feat(SSA): always simplify AsSlice intrinsic in ACIR functions

compiler/noirc_evaluator/src/acir/call/intrinsics/mod.rs

@@ -1,9 +1,6 @@
 use iter_extended::vecmap;
 
-use crate::acir::{
-    arrays,
-    types::{AcirType, AcirValue},
-};
+use crate::acir::{arrays, types::AcirValue};
 use crate::errors::RuntimeError;
 use crate::ssa::ir::{
     dfg::DataFlowGraph,
@@ -92,21 +89,6 @@ impl Context<'_> {
                     .bit_decompose(endian, field, array_length, result_type[0].clone().into())
                     .map(|array| vec![array])
             }
-            Intrinsic::AsSlice => {
-                let array_contents = arguments[0];
-                let array_type = dfg.type_of_value(array_contents);
-                assert!(!array_type.is_nested_slice(), "ICE: Nested slice used in ACIR generation");
-                let Type::Array(_, slice_length) = array_type else {
-                    unreachable!("Expected Array input for `as_slice` intrinsic");
-                };
-                let slice_length = self.acir_context.add_constant(slice_length);
-                let acir_value = self.convert_value(array_contents, dfg);
-                let result = self.read_array(acir_value)?;
-                Ok(vec![
-                    AcirValue::Var(slice_length, AcirType::unsigned(32)),
-                    AcirValue::Array(result),
-                ])
-            }
 
             Intrinsic::SlicePushBack => self.convert_slice_push_back(arguments, dfg),
             Intrinsic::SlicePushFront => self.convert_slice_push_front(arguments, dfg),
@@ -141,7 +123,8 @@ impl Context<'_> {
             | Intrinsic::StaticAssert
             | Intrinsic::AssertConstant
             | Intrinsic::ArrayRefCount
-            | Intrinsic::SliceRefCount => {
+            | Intrinsic::SliceRefCount
+            | Intrinsic::AsSlice => {
                 unreachable!("Expected {intrinsic} to have been removing during SSA optimizations")
             }
         }

compiler/noirc_evaluator/src/acir/tests/arrays.rs

@@ -74,7 +74,9 @@ fn does_not_generate_memory_blocks_without_dynamic_accesses() {
     let src = "
         acir(inline) fn main f0 {
           b0(v0: [Field; 2]):
-            v2, v3 = call as_slice(v0) -> (u32, [Field])
+            v1 = array_get v0, index u32 0 -> Field
+            v2 = array_get v0, index u32 1 -> Field
+            v3 = make_array [v1, v2] : [Field]
             call f1(u32 2, v3)
             v7 = array_get v0, index u32 0 -> Field
             constrain v7 == Field 0

compiler/noirc_evaluator/src/acir/tests/intrinsics.rs

@@ -511,29 +511,3 @@ fn slice_remove_affected_by_predicate() {
     let program_no_side_effects = ssa_to_acir_program(src_no_side_effects);
     assert_ne!(program_side_effects, program_no_side_effects);
 }
-
-#[test]
-fn as_slice_for_composite_slice() {
-    let src = "
-    acir(inline) predicate_pure fn main f0 {
-      b0():
-        v3 = make_array [Field 10, Field 20, Field 30, Field 40] : [(Field, Field); 2]
-        v4, v5 = call as_slice(v3) -> (u32, [(Field, Field)])
-        return v4
-    }
-    ";
-    let program = ssa_to_acir_program(src);
-
-    // Note that 2 is returned, not 4 (as there are two `(Field, Field)` elements)
-    assert_circuit_snapshot!(program, @r"
-    func 0
-    private parameters: []
-    public parameters: []
-    return values: [w0]
-    ASSERT w1 = 10
-    ASSERT w2 = 20
-    ASSERT w3 = 30
-    ASSERT w4 = 40
-    ASSERT w0 = 2
-    ");
-}

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

@@ -106,26 +106,59 @@ pub(super) fn simplify_call(
         // Strings are already arrays of bytes in SSA
         Intrinsic::ArrayAsStrUnchecked => SimplifyResult::SimplifiedTo(arguments[0]),
         Intrinsic::AsSlice => {
+            let array_type = dfg.type_of_value(arguments[0]);
+            let Type::Array(element_types, length) = array_type else {
+                panic!("Expected as_slice input to be an array")
+            };
+
             let array = dfg.get_array_constant(arguments[0]);
-            if let Some((array, array_type)) = array {
-                // Compute the resulting slice length by dividing the flattened
-                // array length by the size of each array element
-                let elements_size = array_type.element_size();
-                let inner_element_types = array_type.element_types();
-                assert_eq!(
-                    0,
-                    array.len() % elements_size,
-                    "expected array length to be multiple of its elements size"
-                );
-                let slice_length_value = array.len() / elements_size;
-                let slice_length =
-                    dfg.make_constant(slice_length_value.into(), NumericType::length_type());
+            if let Some((array, _array_type)) = array {
+                let slice_length = dfg.make_constant(length.into(), NumericType::length_type());
                 let new_slice =
-                    make_array(dfg, array, Type::Slice(inner_element_types), block, call_stack);
-                SimplifyResult::SimplifiedToMultiple(vec![slice_length, new_slice])
-            } else {
-                SimplifyResult::None
+                    make_array(dfg, array, Type::Slice(element_types), block, call_stack);
+                return SimplifyResult::SimplifiedToMultiple(vec![slice_length, new_slice]);
+            }
+
+            // In ACIR we can simplify `as_slice(array)`, to:
+            //
+            // ```
+            // v0 = array_get array, index u32 0 -> T
+            // v1 = array_get array, index u32 1 -> T
+            // ...
+            // vN = make_array [v0, v1, ...]
+            // ```
+            //
+            // We don't do this for Brillig because it sometimes leads to more opcodes.
+            if dfg.runtime().is_acir() {
+                let mut elements = im::Vector::default();
+                let mut index: u32 = 0;
+                for _ in 0..length {
+                    for element_type in element_types.iter() {
+                        let index_value =
+                            dfg.make_constant(index.into(), NumericType::length_type());
+                        let array_get =
+                            Instruction::ArrayGet { array: arguments[0], index: index_value };
+                        let element = dfg
+                            .insert_instruction_and_results(
+                                array_get,
+                                block,
+                                Some(vec![element_type.clone()]),
+                                call_stack,
+                            )
+                            .first();
+                        elements.push_back(element);
+                        index += 1;
+                    }
+                }
+                let make_array =
+                    Instruction::MakeArray { elements, typ: Type::Slice(element_types.clone()) };
+                let slice_length = dfg.make_constant(length.into(), NumericType::length_type());
+                let new_slice =
+                    dfg.insert_instruction_and_results(make_array, block, None, call_stack).first();
+                return SimplifyResult::SimplifiedToMultiple(vec![slice_length, new_slice]);
             }
+
+            return SimplifyResult::None;
         }
         Intrinsic::SlicePushBack => {
             let slice = dfg.get_array_constant(arguments[1]);
@@ -921,4 +954,69 @@ mod tests {
         "#;
         let _ = Ssa::from_str_simplifying(src).unwrap();
     }
+
+    #[test]
+    fn simplifies_as_slice_for_known_array() {
+        let src = r#"
+        acir(inline) fn main func {
+          b0():
+            v0 = make_array [Field 1, Field 2, Field 3] : [Field; 3]
+            v1, v2 = call as_slice(v0) -> (u32, [Field])
+            return v1, v2
+        }
+        "#;
+        let ssa = Ssa::from_str_simplifying(src).unwrap();
+
+        assert_ssa_snapshot!(ssa, @r"
+        acir(inline) fn main f0 {
+          b0():
+            v3 = make_array [Field 1, Field 2, Field 3] : [Field; 3]
+            v4 = make_array [Field 1, Field 2, Field 3] : [Field]
+            return u32 3, v4
+        }
+        ");
+    }
+
+    #[test]
+    fn simplifies_as_slice_for_unknown_array_in_acir() {
+        let src = r#"
+        acir(inline) fn main func {
+          b0(v0: [Field; 3]):
+            v1, v2 = call as_slice(v0) -> (u32, [Field])
+            return v1, v2
+        }
+        "#;
+        let ssa = Ssa::from_str_simplifying(src).unwrap();
+
+        assert_ssa_snapshot!(ssa, @r"
+        acir(inline) fn main f0 {
+          b0(v0: [Field; 3]):
+            v2 = array_get v0, index u32 0 -> Field
+            v4 = array_get v0, index u32 1 -> Field
+            v6 = array_get v0, index u32 2 -> Field
+            v7 = make_array [v2, v4, v6] : [Field]
+            return u32 3, v7
+        }
+        ");
+    }
+
+    #[test]
+    fn does_not_simplify_as_slice_for_unknown_array_in_brillig() {
+        let src = r#"
+        brillig(inline) fn main func {
+          b0(v0: [Field; 3]):
+            v1, v2 = call as_slice(v0) -> (u32, [Field])
+            return v1, v2
+        }
+        "#;
+        let ssa = Ssa::from_str_simplifying(src).unwrap();
+
+        assert_ssa_snapshot!(ssa, @r"
+        brillig(inline) fn main f0 {
+          b0(v0: [Field; 3]):
+            v2, v3 = call as_slice(v0) -> (u32, [Field])
+            return v2, v3
+        }
+        ");
+    }
 }