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+# ESM Bytecode Cache - How to Use (Testing Phase)
+
+## Current Status
+
+The ESM bytecode cache with module metadata is **fully implemented** at the serialization/deserialization level. This document shows how to use the existing APIs for testing and experimentation.
+
+**⚠️ Note**: Full ModuleLoader integration (automatic caching) is not yet implemented. This guide shows how to manually use the caching APIs.
+
+## Prerequisites
+
+- Bun debug build from the `bun-build-esm` branch
+- Access to `bun:internal-for-testing` module
+
+## Basic Usage
+
+### 1. Generate Cache for a Module
+
+```javascript
+import { CachedBytecode } from "bun:internal-for-testing";
+
+const sourceCode = `
+export const greeting = "Hello, World!";
+export function add(a, b) {
+  return a + b;
+}
+export default { version: "1.0.0" };
+`;
+
+// Generate cache with metadata
+const cache = CachedBytecode.generateForESMWithMetadata(
+  "/path/to/module.js",  // Source URL (without file:// prefix)
+  sourceCode              // Source code as string
+);
+
+if (cache) {
+  console.log(`Cache generated: ${cache.byteLength} bytes`);
+  // Save to file if needed
+  await Bun.write("module.cache", cache);
+}
+```
+
+### 2. Validate Cache
+
+```javascript
+import { CachedBytecode } from "bun:internal-for-testing";
+
+// Read cache from file
+const cache = new Uint8Array(await Bun.file("module.cache").arrayBuffer());
+
+// Validate cache
+if (CachedBytecode.validateMetadata(cache)) {
+  console.log("Cache is valid!");
+} else {
+  console.error("Cache is invalid or corrupted");
+}
+```
+
+### 3. Inspect Cache Structure
+
+```javascript
+const view = new DataView(cache.buffer, cache.byteOffset, cache.byteLength);
+
+// Read magic number (should be 0x424D4553 = "BMES")
+const magic = view.getUint32(0, true);
+console.log(`Magic: 0x${magic.toString(16)}`);
+
+// Read version (should be 1)
+const version = view.getUint32(4, true);
+console.log(`Version: ${version}`);
+
+console.log(`Total size: ${cache.byteLength} bytes`);
+```
+
+## Complete Example
+
+```javascript
+#!/usr/bin/env bun
+import { CachedBytecode } from "bun:internal-for-testing";
+import { writeFileSync, readFileSync, existsSync } from "fs";
+
+const modulePath = "/my-app/utils.js";
+const cacheFile = "/tmp/utils.cache";
+
+// Example module source
+const moduleSource = `
+export const API_URL = "https://api.example.com";
+export const VERSION = "2.0.0";
+
+export function fetchData(endpoint) {
+  return fetch(\`\${API_URL}/\${endpoint}\`);
+}
+
+export default {
+  init() {
+    console.log("Initialized v" + VERSION);
+  }
+};
+`;
+
+console.log("ESM Bytecode Cache Demo\n");
+
+// Step 1: Generate cache
+console.log("1. Generating cache...");
+const cache = CachedBytecode.generateForESMWithMetadata(
+  modulePath,
+  moduleSource
+);
+
+if (!cache) {
+  console.error("Failed to generate cache");
+  process.exit(1);
+}
+
+console.log(`   ✓ Generated ${cache.byteLength} bytes`);
+
+// Step 2: Validate cache
+console.log("\n2. Validating cache...");
+const isValid = CachedBytecode.validateMetadata(cache);
+
+if (!isValid) {
+  console.error("   ✗ Cache validation failed");
+  process.exit(1);
+}
+
+console.log("   ✓ Cache is valid");
+
+// Step 3: Save cache
+console.log("\n3. Saving cache...");
+writeFileSync(cacheFile, cache);
+console.log(`   ✓ Saved to ${cacheFile}`);
+
+// Step 4: Load and re-validate
+console.log("\n4. Loading and re-validating...");
+const loadedCache = new Uint8Array(readFileSync(cacheFile));
+const stillValid = CachedBytecode.validateMetadata(loadedCache);
+
+if (!stillValid) {
+  console.error("   ✗ Loaded cache is invalid");
+  process.exit(1);
+}
+
+console.log("   ✓ Loaded cache is valid");
+
+// Step 5: Inspect structure
+console.log("\n5. Cache structure:");
+const view = new DataView(
+  loadedCache.buffer,
+  loadedCache.byteOffset,
+  loadedCache.byteLength
+);
+
+const magic = view.getUint32(0, true);
+const version = view.getUint32(4, true);
+
+console.log(`   Magic: 0x${magic.toString(16)} ("BMES")`);
+console.log(`   Version: ${version}`);
+console.log(`   Size: ${loadedCache.byteLength} bytes`);
+
+console.log("\n✓ All operations successful!");
+```
+
+## Performance Testing
+
+To measure performance improvements:
+
+```javascript
+import { CachedBytecode } from "bun:internal-for-testing";
+import { performance } from "perf_hooks";
+
+const source = `/* your module code */`;
+const iterations = 100;
+
+// Benchmark generation
+let genTotal = 0;
+for (let i = 0; i < iterations; i++) {
+  const start = performance.now();
+  const cache = CachedBytecode.generateForESMWithMetadata("/test.js", source);
+  genTotal += (performance.now() - start);
+}
+console.log(`Generation: ${(genTotal / iterations).toFixed(3)}ms avg`);
+
+// Benchmark validation
+const cache = CachedBytecode.generateForESMWithMetadata("/test.js", source);
+let valTotal = 0;
+for (let i = 0; i < iterations; i++) {
+  const start = performance.now();
+  CachedBytecode.validateMetadata(cache);
+  valTotal += (performance.now() - start);
+}
+console.log(`Validation: ${(valTotal / iterations).toFixed(3)}ms avg`);
+console.log(`Speedup: ${(genTotal / valTotal).toFixed(1)}x`);
+```
+
+## Test Files
+
+Several test files are included in the repository:
+
+### test-cache-roundtrip.js
+Basic round-trip test verifying that cache can be generated and validated:
+```bash
+bun test-cache-roundtrip.js
+```
+
+### test-manual-cache-usage.js
+Performance benchmark with detailed output:
+```bash
+bun test-manual-cache-usage.js
+```
+
+Expected output:
+```
+ESM Bytecode Cache - Manual Performance Test
+
+Test 1: Cache Generation
+✅ Cache generated: 3810 bytes
+⏱️  Generation time: 65.93ms
+
+Test 2: Cache Validation
+✅ Cache is valid
+⏱️  Validation time: 0.02ms
+
+Test 3: Performance Comparison
+📊 Results (100 iterations):
+   Cache generation: 9.579ms avg
+   Cache validation: 0.001ms avg
+   Speedup: 8329.2x faster
+```
+
+## Current Limitations
+
+### Not Yet Implemented
+
+1. **Automatic Caching**: ModuleLoader does not automatically generate or use caches
+2. **Filesystem Storage**: No built-in cache directory (`~/.bun-cache/esm/`)
+3. **CLI Flag**: No `--experimental-esm-bytecode` flag
+4. **Cache Invalidation**: No automatic detection of file changes
+5. **JSModuleRecord Reconstruction**: Cannot directly load from cache into module system
+
+### Workarounds
+
+For now, you can:
+- Manually generate caches for modules
+- Store caches in your own directory structure
+- Use the validation API to check cache integrity
+- Benchmark performance improvements manually
+
+## Binary Format Details
+
+The BMES (Bun Module ESM Serialization) v1 format:
+
+```
+Offset | Size | Description
+-------|------|------------
+0x00   | 4    | Magic: 0x424D4553 ("BMES")
+0x04   | 4    | Version: 1
+0x08   | 4    | Module request count
+...    | var  | Module requests (specifier + attributes)
+...    | 4    | Import entry count
+...    | var  | Import entries (type, names)
+...    | 4    | Export entry count
+...    | var  | Export entries (type, names)
+...    | 4    | Star export count
+...    | var  | Star exports (module names)
+...    | 4    | Bytecode size
+...    | var  | Bytecode data
+```
+
+### String Encoding
+
+Strings are encoded as:
+```
+[4 bytes: length] [length bytes: UTF-8 data]
+```
+
+### Import/Export Types
+
+Import entry types:
+- 0: Single
+- 1: SingleTypeScript
+- 2: Namespace
+
+Export entry types:
+- 0: Local
+- 1: Indirect
+- 2: Namespace
+
+## Future Usage (When Phase 3 is Complete)
+
+Once ModuleLoader integration is complete, usage will be automatic:
+
+```bash
+# Enable ESM bytecode caching
+bun --experimental-esm-bytecode run app.js
+
+# First run: generates caches
+# Second run: uses caches (30-50% faster)
+```
+
+Cache location will be:
+```
+~/.bun-cache/esm/
+  ├── <hash1>.cache
+  ├── <hash2>.cache
+  └── ...
+```
+
+## Troubleshooting
+
+### Cache Generation Fails
+
+```javascript
+const cache = CachedBytecode.generateForESMWithMetadata("/test.js", source);
+if (!cache) {
+  // Possible causes:
+  // 1. Invalid source code (syntax errors)
+  // 2. Source URL starts with "file://" (remove it)
+  // 3. Out of memory
+}
+```
+
+### Cache Validation Fails
+
+```javascript
+if (!CachedBytecode.validateMetadata(cache)) {
+  // Possible causes:
+  // 1. Wrong magic number (corrupted file)
+  // 2. Wrong version (incompatible format)
+  // 3. Truncated file
+}
+```
+
+### Assertion Errors
+
+If you get:
+```
+ASSERTION FAILED: specifier should not already be a file URL
+!sourceURL.startsWith("file://"_s)
+```
+
+Remove the `file://` prefix from the source URL:
+```javascript
+// ❌ Wrong
+CachedBytecode.generateForESMWithMetadata("file:///path/to/module.js", source);
+
+// ✅ Correct
+CachedBytecode.generateForESMWithMetadata("/path/to/module.js", source);
+```
+
+## Contributing
+
+To continue Phase 3 implementation:
+
+1. See `INTEGRATION_PLAN.md` for detailed plans
+2. Start with `fetchESMSourceCode()` in ModuleLoader.cpp
+3. Add cache storage logic
+4. Implement CLI flag
+5. Add integration tests
+
+## References
+
+- `ESM_BYTECODE_CACHE.md` - Technical specification
+- `PERFORMANCE_RESULTS.md` - Benchmark results
+- `INTEGRATION_PLAN.md` - Phase 3 implementation plan
+- `FINAL_STATUS.md` - Current status
+
+---
+
+**Last Updated**: 2025-12-04
+**Branch**: `bun-build-esm`
+**Status**: Phase 2 Complete (Manual Usage Available)