1. Fetch Directed Prefetching - a Study
CS 752 Project
Gokul Nadathur
Nitin Bahadur
Sambavi Muthukrishnan
Gokul, Nitin, Sambavi

2. Motivation Execution engine limited by fetch bandwidth
effect of memory latency on fetch
correlation between i-cache stalls and branch predictor
rate at which branch predictor and BTB can be cycled
With increase in ILP, there is a need to increase fetch performance
Gokul, Nitin, Sambavi

3. Fetch Directed Architecture
Prefetch
Buffer
Branch
Predictor
Prefetch Filtration Mechanism
L2 Cache
Instruction Fetch
Fetch Target Queue
Prefetch Instruction Queue
Fetch Target Buffer
Gokul, Nitin, Sambavi

4. Decoupled Branch Predictor
has its own PC
runs independent of fetch pipeline stage
makes a prediction each cycle
unaffected by i-cache stalls
Problem!!!
May not have updated branch history
Gokul, Nitin, Sambavi

5. Fetch Target Buffer and Fetch Target Queue
Stores fall through and target address for taken branches
Accessed with a prediction from branch predictor each cycle
Fills in single/multiple cache line blocks into FTQ
Fetch Target Queue
Contains blocks of instruction addresses to be next executed
FTQ entries are dequeued by fetch engine
Gokul, Nitin, Sambavi

6. Prefetch Filter and Prefetch Instruction Queue
Contains queue of cache blocks to be prefetched
Prefetch mechanism dequeues PIQ and performs the prefetching
Prefetch Filter
Takes entries from FTQ, filters them and inserts them into PIQ
Enables intelligent prefetching !
Gokul, Nitin, Sambavi

7. Stream Buffers L1 I-cache L2 I-cache Stream buffer FIFO
Tag and comparator
Cache block
Tag
FIFO
Head
Tail
Gokul, Nitin, Sambavi

8. Prefetching in the Fetch Directed Architecture
Similar to stream buffers
Addresses given by PIQ
Gokul, Nitin, Sambavi

9. Simulation Results
Gokul, Nitin, Sambavi

10. Simulation Results
Gokul, Nitin, Sambavi

11. Simulation Results
Gokul, Nitin, Sambavi

12. Simulation Results
Gokul, Nitin, Sambavi

13. Simulation Results
Gokul, Nitin, Sambavi

14. Simulation Results
Gokul, Nitin, Sambavi

15. Conclusions Prefetching definitely helps
Fetch directed architecture aids prefetching
Optimal results require sophisticated memory hierarchy
Gokul, Nitin, Sambavi