Understand Dynamic Recompilation (Dynarec) for Emulation Speedup

Dynamic Recompilation

Dynamic recompilation (dynarec) translates guest machine code to host machine code at runtime, caching translated blocks. It powers high-performance console emulators when interpretive CPUs are too slow.

Basic Block Cache

Workflow:

Identify a guest basic block entry (guest PC)
Translate instructions until a branch or end
Store host code pointer in a hash map keyed by guest PC
On revisit, jump directly to cached host code

Challenges beyond template JIT:

Guest registers mapped to host registers
Self-modifying code and cache invalidation
Direct block linking to skip dispatcher on taken branches

For example, translating a MIPS block from a PS1 emulator into x86-64 once, then reusing it for every loop iteration, removes interpret overhead entirely.

Profile hot PCs first; cold code can stay interpreted.

Design Notes

Invalidate cached blocks when guest code writes to memory that might contain instructions. Self-modifying code and JIT caches interact badly if you assume ROM is always read-only.

For This Exercise

This exercise asks you to document or prototype a dynarec block cache for a simple guest ISA. You will explain block translation, caching by program counter, and invalidation when guest code changes.

Document dynamic recompilation concepts in C.

Requirements:

Define structs for GuestInstruction, TranslatedBlock, and CodeCache
Implement a function that prints a sample guest instruction and its hypothetical host translation
Implement a basic block detector that scans a guest instruction array and prints block boundaries
Implement a code cache that stores translated blocks keyed by guest PC address
Print how block chaining works: show a guest block ending in a jump and how it links to the next block
Document code cache invalidation: print when and why blocks are removed from the cache
Include a comparison table showing interpreter overhead vs dynarec overhead per instruction
Print a summary of the translation pipeline: fetch guest block, emit host code, store in cache, execute

Dynamic Recompilation

Basic Block Cache

Workflow:

Identify a guest basic block entry (guest PC)
Translate instructions until a branch or end
Store host code pointer in a hash map keyed by guest PC
On revisit, jump directly to cached host code

Challenges beyond template JIT:

Guest registers mapped to host registers
Self-modifying code and cache invalidation
Direct block linking to skip dispatcher on taken branches

For example, translating a MIPS block from a PS1 emulator into x86-64 once, then reusing it for every loop iteration, removes interpret overhead entirely.

Profile hot PCs first; cold code can stay interpreted.

Design Notes

Invalidate cached blocks when guest code writes to memory that might contain instructions. Self-modifying code and JIT caches interact badly if you assume ROM is always read-only.

For This Exercise

This exercise asks you to document or prototype a dynarec block cache for a simple guest ISA. You will explain block translation, caching by program counter, and invalidation when guest code changes.

Document dynamic recompilation concepts in C.

Requirements:

Define structs for GuestInstruction, TranslatedBlock, and CodeCache
Implement a function that prints a sample guest instruction and its hypothetical host translation
Implement a basic block detector that scans a guest instruction array and prints block boundaries
Implement a code cache that stores translated blocks keyed by guest PC address
Print how block chaining works: show a guest block ending in a jump and how it links to the next block
Document code cache invalidation: print when and why blocks are removed from the cache
Include a comparison table showing interpreter overhead vs dynarec overhead per instruction
Print a summary of the translation pipeline: fetch guest block, emit host code, store in cache, execute

Topic Overview

Dynamic Recompilation

Basic Block Cache

Design Notes

For This Exercise

Implementation Task

Progressive Hints

Resources

Open Source

Understand Dynamic Recompilation (Dynarec) for Emulation Speedup

Topic Overview

Dynamic Recompilation

Basic Block Cache

Design Notes

For This Exercise

Implementation Task

Progressive Hints

Resources

Open Source