Speculative Execution

 

How can we avoid blocking issue on branches?

• Tomasulo's scheme allows values to be passed from inst to inst without updating registers
• So we extend it so register update is delayed until conditional branch is resolved
• We need somewhere to stash results while waiting to be committed

(H&P pp.308)

• Re-order buffer
• ...
• Tomasulo extended with ROB
• Sequence of events
• Re-order buffer: commit
• Re-order buffer: branches
• Re-order buffer: subtleties
• Speculative execution: summary