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fix(ssa): Do not replace previously constrained values (#2647)
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Co-authored-by: Tom French <15848336+TomAFrench@users.noreply.github.com>
Co-authored-by: kevaundray <kevtheappdev@gmail.com>
Co-authored-by: jfecher <jake@aztecprotocol.com>
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@vezenovm @TomAFrench
@kevaundray @jfecher
4 people authored Sep 21, 2023
1 parent 68af50b commit d528844
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Showing 2 changed files with 8 additions and 117 deletions.
119 changes: 5 additions & 114 deletions compiler/noirc_evaluator/src/ssa/opt/constant_folding.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,7 @@ use crate::ssa::{
dfg::{DataFlowGraph, InsertInstructionResult},
function::Function,
instruction::{Instruction, InstructionId},
value::{Value, ValueId},
value::ValueId,
},
ssa_gen::Ssa,
};
Expand Down Expand Up @@ -77,15 +77,13 @@ impl Context {

// Cache of instructions without any side-effects along with their outputs.
let mut cached_instruction_results: HashMap<Instruction, Vec<ValueId>> = HashMap::default();
let mut constrained_values: HashMap<ValueId, ValueId> = HashMap::default();

for instruction_id in instructions {
Self::fold_constants_into_instruction(
&mut function.dfg,
block,
instruction_id,
&mut cached_instruction_results,
&mut constrained_values,
);
}
self.block_queue.extend(function.dfg[block].successors());
Expand All @@ -96,9 +94,8 @@ impl Context {
block: BasicBlockId,
id: InstructionId,
instruction_result_cache: &mut HashMap<Instruction, Vec<ValueId>>,
constrained_values: &mut HashMap<ValueId, ValueId>,
) {
let instruction = Self::resolve_instruction(id, dfg, constrained_values);
let instruction = Self::resolve_instruction(id, dfg);
let old_results = dfg.instruction_results(id).to_vec();

// If a copy of this instruction exists earlier in the block, then reuse the previous results.
Expand All @@ -112,42 +109,15 @@ impl Context {

Self::replace_result_ids(dfg, &old_results, &new_results);

Self::cache_instruction(
instruction,
new_results,
dfg,
instruction_result_cache,
constrained_values,
);
Self::cache_instruction(instruction, new_results, dfg, instruction_result_cache);
}

/// Fetches an [`Instruction`] by its [`InstructionId`] and fully resolves its inputs.
fn resolve_instruction(
instruction_id: InstructionId,
dfg: &DataFlowGraph,
constrained_values: &HashMap<ValueId, ValueId>,
) -> Instruction {
fn resolve_instruction(instruction_id: InstructionId, dfg: &DataFlowGraph) -> Instruction {
let instruction = dfg[instruction_id].clone();

// Alternate between resolving `value_id` in the `dfg` and checking to see if the resolved value
// has been constrained to be equal to some simpler value in the current block.
//
// This allows us to reach a stable final `ValueId` for each instruction input as we add more
// constraints to the cache.
fn resolve_cache(
dfg: &DataFlowGraph,
cache: &HashMap<ValueId, ValueId>,
value_id: ValueId,
) -> ValueId {
let resolved_id = dfg.resolve(value_id);
match cache.get(&resolved_id) {
Some(cached_value) => resolve_cache(dfg, cache, *cached_value),
None => resolved_id,
}
}

// Resolve any inputs to ensure that we're comparing like-for-like instructions.
instruction.map_values(|value_id| resolve_cache(dfg, constrained_values, value_id))
instruction.map_values(|value_id| dfg.resolve(value_id))
}

/// Pushes a new [`Instruction`] into the [`DataFlowGraph`] which applies any optimizations
Expand Down Expand Up @@ -185,35 +155,7 @@ impl Context {
instruction_results: Vec<ValueId>,
dfg: &DataFlowGraph,
instruction_result_cache: &mut HashMap<Instruction, Vec<ValueId>>,
constraint_cache: &mut HashMap<ValueId, ValueId>,
) {
// If the instruction was a constraint, then create a link between the two `ValueId`s
// to map from the more complex to the simpler value.
if let Instruction::Constrain(lhs, rhs, _) = instruction {
// These `ValueId`s should be fully resolved now.
match (&dfg[lhs], &dfg[rhs]) {
// Ignore trivial constraints
(Value::NumericConstant { .. }, Value::NumericConstant { .. }) => (),

// Prefer replacing with constants where possible.
(Value::NumericConstant { .. }, _) => {
constraint_cache.insert(rhs, lhs);
}
(_, Value::NumericConstant { .. }) => {
constraint_cache.insert(lhs, rhs);
}
// Otherwise prefer block parameters over instruction results.
// This is as block parameters are more likely to be a single witness rather than a full expression.
(Value::Param { .. }, Value::Instruction { .. }) => {
constraint_cache.insert(rhs, lhs);
}
(Value::Instruction { .. }, Value::Param { .. }) => {
constraint_cache.insert(lhs, rhs);
}
(_, _) => (),
}
}

// If the instruction doesn't have side-effects, cache the results so we can reuse them if
// the same instruction appears again later in the block.
if instruction.is_pure(dfg) {
Expand Down Expand Up @@ -391,55 +333,4 @@ mod test {

assert_eq!(instruction, &Instruction::Cast(ValueId::test_new(0), Type::unsigned(32)));
}

#[test]
fn constrained_value_replacement() {
// fn main f0 {
// b0(v0: Field):
// constrain v0 == Field 10
// v1 = add v0, Field 1
// constrain v1 == Field 11
// }
//
// After constructing this IR, we run constant folding which should replace references to `v0`
// with the constant `10`. This then allows us to optimize away the rest of the circuit.

let main_id = Id::test_new(0);

// Compiling main
let mut builder = FunctionBuilder::new("main".into(), main_id, RuntimeType::Acir);
let v0 = builder.add_parameter(Type::field());

let field_10 = builder.field_constant(10u128);
builder.insert_constrain(v0, field_10, None);

let field_1 = builder.field_constant(1u128);
let v1 = builder.insert_binary(v0, BinaryOp::Add, field_1);

let field_11 = builder.field_constant(11u128);
builder.insert_constrain(v1, field_11, None);

let mut ssa = builder.finish();
let main = ssa.main_mut();
let instructions = main.dfg[main.entry_block()].instructions();
assert_eq!(instructions.len(), 3);

// Expected output:
//
// fn main f0 {
// b0(v0: Field):
// constrain v0 == Field 10
// }
let ssa = ssa.fold_constants();
let main = ssa.main();
let instructions = main.dfg[main.entry_block()].instructions();

assert_eq!(instructions.len(), 1);
let instruction = &main.dfg[instructions[0]];

assert_eq!(
instruction,
&Instruction::Constrain(ValueId::test_new(0), ValueId::test_new(1), None)
);
}
}
6 changes: 3 additions & 3 deletions tooling/nargo_cli/tests/execution_success/slices/src/main.nr
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Original file line number Diff line number Diff line change
Expand Up @@ -92,16 +92,16 @@ fn regression_merge_slices(x: Field, y: Field) {

fn merge_slices_if(x: Field, y: Field) {
let slice = merge_slices_return(x, y);
assert(slice[2] == 10);
assert(slice.len() == 3);
assert(slice[2] == 10);

let slice = merge_slices_mutate(x, y);
assert(slice[3] == 5);
assert(slice.len() == 4);
assert(slice[3] == 5);

let slice = merge_slices_mutate_in_loop(x, y);
assert(slice[6] == 4);
assert(slice.len() == 7);
assert(slice[6] == 4);

let slice = merge_slices_mutate_two_ifs(x, y);
assert(slice.len() == 6);
Expand Down

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