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Out-of-order execution - what can it do for me? - Patrick Schittekat - NDC TechTown 2023
Discover the power of out-of-order execution (OOE) and how it can improve performance by overlapping long latency operations with shorter ones, but also learn about its limitations and the techniques used to mitigate mistakes.
- Out-of-order execution (OOE) is a hardware feature that allows instructions to be executed in a non-sequential order.
- OOE can improve performance by overlapping long latency operations, such as loading data from memory, with shorter latency operations, such as executing instructions on the CPU.
- The reorder buffer is a key component of OOE, as it holds instructions that have been reordered and are waiting to be executed.
- Load hosting is a technique used by OOE to predict whether a load instruction will be executed before a store instruction, and if so, to execute the load instruction before the store.
- Branch prediction is another important aspect of OOE, as it allows the CPU to predict the outcome of a conditional branch instruction and execute subsequent instructions accordingly.
- OOE can lead to bad speculation if the prediction is incorrect, and the CPU needs to restart from a previous point.
- To mitigate this, CPUs use techniques such as branch prediction tables and machine clears to detect and correct mistakes.
- OOE has been shown to improve performance in various benchmarks, such as clang, and can provide a significant speedup in certain scenarios.
- However, OOE also has limitations, such as the need for complex prediction algorithms and the potential for mistakes, which can lead to reduced performance.
- New CPU architectures, such as the ARM architecture, also implement OOE, which can provide improved performance and efficiency.