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What is the primary issue with the lower bits address indexing in the example provided?
What is the limitation of the 1-bit prediction scheme in the context of a loop?
What is the main purpose of the example shown in Listing 6.2?
What is the consequence of the misprediction in the 1-bit prediction scheme?
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What is the primary issue with the indexing scheme shown in Listing 6.3?
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What is the purpose of the code shown in Listing 6.4?
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What is the main reason why speculative execution is required in certain types of loops?
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What is the consequence of not having efficient branch prediction in speculative superscalar execution?
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What is the main advantage of dynamic branch prediction over static branch prediction?
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What is the result of increasing the number of executed instructions per clock cycle?
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What is the maximum number of times the branch prediction will miss in the given code snippet?
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What is the purpose of reservation stations in speculative superscalar execution?
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What is the primary function of branch-prediction buffers?
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What is the main challenge in controlling dependencies in speculative superscalar execution?
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What is the consequence of a branch outcome misprediction in speculative superscalar execution?
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What is the limitation of the 1-bit branch-prediction buffer?
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What is the main reason why speculative execution is necessary in certain types of loops?
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What is the purpose of using the low-order address bits in the branch-prediction buffer?
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What is the state of the 1-bit branch-prediction buffer when the prediction is correct?
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What happens when the branch prediction is incorrect in the 1-bit branch-prediction buffer?
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What is the name of the approach that considers the aid of a small memory indexed by the lower portion of the branch instruction address?
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What is the primary goal of using branch prediction in the given code snippet?
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What is the issue with the addresses in Listing 6.3?
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What happens to the prediction bit in the 1-bit prediction scheme when a branch is mispredicted?
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What is the purpose of the example in Listing 6.2?
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What is the consequence of the 1-bit prediction scheme's shortcoming in a loop?
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What is the relationship between the branch instruction address and the small memory address in Listing 6.2?
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What is the effect of the misprediction on the branch prediction bit in the 1-bit prediction scheme?
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What is the primary reason why speculative execution is required in certain types of loops?
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What is the main challenge in controlling dependencies in speculative superscalar execution?
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What is the primary benefit of dynamic branch prediction over static branch prediction?
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What is the main purpose of additional memory in speculative superscalar execution?
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What is the result of increasing the number of executed instructions per clock cycle?
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What is the main advantage of using speculative execution in certain types of loops?
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What is the primary function of reservation stations in speculative superscalar execution?
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What is the consequence of not having efficient branch prediction in speculative superscalar execution?
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What is the primary objective of using a branch-prediction buffer?
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What is the main characteristic of a 1-bit branch-prediction buffer?
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What is the role of the low-order address bits in the branch-prediction buffer?
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What happens when the branch prediction is incorrect in the 1-bit branch-prediction buffer?
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What is the primary benefit of using dynamic branch prediction over static branch prediction?
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What is the main limitation of the 1-bit branch-prediction buffer?
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What is the purpose of the branch history table?
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What is the consequence of a misprediction in the 1-bit branch-prediction buffer?
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Study Notes
Speculative Execution
- Speculative execution is required when unrolling loops with conditional branch instructions that have a RAW dependence on the loop iteration.
- It requires more hardware, such as reservation stations and functional units, as well as more complex control and dependencies detection.
- Additional memory is needed to guarantee an eventual correction in case of a prediction error.
- The architecture needs more efficient branch prediction to pay off the new hardware cost with performance.
Control Dependencies
- As the number of executed instructions per clock cycle increases, the potential instructions flow also increases, reducing CPI.
- Delays caused by branches can seriously impact performance.
- A "simple" strategy to minimize this problem is to not stop or reduce speed in branches.
Dynamic Branch Prediction
- Dynamic branch prediction considers the fact that branch instructions can be executed many times during program execution, e.g., inside loops.
- This approach is different from static branch prediction strategies, such as predicted-not-taken and predicted-taken.
- The predicted-not-taken approach may be inefficient in loops, leading to many mispredictions.
Branch-Prediction Buffers
- A possible solution to the problem is to use branch-prediction buffers, which relate one bit to each branch decision.
- The buffer is a small memory indexed by the lower portion of the branch instruction address.
- The memory contains a bit that indicates whether a branch was recently taken or untaken.
- This approach is a hint assumed to be correct, and the next instruction fetch begins in the predicted direction.
- If the hint is wrong, the prediction bit is inverted and stored back.
1-Bit Prediction
- The 1-bit prediction scheme is a simple dynamic branch-prediction scheme.
- It is a 2-state finite-state machine (FSM) with states: TAKEN and UNTAKEN.
- The scheme uses just the low-order address bits to have variations, i.e., different addresses considering the use of just the low-order bits.
- However, there may be an inconvenient or even problematic case regarding these low-order address bits, where two distinct branch instructions end up with the same index.
- The 1-bit prediction scheme has the shortcoming that it may mispredict twice, rather than once, when a branch is not taken in a loop.
Speculative Execution
- Speculative execution is required when unrolling loops with conditional branch instructions that have a RAW dependence on the loop iteration.
- It requires more hardware, such as reservation stations and functional units, as well as more complex control and dependencies detection.
- Additional memory is needed to guarantee an eventual correction in case of a prediction error.
- The architecture needs more efficient branch prediction to pay off the new hardware cost with performance.
Control Dependencies
- As the number of executed instructions per clock cycle increases, the potential instructions flow also increases, reducing CPI.
- Delays caused by branches can seriously impact performance.
- A "simple" strategy to minimize this problem is to not stop or reduce speed in branches.
Dynamic Branch Prediction
- Dynamic branch prediction considers the fact that branch instructions can be executed many times during program execution, e.g., inside loops.
- This approach is different from static branch prediction strategies, such as predicted-not-taken and predicted-taken.
- The predicted-not-taken approach may be inefficient in loops, leading to many mispredictions.
Branch-Prediction Buffers
- A possible solution to the problem is to use branch-prediction buffers, which relate one bit to each branch decision.
- The buffer is a small memory indexed by the lower portion of the branch instruction address.
- The memory contains a bit that indicates whether a branch was recently taken or untaken.
- This approach is a hint assumed to be correct, and the next instruction fetch begins in the predicted direction.
- If the hint is wrong, the prediction bit is inverted and stored back.
1-Bit Prediction
- The 1-bit prediction scheme is a simple dynamic branch-prediction scheme.
- It is a 2-state finite-state machine (FSM) with states: TAKEN and UNTAKEN.
- The scheme uses just the low-order address bits to have variations, i.e., different addresses considering the use of just the low-order bits.
- However, there may be an inconvenient or even problematic case regarding these low-order address bits, where two distinct branch instructions end up with the same index.
- The 1-bit prediction scheme has the shortcoming that it may mispredict twice, rather than once, when a branch is not taken in a loop.
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Description
This quiz covers branch prediction and speculative superscalar processors in computer organisation, focusing on loop unrolling and speculative execution.