Fix invalid local preservation overwrite (#1177)

* properly assert for audit_1_execution test case

* fix invalid local preservation overwrite

crates/wasmi/src/engine/translator/instr_encoder.rs

@@ -796,10 +796,10 @@ impl InstrEncoder {
         };
         if matches!(
             stack.get_register_space(returned_value),
-            RegisterSpace::Local
+            RegisterSpace::Local | RegisterSpace::Preserve
         ) {
             // Can only apply the optimization if the returned value of `last_instr`
-            // is _NOT_ itself a local register due to observable behavior.
+            // is _NOT_ itself a local register due to observable behavior or already preserved.
             return fallback_case(self, stack, local, value, preserved, fuel_info);
         }
         let Some(last_instr) = self.last_instr else {

crates/wasmi/src/engine/translator/tests/fuzz/mod.rs

@@ -7,6 +7,7 @@ use crate::{
         bytecode::{BranchOffset, BranchOffset16, GlobalIdx, RegisterSpan},
         EngineFunc,
     },
+    Val,
 };
 
 #[test]
@@ -510,3 +511,37 @@ fn fuzz_regression_17() {
         ])
         .run()
 }
+
+#[test]
+#[cfg_attr(miri, ignore)]
+fn audit_2_codegen() {
+    let wasm = include_str!("wat/audit_2.wat");
+    TranslationTest::from_wat(wasm)
+        .expect_func_instrs([
+            Instruction::copy(2, 0),
+            Instruction::copy(0, 2),
+            Instruction::copy(1, 0),
+            Instruction::return_many(2, 1, 0),
+            Instruction::register(0),
+        ])
+        .run()
+}
+
+#[test]
+#[cfg_attr(miri, ignore)]
+fn audit_2_execution() {
+    use crate::{Engine, Instance, Store};
+    let wat = include_str!("wat/audit_2.wat");
+    let wasm = wat::parse_str(wat).unwrap();
+    let engine = Engine::default();
+    let mut store = <Store<()>>::new(&engine, ());
+    let module = Module::new(&engine, &wasm[..]).unwrap();
+    let instance = Instance::new(&mut store, &module, &[]).unwrap();
+    let func = instance.get_func(&store, "").unwrap();
+    let inputs = [Val::I32(1)];
+    let mut results = [0_i32; 4].map(Val::from);
+    let expected = [1_i32; 4];
+    func.call(&mut store, &inputs[..], &mut results[..])
+        .unwrap();
+    assert_eq!(results.map(|v| v.i32().unwrap()), expected,);
+}

crates/wasmi/src/engine/translator/tests/fuzz/wat/audit_2.wat

@@ -0,0 +1,15 @@
+(module ;; different result on main than on Wasmtime
+  (func (export "") (param i32) (result i32 i32 i32 i32)
+    local.get 0
+    local.get 0
+    block (param i32 i32)
+      local.tee 0
+      block (param i32 i32)
+        local.get 0
+        local.get 0
+        br 2 ;; returns
+      end
+    end
+    unreachable
+  )
+)