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+# LLVM Backend Implementation Guide
+
+## Overview
+
+The LLVM backend provides native code compilation for Nyash programs through the inkwell LLVM wrapper. This document covers the implementation completed in Phase 9.78 Week 1.
+
+## Current Implementation Status
+
+### ✅ Completed (Phase 9.78 Week 1)
+
+- **Infrastructure Setup**: Complete LLVM backend directory structure
+- **Mock Implementation**: Fully functional mock that demonstrates all integration points
+- **CLI Integration**: `--backend llvm` option support
+- **MIR Integration**: Connection between MIR compiler and LLVM backend
+- **Error Handling**: Proper error messages and user feedback
+
+### 🔄 In Progress/Planned
+
+- **Real LLVM Integration**: Requires LLVM development libraries
+- **Advanced Code Generation**: Full MIR instruction set support
+- **Optimization Passes**: LLVM optimization pipeline integration
+
+## Architecture
+
+### Directory Structure
+
+```
+src/backend/llvm/
+├── mod.rs          # Main module interface and exports
+├── context.rs      # LLVM context, module, and target management  
+└── compiler.rs     # MIR to LLVM IR compilation logic
+```
+
+### Key Components
+
+1. **LLVMCompiler**: Main compilation orchestrator
+2. **CodegenContext**: LLVM context and target machine management
+3. **Integration Layer**: Connects with existing MIR → Backend pipeline
+
+## Usage
+
+### Mock Implementation (Current)
+
+```bash
+# Run with mock LLVM backend
+cargo run -- --backend llvm test_program.nyash
+
+# This will:
+# 1. Parse Nyash source to AST
+# 2. Compile AST to MIR
+# 3. Analyze MIR structure (mock)
+# 4. Display mock compilation results
+# 5. Return appropriate exit code
+```
+
+### Real Implementation (Future)
+
+```bash
+# Install LLVM development libraries first
+sudo apt install llvm-17-dev clang-17
+
+# Enable LLVM feature and build
+cargo build --features llvm --release
+
+# Run with real LLVM backend  
+cargo run --features llvm -- --backend llvm test_program.nyash
+
+# This will:
+# 1. Parse Nyash source to AST
+# 2. Compile AST to MIR
+# 3. Generate LLVM IR from MIR
+# 4. Compile to object file
+# 5. Link with system linker
+# 6. Execute native binary
+```
+
+## Test Cases
+
+### Basic Return Test
+
+**File**: `local_tests/test_return_42.nyash`
+```nyash
+static box Main {
+    main() {
+        return 42
+    }
+}
+```
+
+**Expected Behavior**:
+- Mock: Analyzes MIR and returns appropriate exit code
+- Real: Compiles to native code that returns exit code 42
+
+### Running Tests
+
+```bash
+# Test mock implementation
+cargo run -- --backend llvm local_tests/test_return_42.nyash
+echo "Exit code: $?"
+
+# Should show mock execution and exit code 0 (42 when real implementation is complete)
+```
+
+## Implementation Details
+
+### Mock vs Real Implementation
+
+The current implementation uses conditional compilation to provide both mock and real implementations:
+
+```rust
+#[cfg(feature = "llvm")]
+// Real inkwell-based implementation
+use inkwell::context::Context;
+
+#[cfg(not(feature = "llvm"))]  
+// Mock implementation for demonstration
+pub struct CodegenContext { /* ... */ }
+```
+
+### MIR Integration
+
+The LLVM backend integrates with the existing MIR compilation pipeline:
+
+1. **AST → MIR**: Uses existing `MirCompiler`
+2. **MIR → LLVM**: New `LLVMCompiler` handles MIR instruction translation
+3. **LLVM → Native**: Uses LLVM's code generation and system linker
+
+### Error Handling
+
+The implementation provides comprehensive error handling:
+
+- **Environment Errors**: Missing LLVM libraries, compilation failures
+- **User Feedback**: Clear messages about requirements and next steps
+- **Graceful Degradation**: Mock implementation when LLVM unavailable
+
+## Dependencies
+
+### System Requirements
+
+- **LLVM 17+**: Development libraries and headers
+- **Clang**: System C compiler for linking
+- **Target Support**: Native target architecture support
+
+### Rust Dependencies
+
+```toml
+[dependencies]
+inkwell = { version = "0.5", features = ["target-x86"], optional = true }
+
+[features]
+llvm = ["dep:inkwell"]
+```
+
+## Future Enhancements
+
+### Week 2+ Roadmap
+
+1. **Real LLVM Integration**: Replace mock with inkwell implementation
+2. **Instruction Support**: Complete MIR instruction set coverage
+3. **Type System**: Proper LLVM type mapping for Nyash Box types
+4. **Optimization**: LLVM optimization pass integration
+5. **Debug Info**: Debug symbol generation for debugging support
+
+### Performance Goals
+
+- **Compilation Speed**: Sub-second compilation for small programs
+- **Runtime Performance**: 2x+ improvement over VM backend
+- **Binary Size**: Reasonable binary sizes with optional optimization
+
+## Contributing
+
+### Development Setup
+
+1. Install LLVM development libraries
+2. Enable LLVM feature flag
+3. Run tests with `cargo test --features llvm`
+4. Implement missing MIR instruction handlers
+
+### Code Style
+
+- Follow existing Nyash code patterns
+- Use conditional compilation for feature gates
+- Provide comprehensive error messages
+- Include tests for new functionality
+
+## References
+
+- [inkwell Documentation](https://thedan64.github.io/inkwell/)
+- [LLVM Language Reference](https://llvm.org/docs/LangRef.html)
+- [Nyash MIR Specification](../mir/README.md)
+- [Phase 9.78 Implementation Plan](../../予定/native-plan/llvm/issue/001-setup-inkwell-hello-world.md)
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