chore: debug mem2reg for load in loop

compiler/noirc_evaluator/src/ssa/opt/mem2reg.rs

@@ -68,7 +68,7 @@
 //!
 //! Repeating this algorithm for each block in the function in program order should result in
 //! optimizing out most known loads. However, identifying all aliases correctly has been proven
 //! undecidable in general (Landi, 1992). So this pass will not always optimize out all loads
 //! that could theoretically be optimized out. This pass can be performed at any time in the
 //! SSA optimization pipeline, although it will be more successful the simpler the program's CFG is.
 //! This pass is currently performed several times to enable other passes - most notably being
@@ -129,7 +129,7 @@
     /// Load and Store instructions that should be removed at the end of the pass.
     ///
     /// We avoid removing individual instructions as we go since removing elements
     /// from the middle of Vecs many times will be slower than a single call to `retain`.
     instructions_to_remove: HashSet<InstructionId>,
 
     /// Track a value's last load across all blocks.
@@ -1228,4 +1228,29 @@
         // We expect the program to be unchanged
         assert_normalized_ssa_equals(ssa, src);
     }
+
+    #[test]
+    fn optimizes_out_load_in_loop_if_var_is_read_only_in_loop() {
+        let src = "
+        brillig(inline) fn main f0 {
+          b0(v0: [u8; 1]):
+            v2 = allocate -> &mut u32
+            store u32 1 at v2
+            jmp b1(u32 0)
+          b1(v1: u32):
+            jmpif u1 1 then: b3, else: b2
+          b2():
+            return Field 0
+          b3():
+            // TODO: this load should be optimized out to `u32 1`
+            v7 = load v2 -> u32
+            jmp b1(v7)
+        }
+        ";
+        let ssa = Ssa::from_str(src).unwrap();
+
+        let mut ssa = ssa.mem2reg();
+        ssa.normalize_ids();
+        println!("{ssa}");
+    }
 }