CSC 4250 Computer Architectures

1. CSC 4250Computer Architectures September 29, 2006Appendix A. Pipelining

2. Static Pipeline Scheduling • Simple pipeline fetches an instruction, decodes it, and checks for hazards (structural and data) • If no hazard, then issue instruction • If there is hazard, then stall pipeline ─ no new instructions will be fetched or issued • Compiler may schedule instructions to avoid the hazard ─ static scheduling

3. Dynamic Pipeline Scheduling • Hardware rearranges instruction execution to reduce stalls • Scoreboarding technique of CDC6600 • Tomasulo’s algorithm (Chapter 3) • We do in-order instruction issue ─ if an instruction is stalled in the pipeline, then no later instructions can proceed • What if later instructions are independent? • Example: DIV.D F0,F2,F4 ADD.D F10,F0,F8 MUL.D F6,F6,F14 We want to issue and execute MUL instruction while ADD instruction waits for the result of DIV

4. Scoreboarding • In a dynamically scheduled pipeline, all instructions pass through the issue stage in order (in-order issue); however, they can be stalled or they can bypass each other in the second stage (read operands) and enter execution out of order • Scoreboarding is a technique for allowing instructions to execute out of order when there are sufficient resources and no data dependences; it is named after the CDC 6600 scoreboard, which developed this capability

5. First Supercomputer • CDC = Control Data Corporation • In 1964 CDC delivered the first CDC6600 • The machine was unique in many ways • It introduced scoreboarding • It was the first processor to make extensive use of multiple functional units. It had 16 separate FUs, including 4 FP units, 5 units for memory references and 7 units for integer operations • It had peripheral processors that used multithreading • The interaction between pipelining and IS design was understood, and a simple, load-store instruction set was used to promote pipelining

6. Structural and Data Hazards • Before, no instruction issue if there is either structural or data hazard • Data hazards include WAW, RAW and WAR • Now, issue instruction if no structural hazard and no WAW data hazard • Example: DIV.D F0,F2,F4 ADD.D F10,F0,F8 MUL.D F6,F6,F14 • So, all three instructions will be issued • Read operands when no RAW hazards

7. Record Keeping • Every instruction goes through the scoreboard, where a record of the data dependences is constructed; this step corresponds to instruction issue and replaces part of the ID step in the MIPS pipeline • The scoreboard determines when the instruction can read its operands and begin operation (RAW hazards) • If the scoreboard decides that the instruction cannot execute immediately, it monitors every change in the hardware and decides when the instruction can execute • The scoreboard controls when an instruction can write its result into the destination register (WAR hazards)

8. Split ID Stage into Two Stages • Issue ─ Decode instructions; check for structural and WAW hazards • Read operands ─ Wait until no RAW hazards; then read operands No Issue: DIV.D F0,F2,F4 ADD.D F10,F0,F8 SUB.D F6,F6,F14 (why no issue?) No Issue: DIV.D F0,F2,F4 ADD.D F10,F0,F8 MUL.D F0,F6,F14 (why no issue?)

9. MIPS Processor with Scoreboard

10. Four Steps in Execution • Issue ─ if no structural nor WAW hazards • Read operands ─ if no RAW hazards • Execute ─ if both operands are received • Write result ─ if no WAR hazards • We concentrate on FP operations and do not consider a step for memory access

11. Step One. Issue • If a functional unit (FU) for the instruction is free and no other active instruction has the same destination register, the scoreboard issues the instruction to the FU and updates its internal data structure • By ensuring that no other active FU wants to write its result into the destination register, we guarantee that WAW hazards cannot be present • If a structural or WAW hazard exists, then the instruction issue stalls, and no further instructions will issue until these hazards are cleared

12. Step Two. Read Operands • The scoreboard monitors the availability of the source operands. A source operand is available if no earlier issued active instruction is going to write it. • When the source operands are available, the scoreboard tells the FU to proceed to read the operands from the registers and begin execution. • The scoreboard resolves RAW hazards dynamically in this step, and instructions may be sent into execution out of order. • The operands for an instruction are read only when both operands are available in the register file. The scoreboard does not take advantage of forwarding. • Issue and Read Operands together replace the ID stage of the simple MIPS pipeline.

13. Step Three. Execution • The FU begins execution upon receiving operands • When the result is ready, the FU notifies the scoreboard that it has completed execution • This step replaces the EX stage in the MIPS pipeline and takes multiple cycles in the MIPS FP pipeline

14. Step Four. Write Result • Once it is aware that the FU has completed execution, the scoreboard checks for WAR hazards and stalls the completing instruction, if necessary • In general, a completing instruction cannot be allowed to write its results when • There is an instruction that has not read its operands that precedes (i.e., in order of issue) the completing instruction, and • One of the operands is the same register as the result of the completing instruction • If WAR hazard does not exist, or when it clears, the scoreboard tells the FU to store its result to the destination register • This step replaces the WB step in the simple MIPS pipeline

15. Example (p. A-72) L.D F6,34(R2) L.D F2,45(R3) MUL.D F0,F2,F4 SUB.D F8,F6,F2 DIV.D F10,F0,F6 ADD.D F6,F8,F2

16. Scoreboard • Three parts: • Instruction status ─ indicates which of four steps of instruction • Functional unit status ─ busy, op, Fi, Fj, Fk, Qj, Qk, Rj, Rk • Register result status ─ indicates which functional unit will write each register, if instruction is active

17. Example • Code: L.D F6,34(R2) L.D F2, 45(R3) MUL.D F0,F2,F4 SUB.D F8,F6,F2 DIV.D F10,F0,F6 ADD.D F6,F8,F2

18. Scoreboard Tables 1 (Fill in blanks)

19. Scoreboard Tables 2 (Fill in blanks)

20. Scoreboard Tables 3 (Fill in blanks)

21. Scoreboard Tables 4 (Fill in blanks)

22. Required Checks

23. WAR Hazard • WAR hazard exists • if another instr. has this instr.’s destination (Fi[FU]) as a source (Fj[f] or Fk[f]), and • if some other instruction has flagged the register (Rj = Yes or Rk = Yes) • Test on write-result prevents write if WAR hazard exists

24. Costs and Benefits of Scoreboarding • Reported performance improvement of 1.7 for FORTRAN programs and 2.5 for hand-coded assembly language. • Scoreboard had about as much logic as a FU ─ surprisingly low. • Main cost was large number of buses ─ about four times as many as would be required if CPU only executed instructions in order.

25. Factors Limiting Scoreboarding • Amount of Parallelism available among the instructions ─ This determines whether independent instructions can be found to execute. If each instruction depends on its predecessor, no dynamic scheduling scheme can reduce stalls. • Amount of Scoreboard Entries ─ This determines how far ahead the pipeline can look for independent instructions. The set of instructions examined as candidates for potential execution is called the window. The size of the scoreboard determines the size of the window. • Number and Types of FU’s ─ This determines the importance of structural hazards. • Presence of Antidependences and Output Dependences ─ These lead to WAR and WAW stalls.

26. A.9. Fallacies and Pitfalls Unexpected execution may cause unexpected hazards. It looks like that WAW hazards should never occur in a code sequence because no compiler would ever generate two writes to the same register without an intervening read. But they can occur when the sequence is unexpected. For example, the first write might be in the delay slot of a taken branch. Here is an example: BNEZ R1,foo DIV.D F0,F2,F4; moved into delay slot ; from fall through ….. ….. foo: L.D F0,qrs If the branch is taken, then before DIV.D can complete, the L.D will reach WB, causing a WAW hazard.

27. How Extensive Pipelining Affects Performance • Extensive pipelining can impact other aspects of a design, leading to overall worse cost-performance • The best example of this phenomenon comes from two implementations of the VAX, the 8600 and the 8700 • When the 8600 was initially delivered, it had a cycle time of 80ns. Subsequently, a redesigned version called the 8650 with a 55 ns clock was introduced. • The 8700 had a much simpler pipeline that operated at the microinstruction level, yielding a smaller CPU with a faster clock cycle of 45ns • The overall outcome is that the 8650 had a CPI advantage of about 20%, but the 8700 had a clock rate that was about 20% faster. Thus, the 8700 achieved the same performance with much less hardware