cache ball terms before unifying in handle_ball/3 (#1608)

src/machine/dispatch.rs

@@ -4125,14 +4125,6 @@ impl Machine {
                     self.clean_up_block();
                     self.machine_st.p = self.machine_st.cp;
                 }
-                &Instruction::CallEraseBall(_) => {
-                    self.erase_ball();
-                    self.machine_st.p += 1;
-                }
-                &Instruction::ExecuteEraseBall(_) => {
-                    self.erase_ball();
-                    self.machine_st.p = self.machine_st.cp;
-                }
                 &Instruction::CallFail(_) | &Instruction::ExecuteFail(_) => {
                     self.machine_st.backtrack();
                 }
@@ -4284,6 +4276,30 @@ impl Machine {
                     self.set_ball();
                     self.machine_st.p = self.machine_st.cp;
                 }
+                &Instruction::CallPushBallStack(_) => {
+                    self.push_ball_stack();
+                    step_or_fail!(self, self.machine_st.p += 1);
+                }
+                &Instruction::ExecutePushBallStack(_) => {
+                    self.push_ball_stack();
+                    step_or_fail!(self, self.machine_st.p = self.machine_st.cp);
+                }
+                &Instruction::CallPopBallStack(_) => {
+                    self.pop_ball_stack();
+                    self.machine_st.p += 1;
+                }
+                &Instruction::ExecutePopBallStack(_) => {
+                    self.pop_ball_stack();
+                    self.machine_st.p = self.machine_st.cp;
+                }
+                &Instruction::CallPopFromBallStack(_) => {
+                    self.pop_from_ball_stack();
+                    self.machine_st.p += 1;
+                }
+                &Instruction::ExecutePopFromBallStack(_) => {
+                    self.pop_from_ball_stack();
+                    self.machine_st.p = self.machine_st.cp;
+                }
                 &Instruction::CallSetCutPointByDefault(r, _) => {
                     self.set_cut_point_by_default(r);
                     step_or_fail!(self, self.machine_st.p += 1);
@@ -4970,11 +4986,11 @@ impl Machine {
                 }
                 &Instruction::CallPredicateDefined(_) => {
                     self.machine_st.fail = !self.predicate_defined();
-                    self.machine_st.p += 1;
+                    step_or_fail!(self, self.machine_st.p += 1);
                 }
                 &Instruction::ExecutePredicateDefined(_) => {
                     self.machine_st.fail = !self.predicate_defined();
-                    self.machine_st.p = self.machine_st.cp;
+                    step_or_fail!(self, self.machine_st.p = self.machine_st.cp);
                 }
                 &Instruction::CallStripModule(_) => {
                     let (module_loc, qualified_goal) = self.machine_st.strip_module(

src/machine/machine_state.rs

@@ -75,6 +75,7 @@ pub struct MachineState {
     pub(super) hb: usize,
     pub(super) block: usize, // an offset into the OR stack.
     pub(super) ball: Ball,
+    pub(super) ball_stack: Vec<Ball>, // save current ball before jumping via, e.g., verify_attr interrupt.
     pub(super) lifted_heap: Heap,
     pub(super) interms: Vec<Number>, // intermediate numbers.
     // locations of cleaners, cut points, the previous block. for setup_call_cleanup.
@@ -113,6 +114,7 @@ impl fmt::Debug for MachineState {
             .field("hb", &self.hb)
             .field("block", &self.block)
             .field("ball", &self.ball)
+            .field("ball_stack", &self.ball_stack)
             .field("lifted_heap", &self.lifted_heap)
             .field("interms", &self.interms)
             .field("flags", &self.flags)

src/machine/machine_state_impl.rs

@@ -47,6 +47,7 @@ impl MachineState {
             hb: 0,
             block: 0,
             ball: Ball::new(),
+            ball_stack: vec![],
             lifted_heap: Heap::new(),
             interms: vec![Number::default();256],
             cont_pts: Vec::with_capacity(256),

src/machine/system_calls.rs

@@ -4804,11 +4804,6 @@ impl Machine {
         }
     }
 
-    #[inline(always)]
-    pub(crate) fn erase_ball(&mut self) {
-        self.machine_st.ball.reset();
-    }
-
     #[inline(always)]
     pub(crate) fn get_ball(&mut self) {
         let addr = self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]));
@@ -4828,6 +4823,29 @@ impl Machine {
         };
     }
 
+    #[inline(always)]
+    pub(crate) fn push_ball_stack(&mut self) {
+        if self.machine_st.ball.stub.len() > 0 {
+            self.machine_st.ball_stack.push(
+                mem::replace(&mut self.machine_st.ball, Ball::new())
+            );
+        } else {
+            self.machine_st.fail = true;
+        }
+    }
+
+    #[inline(always)]
+    pub(crate) fn pop_ball_stack(&mut self) {
+        self.machine_st.ball_stack.pop();
+    }
+
+    #[inline(always)]
+    pub(crate) fn pop_from_ball_stack(&mut self) {
+        if let Some(ball) = self.machine_st.ball_stack.pop() {
+            self.machine_st.ball = ball;
+        }
+    }
+
     #[inline(always)]
     pub(crate) fn get_current_block(&mut self) {
         let n = Fixnum::build_with(i64::try_from(self.machine_st.block).unwrap());