Hardware-Based Speculation

Motivation

With branch prediction, instructions after branches get fetched early into processor

• So far, branch prediction lets us fetch insturctions after a branch, but we cannot execute them

If we execute instructions after a branch, what happens if the prediction is incorrect?

lw $r2, 0($r4)

beq $r2, $r3, target

add __,__,__

add __,__,__

; 100 more instructions that do not use $r2

target: add $r5, $r2, $r4

• Branch Prediction: saving cycles because you can fetch before knowing the outcome
• Speculation (speculative execution): Saving cycles because you can execute before knowing branch outcome

Storing Speculative Values

• For this to work, you need a place to store speculative values
• Reorder Buffer
  • In order (like a queue)
  • Four fields
    • Instruction type
    • Destination Register
    • Value field
    • Read field (is instruction complete?)

Four Stages with Speculation

1. Issue: Send an instruction to a reservation station
2. Execute: If operands are ready, then execute
3. Write Result: Write to common data bus and the result is stored in the reorder buffer
4. Commit: Write result from the reorder buffer into the register file
   1. On a branch mispredict, clear out instructions after a branch