1. ECE 721 Alternatives to ROB-based Retirement
Spring 2019
Prof. Eric Rotenberg

2. ROB-based Retirement Benefit At what price?
* ROB = Reorder Buffer = Active List
Benefit
Can roll back to any offending instruction (exception or mispredicted branch)
Don’t lose any work prior to the offending instruction
At what price?
The values of all instructions in the instruction window must be retained so that state can be rolled back to any instruction
Since these values are held in the Physical Register File (PRF), the instruction window size (scope for parallelism) is limited by the size of the PRF
ECE 721, Spring 2019
Prof. Eric Rotenberg

3. Checkpoint-based Retirement
Idea: Relax requirement that state can be rolled back to any instruction
Create periodic checkpoints
A checkpoint “pins” physical registers corresponding to the precise architectural state at that point in the dynamic instruction stream
Physical registers of intermediate values (not pinned in any checkpoint) are aggressively freed
ECE 721, Spring 2019
Prof. Eric Rotenberg

4. Checkpoint-based Retirement (cont.)
Benefit
Do not need to buffer results of all instructions in the instruction window
Logically, can have a very large instruction window (scope for exposing parallelism) with a disproportionately smaller physical register file
Example:
Virtual instruction window: 1000s of instructions
Physical Register File: few 100s of registers
Summary: Decouple perceived size of instruction window from size of PRF. Extract high ILP with efficient resources.
At what price?
Can’t roll back to any offending instruction (exception or mispredicted branch)
May lose work prior to the offending instruction
Must judiciously select checkpoint placement
Exceptions are rare anyways
Confidence estimation for branches
ECE 721, Spring 2019
Prof. Eric Rotenberg

5. Aside: ROB + Checkpoints
But didn’t we use checkpoints with ROB?
Shadow Map Tables are checkpoints of Rename Map Table
However:
Shadow Map Tables aren’t involved in the algorithm that determines when physical registers can be freed
The ROB controls freeing of physical registers, and ensures the integrity of physical registers pointed to by a Shadow Map Table
The only purpose of the Shadow Map Table is to accelerate restoring the Rename Map Table, providing a 1-cycle alternative to forward- walking or backward-walking the ROB
ECE 721, Spring 2019
Prof. Eric Rotenberg

6. Summary ROB-based processor Checkpoint-based processor
Fine-grain recovery (roll back to any instruction)
Instruction window size = PRF size
Checkpoint-based processor
Coarse-grain recovery (roll back to selected points)
Instruction window size >> PRF size
ECE 721, Spring 2019
Prof. Eric Rotenberg