How Does Memory Reordering Optimize Processors and Compilers?

How Memory Reordering Benefits Processors and Compilers

Memory reordering plays a crucial role in optimizing performance for both processors and compilers.

Benefits for Processors

StoreLoad Reordering (Essential for Out-of-Order Execution):
Memory reordering allows processors to execute certain stores before loads, making out-of-order execution possible. This allows the processor to exploit instruction-level parallelism and reduce overall execution time.

StoreStore Reordering (Not Essential, but Beneficial):
StoreStore reordering allows stores to be executed out of order, even if they involve different memory locations. This provides more opportunities for optimization by merging consecutive stores into fewer operations.

Benefits for Compilers

Optimization Opportunities for Shared Variables:
Memory reordering enables compilers to perform aggressive optimizations on code that accesses shared memory. Since other threads cannot observe intermediate states of stores, compilers can combine multiple store operations into a single operation.

Reducing Code Size and Improving Performance:
Compilers leverage memory reordering to reduce code size and improve performance. For example, a sequence of incrementing a variable can be simplified into a single store operation, reducing both code size and execution time.

As-If Rule

Memory reordering is based on the "as-if" rule, which states that the observable effects of a program must be the same as if instructions were executed in order. This rule gives the compiler and hardware more flexibility in optimizing code, without violating the requirements of source code correctness.

Conclusion

Memory reordering is a crucial technique that improves processor and compiler performance. By allowing stores to be reordered with respect to loads and other stores, compilers can optimize shared memory operations, and processors can exploit instruction-level parallelism, resulting in faster execution times and improved performance.

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