chore: Initial SSA refactor module (#1113)

* add ssa refactor module

* add more stub code

* move value to its own file

* add function

* Rollback to simpler case

* move types to types module

* review

* make types pub(crate)

* allow dead code

* add offline code

* remove cfg

* clean up

* remove changes to old code

* fix clippy

* remove builder.rs

* cargo fmt

* clippy

crates/noirc_evaluator/src/frontend.rs

@@ -0,0 +1 @@
+pub mod variable;

crates/noirc_evaluator/src/frontend/variable.rs

@@ -0,0 +1,23 @@
+/// A variable in the SSA IR.
+/// By definition, a variable can only be defined once.
+///
+/// As in Cranelift, we also allow variable use before definition.
+/// This will produce side-effects which will need to be handled
+/// before sealing a block.
+pub struct Variable(u32);
+
+impl From<u32> for Variable {
+    fn from(value: u32) -> Self {
+        Variable(value)
+    }
+}
+impl From<u16> for Variable {
+    fn from(value: u16) -> Self {
+        Variable(value as u32)
+    }
+}
+impl From<u8> for Variable {
+    fn from(value: u8) -> Self {
+        Variable(value as u32)
+    }
+}

crates/noirc_evaluator/src/lib.rs

@@ -6,6 +6,13 @@
 mod errors;
 mod ssa;
 
+// SSA code to create the SSA based IR
+// for functions and execute different optimizations.
+pub mod ssa_refactor;
+// Frontend helper module to translate a different AST
+// into the SSA IR.
+pub mod frontend;
+
 use acvm::{
     acir::circuit::{opcodes::Opcode as AcirOpcode, Circuit, PublicInputs},
     acir::native_types::{Expression, Witness},

crates/noirc_evaluator/src/ssa_refactor.rs

@@ -0,0 +1,13 @@
+//! SSA stands for Single Static Assignment
+//! The IR presented in this module will already
+//! be in SSA form and will be used to apply
+//! conventional optimizations like Common Subexpression
+//! elimination and constant folding.
+//!
+//! This module heavily borrows from Cranelift
+#[allow(dead_code)]
+mod basic_block;
+#[allow(dead_code)]
+mod dfg;
+#[allow(dead_code)]
+mod ir;

crates/noirc_evaluator/src/ssa_refactor/basic_block.rs

@@ -0,0 +1,37 @@
+use super::ir::instruction::{Instruction, TerminatorInstruction};
+
+/// A Basic block is a maximal collection of instructions
+/// such that there are only jumps at the end of block
+/// and one can only enter the block from the beginning.
+///
+/// This means that if one instruction is executed in a basic
+/// block, then all instructions are executed. ie single-entry single-exit.
+#[derive(Debug, PartialEq, Eq, Hash, Clone)]
+pub(crate) struct BasicBlock {
+    /// Arguments to the basic block.
+    phi_nodes: Vec<BlockArguments>,
+    /// Instructions in the basic block.
+    instructions: Vec<Instruction>,
+
+    /// A basic block is considered sealed
+    /// if no further predecessors will be added to it.
+    /// Since only filled blocks can have successors,
+    /// predecessors are always filled.
+    is_sealed: bool,
+
+    /// The terminating instruction for the basic block.
+    ///
+    /// This will be a control flow instruction.
+    terminator: TerminatorInstruction,
+}
+
+#[derive(Debug, PartialEq, Eq, Hash, Clone)]
+/// An identifier for a Basic Block.
+pub(crate) struct BasicBlockId;
+
+#[derive(Debug, PartialEq, Eq, Hash, Clone)]
+/// Arguments to the basic block.
+/// We use the modern Crane-lift strategy
+/// of representing phi nodes as basic block
+/// arguments.
+pub(crate) struct BlockArguments;

crates/noirc_evaluator/src/ssa_refactor/dfg.rs

@@ -0,0 +1,186 @@
+use super::{
+    basic_block::{BasicBlock, BasicBlockId},
+    ir::{
+        extfunc::{SigRef, Signature},
+        instruction::{Instruction, InstructionId, Instructions},
+        types::Typ,
+        value::{Value, ValueId},
+    },
+};
+use std::collections::HashMap;
+
+#[derive(Debug, Default)]
+/// A convenience wrapper to store `Value`s.
+pub(crate) struct ValueList(Vec<ValueId>);
+
+impl ValueList {
+    /// Inserts an element to the back of the list and
+    /// returns the `position`
+    pub(crate) fn push(&mut self, value: ValueId) -> usize {
+        self.0.push(value);
+        self.len() - 1
+    }
+    /// Returns the number of values in the list.
+    fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    /// Removes all items from the list.
+    fn clear(&mut self) {
+        self.0.clear();
+    }
+    /// Returns the ValueId's as a slice.
+    pub(crate) fn as_slice(&self) -> &[ValueId] {
+        &self.0
+    }
+}
+#[derive(Debug, Default)]
+pub(crate) struct DataFlowGraph {
+    /// All of the instructions in a function
+    instructions: Instructions,
+
+    /// Stores the results for a particular instruction.
+    ///
+    /// An instruction may return multiple values
+    /// and for this, we will also use the cranelift strategy
+    /// to fetch them via indices.
+    ///
+    /// Currently, we need to define them in a better way
+    /// Call instructions require the func signature, but
+    /// other instructions may need some more reading on my part
+    results: HashMap<InstructionId, ValueList>,
+
+    /// Storage for all of the values defined in this
+    /// function.
+    values: HashMap<ValueId, Value>,
+
+    /// Function signatures of external methods
+    signatures: HashMap<SigRef, Signature>,
+
+    /// All blocks in a function
+    blocks: HashMap<BasicBlockId, BasicBlock>,
+}
+
+impl DataFlowGraph {
+    /// Creates a new `empty` basic block
+    pub(crate) fn new_block(&mut self) -> BasicBlockId {
+        todo!()
+    }
+
+    /// Inserts a new instruction into the DFG.
+    pub(crate) fn make_instruction(&mut self, instruction_data: Instruction) -> InstructionId {
+        let id = self.instructions.add_instruction(instruction_data);
+
+        // Create a new vector to store the potential results
+        // for the instruction.
+        self.results.insert(id, Default::default());
+
+        id
+    }
+
+    /// Attaches results to the instruction.
+    ///
+    /// Returns the number of results that this instruction
+    /// produces.
+    pub(crate) fn make_instruction_results(
+        &mut self,
+        instruction_id: InstructionId,
+        ctrl_typevar: Typ,
+    ) -> usize {
+        // Clear all of the results instructions associated with this
+        // instruction.
+        self.results.get_mut(&instruction_id).expect("all instructions should have a `result` allocation when instruction was added to the DFG").clear();
+
+        // Get all of the types that this instruction produces
+        // and append them as results.
+        let typs = self.instruction_result_types(instruction_id, ctrl_typevar);
+        let num_typs = typs.len();
+
+        for typ in typs {
+            self.append_result(instruction_id, typ);
+        }
+
+        num_typs
+    }
+
+    /// Return the result types of this instruction.
+    ///
+    /// For example, an addition instruction will return
+    /// one type which is the type of the operands involved.
+    /// This is the `ctrl_typevar` in this case.
+    fn instruction_result_types(
+        &self,
+        instruction_id: InstructionId,
+        ctrl_typevar: Typ,
+    ) -> Vec<Typ> {
+        // Check if it is a call instruction. If so, we don't support that yet
+        let ins_data = self.instructions.get_instruction(instruction_id);
+        match ins_data {
+            Instruction::Call { .. } => todo!("function calls are not supported yet"),
+            ins => ins.return_types(ctrl_typevar),
+        }
+    }
+
+    /// Appends a result type to the instruction.
+    pub(crate) fn append_result(&mut self, instruction_id: InstructionId, typ: Typ) -> ValueId {
+        let next_value_id = self.next_value();
+
+        // Add value to the list of results for this instruction
+        let res_position = self.results.get_mut(&instruction_id).unwrap().push(next_value_id);
+
+        self.make_value(Value::Instruction {
+            typ,
+            position: res_position as u16,
+            instruction: instruction_id,
+        })
+    }
+
+    /// Stores a value and returns its `ValueId` reference.
+    fn make_value(&mut self, data: Value) -> ValueId {
+        let next_value = self.next_value();
+
+        self.values.insert(next_value, data);
+
+        next_value
+    }
+
+    /// Returns the next `ValueId`
+    fn next_value(&self) -> ValueId {
+        ValueId(self.values.len() as u32)
+    }
+
+    /// Returns the number of instructions
+    /// inserted into functions.
+    pub(crate) fn num_instructions(&self) -> usize {
+        self.instructions.num_instructions()
+    }
+
+    /// Returns all of result values which are attached to this instruction.
+    pub(crate) fn instruction_results(&self, instruction_id: InstructionId) -> &[ValueId] {
+        self.results.get(&instruction_id).expect("expected a list of Values").as_slice()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::DataFlowGraph;
+    use crate::ssa_refactor::ir::{
+        instruction::Instruction,
+        types::{NumericType, Typ},
+    };
+    use acvm::FieldElement;
+
+    #[test]
+    fn make_instruction() {
+        let mut dfg = DataFlowGraph::default();
+        let ins = Instruction::Immediate { value: FieldElement::from(0u128) };
+        let ins_id = dfg.make_instruction(ins);
+
+        let num_results =
+            dfg.make_instruction_results(ins_id, Typ::Numeric(NumericType::NativeField));
+
+        let results = dfg.instruction_results(ins_id);
+
+        assert_eq!(results.len(), num_results);
+    }
+}

crates/noirc_evaluator/src/ssa_refactor/ir.rs

@@ -0,0 +1,5 @@
+pub(crate) mod extfunc;
+mod function;
+pub(crate) mod instruction;
+pub(crate) mod types;
+pub(crate) mod value;

crates/noirc_evaluator/src/ssa_refactor/ir/extfunc.rs

@@ -0,0 +1,23 @@
+//! Like Crane-lift all functions outside of the current function is seen as
+//! external.
+//! To reference external functions, one uses
+
+use super::types::Typ;
+
+#[derive(Debug, Default, Clone)]
+pub(crate) struct Signature {
+    pub(crate) params: Vec<Typ>,
+    pub(crate) returns: Vec<Typ>,
+}
+/// Reference to a `Signature` in a map inside of
+/// a functions DFG.
+#[derive(Debug, Default, Clone, Copy)]
+pub(crate) struct SigRef(pub(crate) u32);
+
+#[test]
+fn sign_smoke() {
+    let mut signature = Signature::default();
+
+    signature.params.push(Typ::Numeric(super::types::NumericType::NativeField));
+    signature.returns.push(Typ::Numeric(super::types::NumericType::Unsigned { bit_size: 32 }));
+}

crates/noirc_evaluator/src/ssa_refactor/ir/function.rs

@@ -0,0 +1,25 @@
+use crate::ssa_refactor::basic_block::{BasicBlock, BasicBlockId};
+
+use super::instruction::Instruction;
+
+use noirc_errors::Location;
+use std::collections::HashMap;
+
+/// A function holds a list of instructions.
+/// These instructions are further grouped into
+/// Basic blocks
+#[derive(Debug)]
+pub(crate) struct Function {
+    /// Basic blocks associated to this particular function
+    basic_blocks: HashMap<BasicBlockId, BasicBlock>,
+
+    /// Maps instructions to source locations
+    source_locations: HashMap<Instruction, Location>,
+
+    /// The first basic block in the function
+    entry_block: BasicBlockId,
+}
+
+/// FunctionId is a reference for a function
+#[derive(Debug, PartialEq, Eq, Hash, Clone, Copy)]
+pub(crate) struct FunctionId(pub(crate) u32);