SAFIRE (Scalable and Accurate Fault Injection for Parallel Multi-threaded Applications) is a fault injection framework, descending from REFINE, that supports injecting bit-flip faults in both serial and multi-threaded programs. SAFIRE includes a modified LLVM compiler backend for x86 to instrument and inject bit-flip faults on machine instructions. The backend implements several optimizations for fast instrumentation and injection that make SAFIRE the fastest and most accurate tool for fault injection so far.