Difficulties and Potential Benefits of Implementing Load Address Prediction

[zybzzz]

Load address prediction is a specialized form of value prediction in the processor backend. It predicts the physical address of a load instruction and executes that load in advance. In xs-gem5, implementing load address prediction faces several challenges:

1. Additional Execution Path:
   Once the Load Queue allocates an entry for a load instruction, load address prediction can be performed and the memory access request can be issued early. This effectively adds an extra execution path inside the processor, making the management of out-of-order execution states more complex.

2. Ports Contention:
   Because of this early execution, predicted loads will compete with normal loads for execution ports. Hence, when implementing load address prediction, it may be necessary to model additional ports for these predicted loads.

3. Exception Handling:
   Load address prediction bypasses TLB/MMU address translation. However, real loads can raise exceptions during translation or access. Therefore, the prediction mechanism must consider the correct handling of exceptions at runtime.

4. Limited Benefit from Store-Load Bypass:
   Although predicted loads can be executed earlier, they cannot take advantage of store-load bypassing. This may lead to extra (possibly unnecessary) memory accesses.

5. Recovery from Mis-Prediction:
   When a load address prediction is incorrect, instructions that depend on that load’s data might need to be rolled back. While this is not overly difficult at the simulator level, it could introduce additional wake-up circuitry and logic at the RTL level, increasing hardware complexity.

In summary, implementing load address prediction is quite complex. Unless a thorough workload analysis shows substantial performance gains, the time and effort required may not justify the relatively modest benefits.

[tastynoob]

Why not predict the load value directly? Because the load address is more easy to predict?

[tastynoob]

Most of the time, the biggest cost of a load is cache miss, if we can predict load value it will provide more opportunities.

[zybzzz]

Why not predict the load value directly? Because the load address is more easy to predict?

Yes, it is evident that directly predicting the value of a Load instruction can yield significant benefits. However, a possible scenario is that the value of a Load is more difficult to predict compared to its address. There is currently no concrete evidence to support this claim, so I believe I need to analyze the trace data before drawing such a conclusion.