How do superscalar and vliw processors handle branch prediction and speculation?

1 Superscalar processors

Superscalar processors have multiple execution units that can process different types of instructions simultaneously, such as arithmetic, logic, load, store, and branch. To exploit this parallelism, the processor has to fetch, decode, and schedule multiple instructions per cycle, and also resolve any data or control dependencies among them. For branch prediction, superscalar processors use dedicated hardware components, such as branch target buffers, branch history tables, and branch prediction units, that store and update information about the branch addresses, outcomes, and patterns. For speculation, superscalar processors use reorder buffers, reservation stations, and register renaming, that allow them to execute instructions out of order and commit them in order, while avoiding any hazards or exceptions.

2 VLIW processors

VLIW processors have multiple functional units that can execute different types of instructions in parallel, but unlike superscalar processors, they do not have any hardware mechanisms to fetch, decode, or schedule multiple instructions per cycle. Instead, they rely on the compiler to pack multiple independent instructions into a single long instruction word (VLIW) that specifies which functional unit should execute each instruction. For branch prediction, VLIW processors use software techniques, such as loop unrolling, predication, and trace scheduling, that reduce the number of branches or make them explicit in the code. For speculation, VLIW processors use software techniques, such as speculative loading, speculative execution, and software pipelining, that perform some operations ahead of time or overlap them with other operations.

3 Comparison of superscalar and VLIW processors

Superscalar and VLIW processors have different trade-offs between hardware and software complexity, performance and power consumption, and flexibility and portability. Superscalar processors have more hardware complexity, as they need to dynamically analyze and optimize the instruction stream at runtime, but they can achieve higher performance and adapt to different types of code. VLIW processors have less hardware complexity, as they delegate most of the analysis and optimization to the compiler at compile time, but they can have lower performance and require more code size and memory bandwidth. Moreover, VLIW processors are less flexible and portable, as they depend on the compiler to generate code that matches the specific architecture and functional units of the processor.

4 Advantages and disadvantages of branch prediction and speculation

Branch prediction and speculation are techniques that aim to improve the performance of processors by reducing the impact of branches on the instruction flow. However, they also have some advantages and disadvantages that depend on the accuracy and cost of these techniques. Branch prediction can increase the performance of processors by avoiding stalls or pipeline flushes caused by branches, but it can also decrease the performance if the prediction is wrong and the processor has to discard the executed instructions and fetch the correct ones. Speculation can increase the performance of processors by executing instructions that may be useful in the future, but it can also decrease the performance if the speculation is invalid and the processor has to undo the effects of the executed instructions and handle any exceptions.

5 Tips and best practices for branch prediction and speculation

Branch prediction and speculation can be beneficial for certain types of code and processors, but it is important to know how to use them effectively. For example, you should use simple and consistent branch conditions that are easy to predict and avoid complex or random ones. Profile-guided optimization or feedback-directed optimization can help collect information about the branch behavior and frequency and use it to improve the code generation or layout. Additionally, static branch hints or annotations can provide hints to the compiler or the processor about the expected branch outcome or direction. Loop unrolling or loop peeling can also eliminate or reduce the number of branches in loops and increase the instruction level parallelism. Predication or conditional execution can convert some branches into conditional instructions that execute regardless of the branch outcome and avoid branch penalties. Software pipelining or loop splitting can overlap the execution of different iterations of a loop and reduce the dependencies and stalls. Speculative loading or prefetching can load data from memory before it is needed and reduce the memory latency. Speculative execution or guarded execution can execute instructions that may not be needed and discard them if they are not valid. Finally, check instructions or recovery code can verify the validity of the speculated instructions and handle any exceptions or errors.

6 Here’s what else to consider

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