feat(optimization): Redirect array gets to earlier arrays if possible

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

@@ -623,8 +623,8 @@ impl Instruction {
                 }
             }
             Instruction::ArrayGet { array, index } => {
-                if let Some(index) = dfg.get_numeric_constant(*index) {
-                    try_optimize_array_get_from_previous_set(dfg, *array, index)
+                if let Some(index_value) = dfg.get_numeric_constant(*index) {
+                    try_optimize_array_get_from_previous_set(dfg, *array, index_value, *index)
                 } else {
                     None
                 }
@@ -776,8 +776,9 @@ impl Instruction {
 ///   - If the array value is from a previous array-set, we recur.
 fn try_optimize_array_get_from_previous_set(
     dfg: &DataFlowGraph,
-    mut array_id: Id<Value>,
+    mut array_id: ValueId,
     target_index: FieldElement,
+    index_id: ValueId,
 ) -> SimplifyResult {
     let mut elements = None;
 
@@ -805,6 +806,12 @@ fn try_optimize_array_get_from_previous_set(
                 elements = Some(array.clone());
                 break;
             }
+            // None of the past array sets changed this index so redirect this array get
+            // to the original param. This lets it potentially be de-duplicated later.
+            Value::Param { .. } => {
+                let new_get = Instruction::ArrayGet { array: array_id, index: index_id };
+                return SimplifyResult::SimplifiedToInstruction(new_get);
+            }
             _ => return SimplifyResult::None,
         }
     }

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

@@ -828,20 +828,16 @@ mod test {
         //     enable_side_effects v0
         //     v3 = array_get v2, index Field 0
         //     v4 = array_set v2, index Field 1, value: Field 2
-        //     v5 = array_get v4, index Field 0
-        //     constrain_eq v3, v5
         //     enable_side_effects v1
         //     v7 = array_set v2, index Field 1, value: Field 2
-        //     v8 = array_get v7, index Field 0
-        //     constrain_eq v3, v8
         //     enable_side_effects v0
         // }
         let ssa = ssa.fold_constants_using_constraints();
         println!("{ssa}");
 
         let main = ssa.main();
         let instructions = main.dfg[main.entry_block()].instructions();
-        assert_eq!(instructions.len(), 10);
+        assert_eq!(instructions.len(), 6);
     }
 
     // This test currently fails. It being fixed will address the issue https://github.com/noir-lang/noir/issues/5756