compilerscheduling

Compiler scheduling, or instruction scheduling, is the process by which a compiler determines the order in which instructions are issued for execution on a target processor. The aim is to minimize stalls, hide latencies, and balance resource use while preserving program semantics.

Scheduling can occur at different scopes. Local scheduling operates within a single basic block, while global or regional scheduling attempts to optimize across multiple blocks, often around loops. Techniques include trace scheduling, which follows likely execution paths, and software pipelining, which overlaps iterations of a loop to improve throughput.

Scheduling must respect data dependencies (true dependencies, anti-dependencies, and output dependencies) and resource constraints on functional units, ports, and memory. It constructs a dependency graph or DAG and applies algorithms such as list scheduling, topological ordering, or heuristic search. Some systems pursue exact optimization; most rely on fast heuristics for practical compile times.

Architectures influence scheduling choices. In superscalar or out-of-order CPUs, the hardware can exploit instruction-level parallelism but compilers still reorder to reduce stalls and to present independent instructions that the runtime can exploit. In VLIW architectures, the compiler must schedule instructions to fill parallel issue slots. For SIMD, the scheduler aligns operations with vector lanes and data widths.

Scheduling interacts with other back-end optimizations, particularly register allocation and instruction selection. Heavy scheduling can increase register pressure and spills, or expand code size. Many compilers perform scheduling after register allocation, or they perform co-optimized passes to balance trade-offs.

The primary outcome is improved execution time and processor utilization, measured by lower cycles per instruction or higher instruction throughput. Compile-time cost and code size are common trade-offs.