feat(ssa): Switch mem2reg pass to be per function rather than per block

crates/nargo_cli/tests/execution_success/references/src/main.nr

@@ -1,4 +1,6 @@
 fn main(mut x: Field) {
+    regression_2218(x, 10);
+
     add1(&mut x);
     assert(x == 3);
 
@@ -97,3 +99,20 @@ fn regression_2030() {
     let _ = *array[0];
     *array[0] = 1;
 }
+
+fn regression_2218(x: Field, y: Field) {
+    let q = &mut &mut 0;
+    let q1 = *q;
+    let q2 = *q;
+
+    if x != y {
+        *q1 = 1;
+        // Make sure that we correct load reference aliases through multiple blocks
+        if x != 20 {
+            *q1 = 10;
+            *q2 = 2; // now we'd expect q1 == q2 == 2
+
+            assert(*q1 == 2);  
+        }
+    }
+}

crates/noirc_evaluator/src/ssa.rs

@@ -49,12 +49,16 @@ pub(crate) fn optimize_into_acir(
         ssa = ssa
             .inline_functions()
             .print(print_ssa_passes, "After Inlining:")
+            // Run mem2reg with the CFG separated into blocks
+            .mem2reg()
+            .print(print_ssa_passes, "After Mem2Reg:")
             .unroll_loops()?
             .print(print_ssa_passes, "After Unrolling:")
             .simplify_cfg()
             .print(print_ssa_passes, "After Simplifying:")
             .flatten_cfg()
             .print(print_ssa_passes, "After Flattening:")
+            // Run mem2reg once more with the flattened CFG to catch any remaining loads/stores
             .mem2reg()
             .print(print_ssa_passes, "After Mem2Reg:")
             .fold_constants()

crates/noirc_evaluator/src/ssa/ir/function.rs

@@ -1,4 +1,4 @@
-use std::collections::HashSet;
+use std::collections::BTreeSet;
 
 use iter_extended::vecmap;
 
@@ -107,8 +107,8 @@ impl Function {
     /// Note that self.dfg.basic_blocks_iter() iterates over all blocks,
     /// whether reachable or not. This function should be used if you
     /// want to iterate only reachable blocks.
-    pub(crate) fn reachable_blocks(&self) -> HashSet<BasicBlockId> {
-        let mut blocks = HashSet::new();
+    pub(crate) fn reachable_blocks(&self) -> BTreeSet<BasicBlockId> {
+        let mut blocks = BTreeSet::new();
         let mut stack = vec![self.entry_block];
 
         while let Some(block) = stack.pop() {