# Code Review: register-state-optimization

## Summary
✅ **APPROVED** - No blocking issues found. All acceptance criteria met. The implementation correctly tracks per-function max_register_used and uses it to optimize register saving during function calls.

## Acceptance Criteria Review
✅ CompilerMetadata struct with max_register_used: u8 field - Implemented in bytecode.rs:78-83
✅ Compiler tracks max_register_used during compilation - Implemented in compiler.rs:339-356
✅ FunctionMetadata includes max_register_used: Option<u8> - Implemented in vm.rs:22
✅ VM save_register_state(max_reg: u8) - Implemented in vm.rs:141-144
✅ VM restore_register_state(saved: Vec<Value>) - Implemented in vm.rs:147-154
✅ CallFrame stores saved_registers, saved_register_valid, max_saved_reg - Implemented in vm.rs:33-37
✅ Call instruction handler uses max_register_used from metadata - Implemented in vm.rs:344-345

## Code Quality Assessment

### Strengths
1. **Correct per-function tracking**: The compiler correctly resets max_register_used for each function body (line 339), tracks it during compilation, and stores it in the DefineFunction instruction (line 391). This is the core fix described in the coder's summary.

2. **Backward compatibility**: The implementation uses Option<u8> for max_register_used in FunctionMetadata (vm.rs:22), defaulting to 255 if not present (vm.rs:344), ensuring older bytecode still works.

3. **Proper state restoration**: The restore_register_state method correctly restores both register values and the validity bitmap (vm.rs:147-154).

4. **Clean separation of concerns**: Register state management is encapsulated in save_register_state and restore_register_state methods.

### Non-Blocking Issues

#### SHOULD_FIX

None identified.

#### SUGGESTIONS

1. **Minor: Consider adding a comment in compiler.rs**
   - Location: compiler.rs:339
   - The line `self.max_register_used = if params.len() > 0 { params.len() as u8 - 1 } else { 0 };` resets max_register_used for per-function tracking. A comment explaining this critical logic would improve code clarity.
   - Example: `// Reset max_register_used for this function scope (parameters occupy registers 0..param_count-1)`

2. **Minor: Test coverage for edge case**
   - While the existing tests cover basic function calls, consider adding a test that explicitly verifies that a function using only registers 0-2 saves only 3 registers (not all 256).
   - This would directly validate the optimization's effect.

## Verification

The code correctly implements the optimization described in the PRD:
- Before: All 256 registers saved on every function call (~2000 cycles)
- After: Only registers [0..=max_register_used] saved (~50-150 cycles for typical functions)
- This represents the claimed 90-95% improvement in function call overhead

## Conclusion

**Status**: APPROVED ✅
**Blocking**: No
**Summary**: Implementation correctly tracks per-function max_register_used and optimizes register state saving during function calls. All acceptance criteria met with high code quality. Minor suggestions provided for improved documentation and test coverage.
