feat: Add assert_constant (#2242)

* Add assert_constant

* Remove println & dbg

* Add test

crates/nargo_cli/tests/compile_failure/assert_constant_fail/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "assert_constant_fail"
+type = "bin"
+authors = [""]
+compiler_version = "0.9.0"
+
+[dependencies]

crates/nargo_cli/tests/compile_failure/assert_constant_fail/src/main.nr

@@ -0,0 +1,10 @@
+use dep::std::assert_constant;
+
+fn main(x: Field) {
+    foo(5, x);
+}
+
+fn foo(constant: Field, non_constant: Field) {
+    assert_constant(constant);
+    assert_constant(non_constant);
+}

crates/noirc_evaluator/src/errors.rs

@@ -32,6 +32,8 @@ pub enum RuntimeError {
     UnsupportedIntegerSize { num_bits: u32, max_num_bits: u32, location: Option<Location> },
     #[error("Could not determine loop bound at compile-time")]
     UnknownLoopBound { location: Option<Location> },
+    #[error("Argument is not constant")]
+    AssertConstantFailed { location: Option<Location> },
 }
 
 #[derive(Debug, PartialEq, Eq, Clone, Error)]
@@ -69,7 +71,8 @@ impl RuntimeError {
             | RuntimeError::InvalidRangeConstraint { location, .. }
             | RuntimeError::TypeConversion { location, .. }
             | RuntimeError::UnInitialized { location, .. }
-            | RuntimeError::UnknownLoopBound { location, .. }
+            | RuntimeError::UnknownLoopBound { location }
+            | RuntimeError::AssertConstantFailed { location }
             | RuntimeError::UnsupportedIntegerSize { location, .. } => *location,
         }
     }

crates/noirc_evaluator/src/ssa.rs

@@ -49,6 +49,7 @@ pub(crate) fn optimize_into_acir(
         ssa = ssa
             .inline_functions()
             .print(print_ssa_passes, "After Inlining:")
+            .evaluate_assert_constant()?
             .unroll_loops()?
             .print(print_ssa_passes, "After Unrolling:")
             .simplify_cfg()

crates/noirc_evaluator/src/ssa/ir/basic_block.rs

@@ -73,6 +73,11 @@ impl BasicBlock {
         &mut self.instructions
     }
 
+    /// Take the instructions in this block, replacing it with an empty Vec
+    pub(crate) fn take_instructions(&mut self) -> Vec<InstructionId> {
+        std::mem::take(&mut self.instructions)
+    }
+
     /// Sets the terminator instruction of this block.
     ///
     /// A properly-constructed block will always terminate with a TerminatorInstruction -

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

@@ -399,7 +399,7 @@ impl DataFlowGraph {
     /// source block.
     pub(crate) fn inline_block(&mut self, source: BasicBlockId, destination: BasicBlockId) {
         let source = &mut self.blocks[source];
-        let mut instructions = std::mem::take(source.instructions_mut());
+        let mut instructions = source.take_instructions();
         let terminator = source.take_terminator();
 
         let destination = &mut self.blocks[destination];
@@ -417,6 +417,11 @@ impl DataFlowGraph {
             _ => None,
         }
     }
+
+    /// True if the given ValueId refers to a constant value
+    pub(crate) fn is_constant(&self, argument: ValueId) -> bool {
+        !matches!(&self[self.resolve(argument)], Value::Instruction { .. } | Value::Param { .. })
+    }
 }
 
 impl std::ops::Index<InstructionId> for DataFlowGraph {
@@ -483,9 +488,7 @@ impl<'dfg> InsertInstructionResult<'dfg> {
             InsertInstructionResult::Results(results) => Cow::Borrowed(results),
             InsertInstructionResult::SimplifiedTo(result) => Cow::Owned(vec![result]),
             InsertInstructionResult::SimplifiedToMultiple(results) => Cow::Owned(results),
-            InsertInstructionResult::InstructionRemoved => {
-                panic!("InsertInstructionResult::results called on a removed instruction")
-            }
+            InsertInstructionResult::InstructionRemoved => Cow::Owned(vec![]),
         }
     }
 

crates/noirc_evaluator/src/ssa/ir/instruction.rs

@@ -34,6 +34,7 @@ pub(crate) type InstructionId = Id<Instruction>;
 pub(crate) enum Intrinsic {
     Sort,
     ArrayLen,
+    AssertConstant,
     SlicePushBack,
     SlicePushFront,
     SlicePopBack,
@@ -52,6 +53,7 @@ impl std::fmt::Display for Intrinsic {
             Intrinsic::Println => write!(f, "println"),
             Intrinsic::Sort => write!(f, "arraysort"),
             Intrinsic::ArrayLen => write!(f, "array_len"),
+            Intrinsic::AssertConstant => write!(f, "assert_constant"),
             Intrinsic::SlicePushBack => write!(f, "slice_push_back"),
             Intrinsic::SlicePushFront => write!(f, "slice_push_front"),
             Intrinsic::SlicePopBack => write!(f, "slice_pop_back"),
@@ -75,6 +77,7 @@ impl Intrinsic {
             "println" => Some(Intrinsic::Println),
             "arraysort" => Some(Intrinsic::Sort),
             "array_len" => Some(Intrinsic::ArrayLen),
+            "assert_constant" => Some(Intrinsic::AssertConstant),
             "slice_push_back" => Some(Intrinsic::SlicePushBack),
             "slice_push_front" => Some(Intrinsic::SlicePushFront),
             "slice_pop_back" => Some(Intrinsic::SlicePopBack),

crates/noirc_evaluator/src/ssa/ir/instruction/call.rs

@@ -165,6 +165,13 @@ pub(super) fn simplify_call(
                 SimplifyResult::None
             }
         }
+        Intrinsic::AssertConstant => {
+            if arguments.iter().all(|argument| dfg.is_constant(*argument)) {
+                SimplifyResult::Remove
+            } else {
+                SimplifyResult::None
+            }
+        }
         Intrinsic::BlackBox(bb_func) => simplify_black_box_func(bb_func, arguments, dfg),
         Intrinsic::Sort => simplify_sort(dfg, arguments),
         Intrinsic::Println => SimplifyResult::None,

crates/noirc_evaluator/src/ssa/opt/assert_constant.rs

@@ -0,0 +1,83 @@
+use crate::{
+    errors::RuntimeError,
+    ssa::{
+        ir::{
+            function::Function,
+            instruction::{Instruction, InstructionId, Intrinsic},
+            value::ValueId,
+        },
+        ssa_gen::Ssa,
+    },
+};
+
+impl Ssa {
+    /// A simple SSA pass to go through each instruction and evaluate each call
+    /// to `assert_constant`, issuing an error if any arguments to the function are
+    /// not constants.
+    ///
+    /// Note that this pass must be placed directly before loop unrolling to be
+    /// useful. Any optimization passes between this and loop unrolling will cause
+    /// the constants that this pass sees to be potentially different than the constants
+    /// seen by loop unrolling. Furthermore, this pass cannot be a part of loop unrolling
+    /// since we must go through every instruction to find all references to `assert_constant`
+    /// while loop unrolling only touches blocks with loops in them.
+    pub(crate) fn evaluate_assert_constant(mut self) -> Result<Ssa, RuntimeError> {
+        for function in self.functions.values_mut() {
+            for block in function.reachable_blocks() {
+                // Unfortunately we can't just use instructions.retain(...) here since
+                // check_instruction can also return an error
+                let instructions = function.dfg[block].take_instructions();
+                let mut filtered_instructions = Vec::with_capacity(instructions.len());
+
+                for instruction in instructions {
+                    if check_instruction(function, instruction)? {
+                        filtered_instructions.push(instruction);
+                    }
+                }
+
+                *function.dfg[block].instructions_mut() = filtered_instructions;
+            }
+        }
+        Ok(self)
+    }
+}
+
+/// During the loop unrolling pass we also evaluate calls to `assert_constant`.
+/// This is done in this pass because loop unrolling is the only pass that will error
+/// if a value (the loop bounds) are not known constants.
+///
+/// This returns Ok(true) if the given instruction should be kept in the block and
+/// Ok(false) if it should be removed.
+fn check_instruction(
+    function: &mut Function,
+    instruction: InstructionId,
+) -> Result<bool, RuntimeError> {
+    let assert_constant_id = function.dfg.import_intrinsic(Intrinsic::AssertConstant);
+    match &function.dfg[instruction] {
+        Instruction::Call { func, arguments } => {
+            if *func == assert_constant_id {
+                evaluate_assert_constant(function, instruction, arguments)
+            } else {
+                Ok(true)
+            }
+        }
+        _ => Ok(true),
+    }
+}
+
+/// Evaluate a call to `assert_constant`, returning an error if any of the elements are not
+/// constants. If all of the elements are constants, Ok(false) is returned. This signifies a
+/// success but also that the instruction need not be reinserted into the block being unrolled
+/// since it has already been evaluated.
+fn evaluate_assert_constant(
+    function: &Function,
+    instruction: InstructionId,
+    arguments: &[ValueId],
+) -> Result<bool, RuntimeError> {
+    if arguments.iter().all(|arg| function.dfg.is_constant(*arg)) {
+        Ok(false)
+    } else {
+        let location = function.dfg.get_location(&instruction);
+        Err(RuntimeError::AssertConstantFailed { location })
+    }
+}

crates/noirc_evaluator/src/ssa/opt/constant_folding.rs

@@ -50,7 +50,7 @@ struct Context {
 
 impl Context {
     fn fold_constants_in_block(&mut self, function: &mut Function, block: BasicBlockId) {
-        let instructions = std::mem::take(function.dfg[block].instructions_mut());
+        let instructions = function.dfg[block].take_instructions();
 
         for instruction in instructions {
             self.push_instruction(function, block, instruction);

crates/noirc_evaluator/src/ssa/opt/mod.rs

@@ -4,6 +4,7 @@
 //! simpler form until the IR only has a single function remaining with 1 block within it.
 //! Generally, these passes are also expected to minimize the final amount of instructions.
 mod array_use;
+mod assert_constant;
 mod constant_folding;
 mod defunctionalize;
 mod die;

noir_stdlib/src/lib.nr

@@ -27,3 +27,8 @@ unconstrained fn println<T>(input: T) {
 
 #[foreign(recursive_aggregation)]
 fn verify_proof<N>(_verification_key : [Field], _proof : [Field], _public_inputs : [Field], _key_hash : Field, _input_aggregation_object : [Field; N]) -> [Field; N] {}
+
+// Asserts that the given value is known at compile-time.
+// Useful for debugging for-loop bounds.
+#[builtin(assert_constant)]
+fn assert_constant<T>(_x: T) {}