fix(ssa refactor): resolve replaced value ids for printing

crates/noirc_evaluator/src/ssa_refactor/ir/dfg.rs

@@ -59,6 +59,11 @@ pub(crate) struct DataFlowGraph {
 
     /// All blocks in a function
     blocks: DenseMap<BasicBlock>,
+
+    /// Debugging information about which `ValueId`s have had their underlying `Value` substituted
+    /// for that of another. This information is purely used for printing the SSA, and has no
+    /// material effect on the SSA itself.
+    replaced_value_ids: HashMap<ValueId, ValueId>,
 }
 
 impl DataFlowGraph {
@@ -146,21 +151,32 @@ impl DataFlowGraph {
         self.values.insert(value)
     }
 
-    /// Replaces the value specified by the given ValueId with a new Value.
+    /// Set the value of value_to_replace to refer to the value referred to by new_value.
     ///
     /// This is the preferred method to call for optimizations simplifying
     /// values since other instructions referring to the same ValueId need
     /// not be modified to refer to a new ValueId.
-    pub(crate) fn set_value(&mut self, value_id: ValueId, new_value: Value) {
-        self.values[value_id] = new_value;
-    }
-
-    /// Set the value of value_to_replace to refer to the value referred to by new_value.
     pub(crate) fn set_value_from_id(&mut self, value_to_replace: ValueId, new_value: ValueId) {
+        // Replaced `ValueId`s are tracked purely for debugging purposes.
+        // We first call `resolve_replaced_value_id(new_value)` in case `new_value` is also a
+        // `ValueId` that has previously had its corresponding `Value` substituted.
+        self.replaced_value_ids.insert(value_to_replace, self.resolve_replaced_value_id(new_value));
+
         let new_value = self.values[new_value];
         self.values[value_to_replace] = new_value;
     }
 
+    /// If `original_value_id`'s underlying `Value` has been substituted for that of another
+    /// `ValueId`, this function will return the `ValueId` from which the substitution was taken.
+    /// If `original_value_id`'s underlying `Value` has not been substituted, the same `ValueId`
+    /// is returned.
+    ///
+    /// The function exists purely for debugging purposes. Only the SSA printer is concerned with
+    /// this information.
+    pub(crate) fn resolve_replaced_value_id(&self, original_value_id: ValueId) -> ValueId {
+        *self.replaced_value_ids.get(&original_value_id).unwrap_or(&original_value_id)
+    }
+
     /// Creates a new constant value, or returns the Id to an existing one if
     /// one already exists.
     pub(crate) fn make_constant(&mut self, constant: FieldElement, typ: Type) -> ValueId {

crates/noirc_evaluator/src/ssa_refactor/ir/printer.rs

@@ -67,7 +67,7 @@ fn value(function: &Function, id: ValueId) -> String {
         }
         Value::Function(id) => id.to_string(),
         Value::Intrinsic(intrinsic) => intrinsic.to_string(),
-        _ => id.to_string(),
+        _ => function.dfg.resolve_replaced_value_id(id).to_string(),
     }
 }
 

crates/noirc_evaluator/src/ssa_refactor/opt/mem2reg.rs

@@ -102,8 +102,7 @@ impl PerBlockContext {
                 Instruction::Load { address } => {
                     if let Some(address) = self.try_const_address(*address, dfg) {
                         if let Some(last_value) = self.last_stores.get(&address) {
-                            let last_value = dfg[*last_value];
-                            loads_to_substitute.push((*instruction_id, last_value));
+                            loads_to_substitute.push((*instruction_id, *last_value));
                         } else {
                             self.failed_substitutes.insert(address);
                         }
@@ -128,7 +127,7 @@ impl PerBlockContext {
                 .instruction_results(*instruction_id)
                 .first()
                 .expect("ICE: Load instructions should have single result");
-            dfg.set_value(result_value, *new_value);
+            dfg.set_value_from_id(result_value, *new_value);
         }
 
         // Delete load instructions