feat: add as_slice builtin function, add execution test (#4523)

# Description

## Problem\*

Expected to resolve slowdown in
#4504

## Summary\*

Adds builtin function for `as_slice`, allowing it to be much more
efficient (`O(1)` instead of `O(n)`)

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] 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: vezenovm <mvezenov@gmail.com>

compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_block.rs

@@ -453,6 +453,29 @@ impl<'block> BrilligBlock<'block> {
                         self.convert_ssa_array_len(arguments[0], result_variable.address, dfg);
                     }
                 }
+                Value::Intrinsic(Intrinsic::AsSlice) => {
+                    let source_variable = self.convert_ssa_value(arguments[0], dfg);
+                    let result_ids = dfg.instruction_results(instruction_id);
+                    let destination_len_variable = self.variables.define_single_addr_variable(
+                        self.function_context,
+                        self.brillig_context,
+                        result_ids[0],
+                        dfg,
+                    );
+                    let destination_variable = self.variables.define_variable(
+                        self.function_context,
+                        self.brillig_context,
+                        result_ids[1],
+                        dfg,
+                    );
+                    let source_size_as_register =
+                        self.convert_ssa_array_set(source_variable, destination_variable, None);
+
+                    // we need to explicitly set the destination_len_variable
+                    self.brillig_context
+                        .mov_instruction(destination_len_variable.address, source_size_as_register);
+                    self.brillig_context.deallocate_register(source_size_as_register);
+                }
                 Value::Intrinsic(
                     Intrinsic::SlicePushBack
                     | Intrinsic::SlicePopBack
@@ -612,13 +635,12 @@ impl<'block> BrilligBlock<'block> {
                     dfg,
                 );
                 self.validate_array_index(source_variable, index_register);
-
-                self.convert_ssa_array_set(
+                let source_size_as_register = self.convert_ssa_array_set(
                     source_variable,
                     destination_variable,
-                    index_register.address,
-                    value_variable,
+                    Some((index_register.address, value_variable)),
                 );
+                self.brillig_context.deallocate_register(source_size_as_register);
             }
             Instruction::RangeCheck { value, max_bit_size, assert_message } => {
                 let value = self.convert_ssa_single_addr_value(*value, dfg);
@@ -806,23 +828,25 @@ impl<'block> BrilligBlock<'block> {
 
     /// Array set operation in SSA returns a new array or slice that is a copy of the parameter array or slice
     /// With a specific value changed.
+    ///
+    /// Returns `source_size_as_register`, which is expected to be deallocated with:
+    /// `self.brillig_context.deallocate_register(source_size_as_register)`
     fn convert_ssa_array_set(
         &mut self,
         source_variable: BrilligVariable,
         destination_variable: BrilligVariable,
-        index_register: MemoryAddress,
-        value_variable: BrilligVariable,
-    ) {
+        opt_index_and_value: Option<(MemoryAddress, BrilligVariable)>,
+    ) -> MemoryAddress {
         let destination_pointer = match destination_variable {
             BrilligVariable::BrilligArray(BrilligArray { pointer, .. }) => pointer,
             BrilligVariable::BrilligVector(BrilligVector { pointer, .. }) => pointer,
-            _ => unreachable!("ICE: array set returns non-array"),
+            _ => unreachable!("ICE: array_set SSA returns non-array"),
         };
 
         let reference_count = match source_variable {
             BrilligVariable::BrilligArray(BrilligArray { rc, .. })
             | BrilligVariable::BrilligVector(BrilligVector { rc, .. }) => rc,
-            _ => unreachable!("ICE: array set on non-array"),
+            _ => unreachable!("ICE: array_set SSA on non-array"),
         };
 
         let (source_pointer, source_size_as_register) = match source_variable {
@@ -836,7 +860,7 @@ impl<'block> BrilligBlock<'block> {
                 self.brillig_context.mov_instruction(source_size_register, size);
                 (pointer, source_size_register)
             }
-            _ => unreachable!("ICE: array set on non-array"),
+            _ => unreachable!("ICE: array_set SSA on non-array"),
         };
 
         // Here we want to compare the reference count against 1.
@@ -879,18 +903,20 @@ impl<'block> BrilligBlock<'block> {
                 self.brillig_context.mov_instruction(target_size, source_size_as_register);
                 self.brillig_context.usize_const(target_rc, 1_usize.into());
             }
-            _ => unreachable!("ICE: array set on non-array"),
+            _ => unreachable!("ICE: array_set SSA on non-array"),
         }
 
-        // Then set the value in the newly created array
-        self.store_variable_in_array(
-            destination_pointer,
-            SingleAddrVariable::new_usize(index_register),
-            value_variable,
-        );
+        if let Some((index_register, value_variable)) = opt_index_and_value {
+            // Then set the value in the newly created array
+            self.store_variable_in_array(
+                destination_pointer,
+                SingleAddrVariable::new_usize(index_register),
+                value_variable,
+            );
+        }
 
-        self.brillig_context.deallocate_register(source_size_as_register);
         self.brillig_context.deallocate_register(condition);
+        source_size_as_register
     }
 
     pub(crate) fn store_variable_in_array_with_ctx(
@@ -1319,6 +1345,7 @@ impl<'block> BrilligBlock<'block> {
             Value::Param { .. } | Value::Instruction { .. } => {
                 // All block parameters and instruction results should have already been
                 // converted to registers so we fetch from the cache.
+
                 self.variables.get_allocation(self.function_context, value_id, dfg)
             }
             Value::NumericConstant { constant, .. } => {

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

@@ -1025,7 +1025,13 @@ impl Context {
         self.array_set_value(&store_value, result_block_id, &mut var_index)?;
 
         let element_type_sizes = if !can_omit_element_sizes_array(&array_typ) {
-            Some(self.init_element_type_sizes_array(&array_typ, array_id, None, dfg)?)
+            let acir_value = self.convert_value(array_id, dfg);
+            Some(self.init_element_type_sizes_array(
+                &array_typ,
+                array_id,
+                Some(&acir_value),
+                dfg,
+            )?)
         } else {
             None
         };
@@ -1246,7 +1252,8 @@ impl Context {
             let read = self.acir_context.read_from_memory(source, &index_var)?;
             Ok::<AcirValue, RuntimeError>(AcirValue::Var(read, AcirType::field()))
         })?;
-        self.initialize_array(destination, array_len, Some(AcirValue::Array(init_values.into())))?;
+        let array: im::Vector<AcirValue> = init_values.into();
+        self.initialize_array(destination, array_len, Some(AcirValue::Array(array)))?;
         Ok(())
     }
 
@@ -1663,6 +1670,49 @@ impl Context {
                 };
                 Ok(vec![AcirValue::Var(self.acir_context.add_constant(len), AcirType::field())])
             }
+            Intrinsic::AsSlice => {
+                let (slice_contents, slice_typ, block_id) =
+                    self.check_array_is_initialized(arguments[0], dfg)?;
+                assert!(!slice_typ.is_nested_slice(), "ICE: Nested slice used in ACIR generation");
+
+                let result_block_id = self.block_id(&result_ids[1]);
+                let acir_value = self.convert_value(slice_contents, dfg);
+
+                let array_len = if !slice_typ.contains_slice_element() {
+                    slice_typ.flattened_size()
+                } else {
+                    self.flattened_slice_size(slice_contents, dfg)
+                };
+                let slice_length = self.acir_context.add_constant(array_len);
+                self.copy_dynamic_array(block_id, result_block_id, array_len)?;
+
+                let element_type_sizes = if !can_omit_element_sizes_array(&slice_typ) {
+                    Some(self.init_element_type_sizes_array(
+                        &slice_typ,
+                        slice_contents,
+                        Some(&acir_value),
+                        dfg,
+                    )?)
+                } else {
+                    None
+                };
+
+                let value_types = self.convert_value(slice_contents, dfg).flat_numeric_types();
+                assert!(
+                    array_len == value_types.len(),
+                    "AsSlice: unexpected length difference: {:?} != {:?}",
+                    array_len,
+                    value_types.len()
+                );
+
+                let result = AcirValue::DynamicArray(AcirDynamicArray {
+                    block_id: result_block_id,
+                    len: value_types.len(),
+                    value_types,
+                    element_type_sizes,
+                });
+                Ok(vec![AcirValue::Var(slice_length, AcirType::field()), result])
+            }
             Intrinsic::SlicePushBack => {
                 // arguments = [slice_length, slice_contents, ...elements_to_push]
                 let slice_length = self.convert_value(arguments[0], dfg).into_var()?;

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

@@ -37,6 +37,7 @@ pub(crate) type InstructionId = Id<Instruction>;
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub(crate) enum Intrinsic {
     ArrayLen,
+    AsSlice,
     AssertConstant,
     SlicePushBack,
     SlicePushFront,
@@ -57,6 +58,7 @@ impl std::fmt::Display for Intrinsic {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
             Intrinsic::ArrayLen => write!(f, "array_len"),
+            Intrinsic::AsSlice => write!(f, "as_slice"),
             Intrinsic::AssertConstant => write!(f, "assert_constant"),
             Intrinsic::SlicePushBack => write!(f, "slice_push_back"),
             Intrinsic::SlicePushFront => write!(f, "slice_push_front"),
@@ -89,6 +91,7 @@ impl Intrinsic {
             Intrinsic::ToBits(_) | Intrinsic::ToRadix(_) => true,
 
             Intrinsic::ArrayLen
+            | Intrinsic::AsSlice
             | Intrinsic::SlicePushBack
             | Intrinsic::SlicePushFront
             | Intrinsic::SlicePopBack
@@ -109,6 +112,7 @@ impl Intrinsic {
     pub(crate) fn lookup(name: &str) -> Option<Intrinsic> {
         match name {
             "array_len" => Some(Intrinsic::ArrayLen),
+            "as_slice" => Some(Intrinsic::AsSlice),
             "assert_constant" => Some(Intrinsic::AssertConstant),
             "apply_range_constraint" => Some(Intrinsic::ApplyRangeConstraint),
             "slice_push_back" => Some(Intrinsic::SlicePushBack),

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

@@ -84,6 +84,16 @@ pub(super) fn simplify_call(
                 SimplifyResult::None
             }
         }
+        Intrinsic::AsSlice => {
+            let slice = dfg.get_array_constant(arguments[0]);
+            if let Some((slice, element_type)) = slice {
+                let slice_length = dfg.make_constant(slice.len().into(), Type::length_type());
+                let new_slice = dfg.make_array(slice, element_type);
+                SimplifyResult::SimplifiedToMultiple(vec![slice_length, new_slice])
+            } else {
+                SimplifyResult::None
+            }
+        }
         Intrinsic::SlicePushBack => {
             let slice = dfg.get_array_constant(arguments[1]);
             if let Some((mut slice, element_type)) = slice {

noir_stdlib/src/array.nr

@@ -52,14 +52,8 @@ impl<T, N> [T; N] {
         result
     }
 
-    // Converts an array into a slice.
-    pub fn as_slice(self) -> [T] {
-        let mut slice = [];
-        for elem in self {
-            slice = slice.push_back(elem);
-        }
-        slice
-    }
+    #[builtin(as_slice)]
+    pub fn as_slice(self) -> [T] {}
 
     // Apply a function to each element of an array, returning a new array
     // containing the mapped elements.

test_programs/execution_success/array_to_slice/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "array_to_slice"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.24.0"
+
+[dependencies]

test_programs/execution_success/array_to_slice/Prover.toml

@@ -0,0 +1,2 @@
+x = "0"
+y = "1"

test_programs/execution_success/array_to_slice/src/main.nr

@@ -0,0 +1,33 @@
+// Converts an array into a slice.
+fn as_slice_push<T, N>(xs: [T; N]) -> [T] {
+    let mut slice = [];
+    for elem in xs {
+        slice = slice.push_back(elem);
+    }
+    slice
+}
+
+fn main(x: Field, y: pub Field) {
+    let xs: [Field; 0] = [];
+    let ys: [Field; 1] = [1];
+    let zs: [Field; 2] = [1, 2];
+    let ws: [Field; 3] = [1; 3];
+    let qs: [Field; 4] = [3, 2, 1, 0];
+
+    let mut dynamic: [Field; 4] = [3, 2, 1, 0];
+    let dynamic_expected: [Field; 4] = [1000, 2, 1, 0];
+    dynamic[x] = 1000;
+
+    assert(x != y);
+    assert(xs.as_slice() == as_slice_push(xs));
+    assert(ys.as_slice() == as_slice_push(ys));
+    assert(zs.as_slice() == as_slice_push(zs));
+    assert(ws.as_slice() == as_slice_push(ws));
+    assert(qs.as_slice() == as_slice_push(qs));
+
+    assert(dynamic.as_slice()[0] == dynamic_expected[0]);
+    assert(dynamic.as_slice()[1] == dynamic_expected[1]);
+    assert(dynamic.as_slice()[2] == dynamic_expected[2]);
+    assert(dynamic.as_slice()[3] == dynamic_expected[3]);
+    assert(dynamic.as_slice().len() == 4);
+}

test_programs/execution_success/brillig_array_to_slice/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "brillig_array_to_slice"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.25.0"
+
+[dependencies]

test_programs/execution_success/brillig_array_to_slice/Prover.toml

@@ -0,0 +1 @@
+x = "0"

test_programs/execution_success/brillig_array_to_slice/src/main.nr

@@ -0,0 +1,18 @@
+unconstrained fn brillig_as_slice(x: Field) -> (u64, Field, Field) {
+    let mut dynamic: [Field; 1] = [1];
+    dynamic[x] = 2;
+    assert(dynamic[0] == 2);
+
+    let brillig_slice = dynamic.as_slice();
+    assert(brillig_slice.len() == 1);
+
+    (brillig_slice.len(), dynamic[0], brillig_slice[0])
+}
+
+fn main(x: Field) {
+    let (slice_len, dynamic_0, slice_0) = brillig_as_slice(x);
+    assert(slice_len == 1);
+    assert(dynamic_0 == 2);
+    assert(slice_0 == 2);
+}
+