Illuminating the processor core with LLVM-mca
2 days ago
- #processor-optimization
- #llvm-mca
- #performance-analysis
- The RISC vs CISC debate concluded with modern processors using micro-ops for execution.
- llvm-mca is a tool within LLVM that analyzes machine code to simulate processor behavior and identify performance insights.
- llvm-mca uses the same datasets as the compiler for instruction scheduling, ensuring improvements in compiler optimizations are reflected.
- The tool has limitations, such as not modeling dynamic properties like cache misses or branch mispredictions.
- Example analysis of Protobuf's VarintSize64 method shows differences in execution between bsr and lzcnt instructions.
- llvm-mca provides detailed timeline views of instruction execution, showing cycles, dispatch, and retirement phases.
- Throughput vs latency considerations are important; llvm-mca can model both scenarios.
- Memory access patterns and critical paths can be analyzed to identify optimization opportunities.
- CRC32C optimization example demonstrates using parallel streams to reduce latency by leveraging instruction-level parallelism.
- llvm-mca's limitations include not modeling memory hierarchy beyond L1, branch prediction, or instruction fetch/decode.