fix(ssa_gen): Do not code gen fetching of empty arrays when initializing the data bus

compiler/noirc_evaluator/src/ssa/function_builder/data_bus.rs

@@ -156,6 +156,11 @@ impl FunctionBuilder {
                         let index_var = FieldElement::from(index as i128);
                         let index_var =
                             self.current_function.dfg.make_constant(index_var, length_type);
+                        // If we do not check for an empty array we will have an unused array get
+                        // as an array of length zero will not be actually added to the databus' values.
+                        if let Type::Array(_, 0) = subitem_typ {
+                            continue;
+                        }
                         let element = self.insert_array_get(value, index_var, subitem_typ.clone());
                         index += match subitem_typ {
                             Type::Array(_, _) | Type::Slice(_) => subitem_typ.element_size(),
@@ -230,6 +235,7 @@ impl FunctionBuilder {
                 }
             }
         }
+
         // create the call-data-bus from the filtered lists
         let mut result = Vec::new();
         for id in databus_param.keys() {

compiler/noirc_evaluator/src/ssa/ssa_gen/tests.rs

@@ -107,3 +107,40 @@ fn basic_loop() {
 
     assert_normalized_ssa_equals(ssa, expected);
 }
+
+#[named]
+#[test]
+fn databus_no_dead_param_on_empty_array() {
+    let src = "
+    fn main(a: (i8, u32, i8), b: call_data(0) [(i8, i8, bool, bool, str<0>); 2]) -> pub [(bool, str<3>, str<0>, u32); 0] {
+        []
+    }
+    ";
+
+    let ssa = get_initial_ssa(src, function_path!()).unwrap();
+
+    // We expect that there to be no `array_get` attempting to fetch from v3
+    // the empty nested array `[u8; 0]`.
+    // The databus is only going to be initialized with actual numeric values so keeping
+    // an empty array in the databus is pointless.
+    // The databus is not mutated after initialization as well. So if we have instructions
+    // on the data bus (such as an `array_get` on an empty array) that go unused, it becomes
+    // more difficult to eliminate those unused instructions. Thus, we just do not generate them.
+    let expected = "
+    acir(inline) fn main f0 {
+      b0(v0: i8, v1: u32, v2: i8, v3: [(i8, i8, u1, u1, [u8; 0]); 2]):
+        v5 = array_get v3, index u32 0 -> i8
+        v7 = array_get v3, index u32 1 -> i8
+        v9 = array_get v3, index u32 2 -> u1
+        v11 = array_get v3, index u32 3 -> u1
+        v13 = array_get v3, index u32 4 -> i8
+        v15 = array_get v3, index u32 5 -> i8
+        v17 = array_get v3, index u32 6 -> u1
+        v19 = array_get v3, index u32 7 -> u1
+        v20 = make_array [v5, v7, v9, v11, v13, v15, v17, v19] : [Field; 8]
+        v21 = make_array [] : [(u1, [u8; 3], [u8; 0], u32); 0]
+        return v21
+    }
+    ";
+    assert_normalized_ssa_equals(ssa, expected);
+}

test_programs/compile_success_no_bug/noirc_evaluator_ssa_ssa_gen_tests_databus_no_dead_param_on_empty_array/Nargo.toml

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

test_programs/compile_success_no_bug/noirc_evaluator_ssa_ssa_gen_tests_databus_no_dead_param_on_empty_array/src/main.nr

@@ -0,0 +1,5 @@
+
+    fn main(a: (i8, u32, i8), b: call_data(0) [(i8, i8, bool, bool, str<0>); 2]) -> pub [(bool, str<3>, str<0>, u32); 0] {
+        []
+    }
+

test_programs/compile_success_no_bug/noirc_evaluator_ssa_ssa_gen_tests_databus_no_dead_param_on_empty_array/src_hash.txt

@@ -0,0 +1 @@
+8959072883599795205

test_programs/execution_success/databus_dead_param/Nargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "databus_dead_param"
+type = "bin"
+authors = [""]
+
+[dependencies]

test_programs/execution_success/databus_dead_param/Prover.toml

@@ -0,0 +1,2 @@
+a = ["1", "2", "3"]
+b = [["0", "0", true, true, ""], ["0", "0", true, true, ""]]

test_programs/execution_success/databus_dead_param/src/main.nr

@@ -0,0 +1,7 @@
+// Regression for issue #8398 (https://github.com/noir-lang/noir/issues/8398)
+fn main(
+    a: (i8, u32, i8),
+    b: call_data(0) [(i8, i8, bool, bool, str<0>); 2],
+) -> pub [(bool, str<3>, str<0>, u32); 0] {
+    []
+}

test_programs/execution_success/databus_dead_param/stdout.txt

@@ -0,0 +1 @@
+[databus_dead_param] Circuit output: Vec([])