1. CS 152 Computer Architecture & Engineering Andrew Waterman University of California, Berkeley Section 8 Spring 2010

2. Mystery Die

3. DEC Alpha 21264 15M transistors 600 MHz in 350 nm Highly speculative OoO superscalar

4. Mystery Die DEC Alpha 21264 15M transistors 600 MHz in 350 nm Highly speculative OoO superscalar Fetch I$D$ Bus

5. Alpha 21264 Pipeline

6. Branch Prediction Two kinds of correlating branch predictors: LocalGlobal PC Local History Table Branch History Table Global History Branch History Table

7. Branch Prediction 21264 uses both! (tournament predictor) LocalGlobal Prediction PC Local History Table Branch History Table Global History Branch History Table Tournament Predictor

8. 21264 Fetch Line/way prediction keeps fetch loop short

9. Alpha 21264 Pipeline

10. 21264 Register Renaming Registers are renamed, then instructions are inserted into the issue queue Map table backed up on every in-flight insn

11. 21264 Register Renaming What hazards does renaming obviate? In what situations is renaming useful? If you had to choose between branch prediction and renaming, which would you pick?

12. 21264 Register Renaming What hazards does renaming obviate? – WAR, WAW In what situations is renaming useful? If you had to choose between branch prediction and renaming, which would you pick?

13. 21264 Register Renaming What hazards does renaming obviate? – WAR, WAW In what situations is renaming useful? – Code with ILP and name dependencies: loops If you had to choose between branch prediction and renaming, which would you pick?

14. 21264 Register Renaming What hazards does renaming obviate? – WAR, WAW In what situations is renaming useful? – Code with ILP and name dependencies: loops If you had to choose between branch prediction and renaming, which would you pick? – Not much ILP within a basic block, so renaming isn’t too useful without branch prediction

15. Alpha 21264 Pipeline

16. 21264 Superscalar Execution The 21264 can decode, rename, issue, execute, and commit 4 insns/cycle How does circuit complexity scale with W in the following operations? – Instruction decode – Register renaming – Result bypassing

17. 21264 Superscalar Execution The 21264 can decode, rename, issue, execute, and commit 4 insns/cycle How does circuit complexity scale with W in the following operations? – Instruction decode: O(W) – Register renaming – Result bypassing

18. 21264 Superscalar Execution The 21264 can decode, rename, issue, execute, and commit 4 insns/cycle How does circuit complexity scale with W in the following operations? – Instruction decode: O(W) – Register renaming: O(W 2 ) – Result bypassing

19. 21264 Superscalar Execution The 21264 can decode, rename, issue, execute, and commit 4 insns/cycle How does circuit complexity scale with W in the following operations? – Instruction decode: O(W) – Register renaming: O(W 2 ) – Result bypassing: O(W 2 )

20. 21264 Superscalar Execution The 21264 can decode, rename, issue, execute, and commit 4 insns/cycle How does circuit complexity scale with W in the following operations? – Instruction decode: O(W) – Register renaming: O(W 2 ) – Result bypassing: O(W 2 ) What about issue window complexity?

21. 21264 Superscalar Execution 21264 couldn’t fit full bypassing into one clock cycle Instead, they fully bypass within each of two clusters; inter-cluster bypass takes another cycle

22. 21264 Instruction Reordering As mentioned earlier, 21264 uses explicit renaming, as opposed to data-in-ROB design What does ROB hold?

23. Memory Ordering in the 21264 To execute the critical instruction path quickly, want to execute loads ASAP Initially, loads speculatively bypass stores On a misspeculation, set a “wait” bit for that load’s PC, so it will behave conservatively from then on Clear wait bits periodically

24. Speculation in the 21264 What does the 21264 speculate on? – Next I$ line/way – Branches, indirect jumps – Exceptions – Load/Store ordering – Load hit/miss Shortens hit time by a cycle – Anything else?