fix(opcode/Ifacmp): compare objects by data pointer only

We store class instances as Rc<dyn ClassInstanc> values.
The pointers to the inner values consist of
1. a pointer to the vtable of the trait
2. a pointer to the data itself
Due to rust internals, the vtable pointer can be different
for Rc<dyn ClassInstance> values that point to the same *data*,
effectively making any comparison involing the vtable pointer moot.
This uses a function rust just introduced to compare such pointers.
(cf. rust-lang/rust#106447,
rust-lang/rust#117717,
rust-lang/rust#116325)

Author: liketechnik

src/executor/op_code.rs

@@ -19,6 +19,12 @@ use crate::{
     heap::Heap,
 };
 
+/// Explicitly compare only the data part of fat/trait/dyn Trait pointers.
+#[allow(clippy::ptr_eq)]
+fn trait_pointer_eq<T: ?Sized, U: ?Sized>(p: *const T, q: *const U) -> bool {
+    (p as *const ()) == (q as *const ())
+}
+
 #[derive(Clone, Debug)]
 pub enum ArrayReferenceKinds {
     Boolean,
@@ -1268,7 +1274,7 @@ got: {:?}",
                     },
                     (None, None) => Update::None,
                     (Some(op1), Some(op2)) => {
-                        if !Rc::<dyn ClassInstance>::ptr_eq(&op1, &op2) {
+                        if !trait_pointer_eq(op1.as_ref(), op2.as_ref()) {
                             Update::GoTo(*size, *direction)
                         } else {
                             Update::None
@@ -1286,7 +1292,7 @@ got: {:?}",
                     (Some(_), None) | (None, Some(_)) => Update::None,
                     (None, None) => Update::GoTo(*size, *direction),
                     (Some(op1), Some(op2)) => {
-                        if Rc::<dyn ClassInstance>::ptr_eq(&op1, &op2) {
+                        if trait_pointer_eq(op1.as_ref(), op2.as_ref()) {
                             Update::GoTo(*size, *direction)
                         } else {
                             Update::None

tests/control_flow.rs

@@ -12,7 +12,9 @@ fn control_flow() -> Result<(), Box<dyn std::error::Error>> {
         .stdout(predicate::str::contains("a / 2 == 5\n"))
         .stdout(predicate::str::contains("l / 2 != 5\n"))
         .stdout(predicate::str::contains("d:\n15\n"))
-        .stdout(predicate::str::contains("f > 10"));
+        .stdout(predicate::str::contains("f > 10"))
+        .stdout(predicate::str::contains("s1 == s2"))
+        .stdout(predicate::str::contains("s1 != s3"));
 
     Ok(())
 }

tests/data/control_flow/Main.class

@@ -37,5 +37,19 @@ public static void main(String[] args) {
         } else {
             System.out.println("f == 10");
         }
+
+        String s1 = "Hello World";
+        String s2 = s1;
+        String s3 = "Goodbye";
+        if (s1 == s2) {
+            System.out.println("s1 == s2");
+        } else {
+            System.out.println("s1 != s2");
+        }
+        if (s1 != s3) {
+            System.out.println("s1 != s3");
+        } else {
+            System.out.println("s1 == s3");
+        }
     }
 }