfg.ebb_args(ebb1)[1], x2);
        let x3 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb1);
            func.dfg.ins(cur).iadd(x2, z2)
        };
        ssa.def_var(x_var, x3, block1);
        let x4 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             x_var,
                             I32,
                             block1)
            .0;
        let y3 = ssa.use_var(&mut func.dfg,
                             &mut func.layout,
                             &mut func.jump_tables,
                             y_var,
                             I32,
                             block1)
            .0;
        assert_eq!(func.dfg.ebb_args(ebb1)[2], y3);
        let y4 = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb1);
            func.dfg.ins(cur).isub(y3, x4)
        };
        ssa.def_var(y_var, y4, block1);
        let jump_inst = {
            let cur = &mut Cursor::new(&mut func.layout);
            cur.goto_bottom(ebb1);
            func.dfg.ins(cur).jump(ebb1, &[])
        };
        ssa.declare_ebb_predecessor(ebb1, block1, jump_inst);
        ssa.seal_ebb_header_block(ebb1, &mut func.dfg, &mut func.layout, &mut func.jump_tables);
        // At sealing the "z" argument disappear but the remaining "x" and "y" args have to be
        // in the right order.
        assert_eq!(func.dfg.ebb_args(ebb1)[1], y3);
        assert_eq!(func.dfg.ebb_args(ebb1)[0], x2);
    }
}