fix: avoid creating unnecessary memory blocks (#7114)

Author: TomAFrench

compiler/noirc_evaluator/src/acir/mod.rs

@@ -1069,8 +1069,7 @@ impl<'a> Context<'a> {
         // Ensure that array id is fully resolved.
         let array = dfg.resolve(array);
 
-        let array_id = dfg.resolve(array);
-        let array_typ = dfg.type_of_value(array_id);
+        let array_typ = dfg.type_of_value(array);
         // Compiler sanity checks
         assert!(!array_typ.is_nested_slice(), "ICE: Nested slice type has reached ACIR generation");
         let (Type::Array(_, _) | Type::Slice(_)) = &array_typ else {
@@ -1125,15 +1124,7 @@ impl<'a> Context<'a> {
         index: ValueId,
         store_value: Option<ValueId>,
     ) -> Result<bool, RuntimeError> {
-        let array_id = dfg.resolve(array);
-        let array_typ = dfg.type_of_value(array_id);
-        // Compiler sanity checks
-        assert!(!array_typ.is_nested_slice(), "ICE: Nested slice type has reached ACIR generation");
-        let (Type::Array(_, _) | Type::Slice(_)) = &array_typ else {
-            unreachable!("ICE: expected array or slice type");
-        };
-
-        match self.convert_value(array_id, dfg) {
+        match self.convert_value(array, dfg) {
             AcirValue::Var(acir_var, _) => {
                 Err(RuntimeError::InternalError(InternalError::Unexpected {
                     expected: "an array value".to_string(),
@@ -2231,45 +2222,41 @@ impl<'a> Context<'a> {
             Intrinsic::AsSlice => {
                 let slice_contents = arguments[0];
                 let slice_typ = dfg.type_of_value(slice_contents);
-                let block_id = self.ensure_array_is_initialized(slice_contents, 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 (slice_length, result) = match acir_value {
+                    AcirValue::Var(_, _) => {
+                        unreachable!("ICE: cannot call `as_slice` on non-array type")
+                    }
+                    array @ AcirValue::Array(_) => {
+                        let array_len = if !slice_typ.contains_slice_element() {
+                            slice_typ.flattened_size() as usize
+                        } else {
+                            self.flattened_slice_size(slice_contents, dfg)
+                        };
+                        (array_len, array)
+                    }
+                    AcirValue::DynamicArray(source_array) => {
+                        let result_block_id = self.block_id(&result_ids[1]);
+                        self.copy_dynamic_array(
+                            source_array.block_id,
+                            result_block_id,
+                            source_array.len,
+                        )?;
 
-                let array_len = if !slice_typ.contains_slice_element() {
-                    slice_typ.flattened_size() as usize
-                } 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 array = AcirValue::DynamicArray(AcirDynamicArray {
+                            block_id: result_block_id,
+                            len: source_array.len,
+                            value_types: source_array.value_types,
+                            element_type_sizes: source_array.element_type_sizes,
+                        });
 
-                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
+                        (source_array.len, array)
+                    }
                 };
 
-                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,
-                });
+                let slice_length = self.acir_context.add_constant(slice_length);
                 Ok(vec![AcirValue::Var(slice_length, AcirType::field()), result])
             }
             Intrinsic::SlicePushBack => {
@@ -3709,4 +3696,35 @@ mod test {
             }
         }
     }
+
+    #[test]
+    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])
+            call f1(u32 2, v3)
+            v7 = array_get v0, index u32 0 -> Field
+            constrain v7 == Field 0
+            return
+        }
+        
+        brillig(inline) fn foo f1 {
+          b0(v0: u32, v1: [Field]):
+              return
+          }
+        ";
+        let ssa = Ssa::from_str(src).unwrap();
+        let brillig = ssa.to_brillig(false);
+
+        let (acir_functions, _brillig_functions, _, _) = ssa
+            .into_acir(&brillig, ExpressionWidth::default())
+            .expect("Should compile manually written SSA into ACIR");
+
+        assert_eq!(acir_functions.len(), 1);
+
+        // Check that no memory opcodes were emitted.
+        let main = &acir_functions[0];
+        assert!(!main.opcodes().iter().any(|opcode| matches!(opcode, Opcode::MemoryOp { .. })));
+    }
 }

test_programs/compile_success_no_bug/regression_7062/Nargo.toml test_programs/execution_success/regression_7062/Nargo.toml

@@ -0,0 +1,2 @@
+index = 1
+value = 1

test_programs/execution_success/regression_7062/Prover.toml

@@ -1,8 +1,10 @@
-fn main(args: [Field; 2]) {
+fn main(value: Field, index: u32) {
+    let mut args = &[0, 1];
+    args[index] = value;
     /// Safety: n/a
     unsafe { store(args) };
     // Dummy test to remove the 'underconstraint bug'
     assert(args[0] + args[1] != 0);
 }
 
-pub unconstrained fn store(_: [Field]) {}
+pub unconstrained fn store(_: [Field]) {}