VirtualMachine: Add checkpoints

Author: adazem009

src/engine/virtualmachine.cpp

@@ -30,6 +30,7 @@ const unsigned int VirtualMachine::instruction_arg_count[] = {
     0, // OP_HALT
     1, // OP_CONST
     0, // OP_NULL
+    0, // OP_CHECKPOINT
     0, // OP_IF
     0, // OP_ELSE
     0, // OP_ENDIF
@@ -168,13 +169,22 @@ void VirtualMachine::reset()
     m_pos = m_bytecode;
 }
 
+/*! Moves back to the last vm::OP_CHECKPOINT instruction in the bytecode. */
+void VirtualMachine::moveToLastCheckpoint()
+{
+    assert(m_checkpoint);
+    m_pos = m_checkpoint;
+    m_updatePos = true;
+}
+
 unsigned int *VirtualMachine::run(unsigned int *pos)
 {
     static const void *dispatch_table[] = {
         nullptr,
         &&do_halt,
         &&do_const,
         &&do_null,
+        &&do_checkpoint,
         &&do_if,
         &&do_else,
         &&do_endif,
@@ -267,6 +277,10 @@ unsigned int *VirtualMachine::run(unsigned int *pos)
     ADD_RET_VALUE(Value());
     DISPATCH();
 
+do_checkpoint:
+    m_checkpoint = pos - 1;
+    DISPATCH();
+
 do_if:
     if (!READ_LAST_REG()->toBool()) {
         while (*pos != OP_ELSE && *pos != OP_ENDIF)
@@ -687,6 +701,10 @@ do_list_get_item : {
 
 do_exec : {
     auto ret = m_functions[*++pos](this);
+    if (m_updatePos) {
+        pos = m_pos;
+        m_updatePos = false;
+    }
     if (m_stop) {
         m_stop = false;
         m_callTree.clear();

src/engine/virtualmachine.h

@@ -19,6 +19,7 @@ enum Opcode
     OP_HALT,             /*!< The last instruction of every bytecode. */
     OP_CONST,            /*!< Adds a constant value with the index in the argument to the next register. */
     OP_NULL,             /*!< Adds a null (zero) value to the next register. */
+    OP_CHECKPOINT,       /*!< A checkpoint for the VirtualMachine::moveToLastCheckpoint() method. */
     OP_IF,               /*!< Jumps to the next instruction if the last register holds "true". If it's false, jumps to OP_ELSE or OP_ENDIF. */
     OP_ELSE,             /*!< Jumps to OP_ENDIF. This instruction is typically reached when the if statement condition was "true". */
     OP_ENDIF,            /*!< Doesn't do anything, but is used by OP_IF and OP_ELSE. */
@@ -113,6 +114,7 @@ class LIBSCRATCHCPP_EXPORT VirtualMachine
 
         void run();
         void reset();
+        void moveToLastCheckpoint();
 
         /*!
          * Use this to stop the script from a function.
@@ -156,6 +158,7 @@ class LIBSCRATCHCPP_EXPORT VirtualMachine
         Engine *m_engine = nullptr;
         Script *m_script = nullptr;
         unsigned int *m_pos = nullptr;
+        unsigned int *m_checkpoint = nullptr;
         bool m_atEnd = false;
         std::vector<Loop> m_loops;
         std::vector<unsigned int *> m_callTree;
@@ -168,6 +171,7 @@ class LIBSCRATCHCPP_EXPORT VirtualMachine
         bool m_savePos = true;
         bool m_goBack = false;
         unsigned int m_freeExecRegs;
+        bool m_updatePos = false;
 
         unsigned int **m_procedures = nullptr;
         BlockFunc *m_functions;