feat: data bus (#3508)

# Description

## Problem\*

aztec private kernel is doing a lot of proof recursion along the call
stack of contract calls, resulting in a lot of public inputs required
for the aztec ABI. These amount of public inputs have an impact on the
performance, which can be mitigated by having instead the public inputs
as private and a public commitment of these inputs. We can then leverage
the proving system capability for proving/verifying efficiently
commitments, outside of the noir program.

## Summary\*
In order to interface with the proving system, the user can declare some
inputs as call_data (instead of pub) and the return values as
return_data (instead of pub). The compiler will abstract these into
specific immutable arrays that the proving system will use.



## Additional Context



## Documentation\*

Check one:
- [ ] No documentation needed.
- [X] Documentation included in this PR.
- [ ] **[Exceptional Case]** Documentation to be submitted in a separate
PR.

# PR Checklist\*

- [X] I have tested the changes locally.
- [X] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

---------

Co-authored-by: Tom French <tom@tomfren.ch>

compiler/noirc_driver/src/abi_gen.rs

@@ -2,11 +2,12 @@ use std::collections::BTreeMap;
 
 use acvm::acir::native_types::Witness;
 use iter_extended::{btree_map, vecmap};
-use noirc_abi::{Abi, AbiParameter, AbiType};
+use noirc_abi::{Abi, AbiParameter, AbiReturnType, AbiType};
 use noirc_frontend::{
     hir::Context,
     hir_def::{function::Param, stmt::HirPattern},
     node_interner::{FuncId, NodeInterner},
+    Visibility,
 };
 use std::ops::Range;
 
@@ -17,9 +18,12 @@ pub(super) fn gen_abi(
     func_id: &FuncId,
     input_witnesses: Vec<Witness>,
     return_witnesses: Vec<Witness>,
+    return_visibility: Visibility,
 ) -> Abi {
     let (parameters, return_type) = compute_function_abi(context, func_id);
     let param_witnesses = param_witnesses_from_abi_param(&parameters, input_witnesses);
+    let return_type = return_type
+        .map(|typ| AbiReturnType { abi_type: typ, visibility: return_visibility.into() });
     Abi { parameters, return_type, param_witnesses, return_witnesses }
 }
 

compiler/noirc_driver/src/lib.rs

@@ -346,11 +346,12 @@ pub fn compile_no_check(
     if !force_compile && hashes_match {
         return Ok(cached_program.expect("cache must exist for hashes to match"));
     }
-
+    let visibility = program.return_visibility;
     let (circuit, debug, input_witnesses, return_witnesses, warnings) =
         create_circuit(program, options.show_ssa, options.show_brillig)?;
 
-    let abi = abi_gen::gen_abi(context, &main_function, input_witnesses, return_witnesses);
+    let abi =
+        abi_gen::gen_abi(context, &main_function, input_witnesses, return_witnesses, visibility);
     let file_map = filter_relevant_files(&[debug.clone()], &context.file_manager);
 
     Ok(CompiledProgram {

compiler/noirc_evaluator/src/ssa/acir_gen/mod.rs

@@ -5,6 +5,7 @@ use std::collections::HashSet;
 use std::fmt::Debug;
 
 use self::acir_ir::acir_variable::{AcirContext, AcirType, AcirVar};
+use super::function_builder::data_bus::DataBus;
 use super::ir::dfg::CallStack;
 use super::{
     ir::{
@@ -90,6 +91,8 @@ struct Context {
     /// Maps SSA array values to their slice size and any nested slices internal to the parent slice.
     /// This enables us to maintain the slice structure of a slice when performing an array get.
     slice_sizes: HashMap<Id<Value>, Vec<usize>>,
+
+    data_bus: DataBus,
 }
 
 #[derive(Clone)]
@@ -199,6 +202,7 @@ impl Context {
             internal_mem_block_lengths: HashMap::default(),
             max_block_id: 0,
             slice_sizes: HashMap::default(),
+            data_bus: DataBus::default(),
         }
     }
 
@@ -226,6 +230,8 @@ impl Context {
         let dfg = &main_func.dfg;
         let entry_block = &dfg[main_func.entry_block()];
         let input_witness = self.convert_ssa_block_params(entry_block.parameters(), dfg)?;
+
+        self.data_bus = dfg.data_bus.to_owned();
         let mut warnings = Vec::new();
         for instruction_id in entry_block.instructions() {
             warnings.extend(self.convert_ssa_instruction(
@@ -892,10 +898,26 @@ impl Context {
         dfg: &DataFlowGraph,
     ) -> Result<AcirValue, RuntimeError> {
         let (array_id, _, block_id) = self.check_array_is_initialized(array, dfg)?;
-
         let results = dfg.instruction_results(instruction);
         let res_typ = dfg.type_of_value(results[0]);
 
+        // Get operations to call-data parameters are replaced by a get to the call-data-bus array
+        if let Some(call_data) = self.data_bus.call_data {
+            if self.data_bus.call_data_map.contains_key(&array_id) {
+                // TODO: the block_id of call-data must be notified to the backend
+                // TODO: should we do the same for return-data?
+                let type_size = res_typ.flattened_size();
+                let type_size =
+                    self.acir_context.add_constant(FieldElement::from(type_size as i128));
+                let offset = self.acir_context.mul_var(var_index, type_size)?;
+                let bus_index = self.acir_context.add_constant(FieldElement::from(
+                    self.data_bus.call_data_map[&array_id] as i128,
+                ));
+                let new_index = self.acir_context.add_var(offset, bus_index)?;
+                return self.array_get(instruction, call_data, new_index, dfg);
+            }
+        }
+
         let value = if !res_typ.contains_slice_element() {
             self.array_get_value(&res_typ, block_id, &mut var_index, &[])?
         } else {

compiler/noirc_evaluator/src/ssa/function_builder/data_bus.rs

@@ -0,0 +1,144 @@
+use std::rc::Rc;
+
+use crate::ssa::ir::{types::Type, value::ValueId};
+use acvm::FieldElement;
+use fxhash::FxHashMap as HashMap;
+use noirc_frontend::hir_def::function::FunctionSignature;
+
+use super::FunctionBuilder;
+
+/// Used to create a data bus, which is an array of private inputs
+/// replacing public inputs
+pub(crate) struct DataBusBuilder {
+    pub(crate) values: im::Vector<ValueId>,
+    index: usize,
+    pub(crate) map: HashMap<ValueId, usize>,
+    pub(crate) databus: Option<ValueId>,
+}
+
+impl DataBusBuilder {
+    pub(crate) fn new() -> DataBusBuilder {
+        DataBusBuilder {
+            index: 0,
+            map: HashMap::default(),
+            databus: None,
+            values: im::Vector::new(),
+        }
+    }
+
+    /// Generates a boolean vector telling which (ssa) parameter from the given function signature
+    /// are tagged with databus visibility
+    pub(crate) fn is_databus(main_signature: &FunctionSignature) -> Vec<bool> {
+        let mut params_is_databus = Vec::new();
+
+        for param in &main_signature.0 {
+            let is_databus = match param.2 {
+                noirc_frontend::Visibility::Public | noirc_frontend::Visibility::Private => false,
+                noirc_frontend::Visibility::DataBus => true,
+            };
+            let len = param.1.field_count() as usize;
+            params_is_databus.extend(vec![is_databus; len]);
+        }
+        params_is_databus
+    }
+}
+
+#[derive(Clone, Default, Debug)]
+pub(crate) struct DataBus {
+    pub(crate) call_data: Option<ValueId>,
+    pub(crate) call_data_map: HashMap<ValueId, usize>,
+    pub(crate) return_data: Option<ValueId>,
+}
+
+impl DataBus {
+    /// Updates the databus values with the provided function
+    pub(crate) fn map_values(&self, mut f: impl FnMut(ValueId) -> ValueId) -> DataBus {
+        let mut call_data_map = HashMap::default();
+        for (k, v) in self.call_data_map.iter() {
+            call_data_map.insert(f(*k), *v);
+        }
+        DataBus {
+            call_data: self.call_data.map(&mut f),
+            call_data_map,
+            return_data: self.return_data.map(&mut f),
+        }
+    }
+
+    /// Construct a databus from call_data and return_data data bus builders
+    pub(crate) fn get_data_bus(call_data: DataBusBuilder, return_data: DataBusBuilder) -> DataBus {
+        DataBus {
+            call_data: call_data.databus,
+            call_data_map: call_data.map,
+            return_data: return_data.databus,
+        }
+    }
+}
+
+impl FunctionBuilder {
+    /// Insert a value into a data bus builder
+    fn add_to_data_bus(&mut self, value: ValueId, databus: &mut DataBusBuilder) {
+        assert!(databus.databus.is_none(), "initialising finalized call data");
+        let typ = self.current_function.dfg[value].get_type().clone();
+        match typ {
+            Type::Numeric(_) => {
+                databus.values.push_back(value);
+                databus.index += 1;
+            }
+            Type::Reference(_) => unreachable!(),
+            Type::Array(typ, len) => {
+                assert!(typ.len() == 1, "unsupported composite type");
+                databus.map.insert(value, databus.index);
+                for i in 0..len {
+                    // load each element of the array
+                    let index = self
+                        .current_function
+                        .dfg
+                        .make_constant(FieldElement::from(i as i128), Type::field());
+                    let element = self.insert_array_get(value, index, typ[0].clone());
+                    self.add_to_data_bus(element, databus);
+                }
+            }
+            Type::Slice(_) => unreachable!(),
+            Type::Function => unreachable!(),
+        }
+    }
+
+    /// Create a data bus builder from a list of values
+    pub(crate) fn initialize_data_bus(
+        &mut self,
+        values: &[ValueId],
+        mut databus: DataBusBuilder,
+    ) -> DataBusBuilder {
+        for value in values {
+            self.add_to_data_bus(*value, &mut databus);
+        }
+        let len = databus.values.len();
+
+        let array = if len > 0 {
+            let array =
+                self.array_constant(databus.values, Type::Array(Rc::new(vec![Type::field()]), len));
+            Some(array)
+        } else {
+            None
+        };
+
+        DataBusBuilder { index: 0, map: databus.map, databus: array, values: im::Vector::new() }
+    }
+
+    /// Generate the data bus for call-data, based on the parameters of the entry block
+    /// and a boolean vector telling which ones are call-data
+    pub(crate) fn call_data_bus(&mut self, is_params_databus: Vec<bool>) -> DataBusBuilder {
+        //filter parameters of the first block that have call-data visilibity
+        let first_block = self.current_function.entry_block();
+        let params = self.current_function.dfg[first_block].parameters();
+        let mut databus_param = Vec::new();
+        for (param, is_databus) in params.iter().zip(is_params_databus) {
+            if is_databus {
+                databus_param.push(param.to_owned());
+            }
+        }
+        // create the call-data-bus from the filtered list
+        let call_data = DataBusBuilder::new();
+        self.initialize_data_bus(&databus_param, call_data)
+    }
+}

compiler/noirc_evaluator/src/ssa/function_builder/mod.rs

@@ -1,3 +1,5 @@
+pub(crate) mod data_bus;
+
 use std::{borrow::Cow, rc::Rc};
 
 use acvm::FieldElement;

compiler/noirc_evaluator/src/ssa/ir/dfg.rs

@@ -1,6 +1,6 @@
 use std::borrow::Cow;
 
-use crate::ssa::ir::instruction::SimplifyResult;
+use crate::ssa::{function_builder::data_bus::DataBus, ir::instruction::SimplifyResult};
 
 use super::{
     basic_block::{BasicBlock, BasicBlockId},
@@ -80,6 +80,8 @@ pub(crate) struct DataFlowGraph {
     /// Instructions inserted by internal SSA passes that don't correspond to user code
     /// may not have a corresponding location.
     locations: HashMap<InstructionId, CallStack>,
+
+    pub(crate) data_bus: DataBus,
 }
 
 pub(crate) type CallStack = im::Vector<Location>;

compiler/noirc_evaluator/src/ssa/opt/die.rs

@@ -33,6 +33,10 @@ impl Ssa {
 /// of its instructions are needed elsewhere.
 fn dead_instruction_elimination(function: &mut Function) {
     let mut context = Context::default();
+    if let Some(call_data) = function.dfg.data_bus.call_data {
+        context.mark_used_instruction_results(&function.dfg, call_data);
+    }
+
     let blocks = PostOrder::with_function(function);
 
     for block in blocks.as_slice() {

compiler/noirc_evaluator/src/ssa/opt/fill_internal_slices.rs

@@ -67,8 +67,11 @@ impl Ssa {
             // The pass is also currently only setup to handle a function with a single flattened block.
             // For complex Brillig functions we can expect this pass to panic.
             if function.runtime() == RuntimeType::Acir {
+                let databus = function.dfg.data_bus.clone();
                 let mut context = Context::new(function);
                 context.process_blocks();
+                // update the databus with the new array instructions
+                function.dfg.data_bus = databus.map_values(|t| context.inserter.resolve(t));
             }
         }
         self

compiler/noirc_evaluator/src/ssa/opt/inlining.rs

@@ -139,11 +139,15 @@ impl InlineContext {
 
         context.blocks.insert(context.source_function.entry_block(), entry_block);
         context.inline_blocks(ssa);
+        // translate databus values
+        let databus = entry_point.dfg.data_bus.map_values(|t| context.translate_value(t));
 
         // Finally, we should have 1 function left representing the inlined version of the target function.
         let mut new_ssa = self.builder.finish();
         assert_eq!(new_ssa.functions.len(), 1);
-        new_ssa.functions.pop_first().unwrap().1
+        let mut new_func = new_ssa.functions.pop_first().unwrap().1;
+        new_func.dfg.data_bus = databus;
+        new_func
     }
 
     /// Inlines a function into the current function and returns the translated return values

compiler/noirc_evaluator/src/ssa/opt/mem2reg.rs

@@ -91,6 +91,7 @@ impl Ssa {
             let mut context = PerFunctionContext::new(function);
             context.mem2reg();
             context.remove_instructions();
+            context.update_data_bus();
         }
         self
     }
@@ -362,6 +363,11 @@ impl<'f> PerFunctionContext<'f> {
         }
     }
 
+    fn update_data_bus(&mut self) {
+        let databus = self.inserter.function.dfg.data_bus.clone();
+        self.inserter.function.dfg.data_bus = databus.map_values(|t| self.inserter.resolve(t));
+    }
+
     fn handle_terminator(&mut self, block: BasicBlockId, references: &mut Block) {
         self.inserter.map_terminator_in_place(block);