1. Is SC + ILP = RC? Chris Gniady, Babak Falsafr, and T.N. Vijaykumar
Presented By Jacob Harer

2. Idea Use large amounts of memory ILP to increase speed in SC
Relax all memory order speculatively in each core.
Appear to all other cores to be non speculative

3. Implementation
Need
Speculate on both loads and stores
Large speculative state
No additional overhead
Well behaved programs
Store all instructions in a Speculative History Que (SHiQ)
Roll back data if speculative data is accessed before it commits

4. Roll Back On Invalidation of speculatively loaded or stored data.
On read of speculatively stored data
On replacement due to a miss
Stored in Block Lookup Table (BLT)
Roll back by restoring from the SHiQ
No speculation until store completes

5. Example Processor 1 speculative load to Block
Processor 1 Does some other work
Processor 1 speculative Store to Block
Processor 2 load to Block
Get shared
Get Exclusive
Roll Back from SHiQ, Send old non speculative data

6. Conclusions Good results
Potential for lots of pathological cases. Where blocks are loaded way ahead of time. Reducing effectiveness of speculation
This is reduced by only speculatively storing once.

7. Questions? How many workloads are “well behaved”
Could RC benefit from the same ILP exploitation?
Can you speculatively load across cores?
Slow down in processor due to additional hardware.