EENG449b/Savvides Lec 4.1 1/25/05 January 25 and 25, 2005 Prof. Andreas Savvides Spring g449b EENG 449b/CPSC 439b Computer Systems Lecture 4 Intro to Pipelining

EENG449b/Savvides Lec 4.2 1/25/05 Announcements Reading for this week –Appendix A Homework #1 (Due Feb 10) –Chapter 2: Problems 2.5, 2.11, 2.12, 2.19 –Appendix A: Problems A.1, A.5, A.6, A.7, A.11 Next week overview –Complete pipelining discussion –Different processor architectures –Requirements for simulator discussion

EENG449b/Savvides Lec 4.3 1/25/05 Fast, Pipelined Instruction Interpretation Instruction Register Operand Registers Instruction Address Result Registers Next Instruction Instruction Fetch Decode & Operand Fetch Execute Store Results NI IF D E W NI IF D E W NI IF D E W NI IF D E W NI IF D E W Time Registers or Mem

EENG449b/Savvides Lec 4.4 1/25/05 Sequential Laundry Sequential laundry takes 6 hours for 4 loads If they learned pipelining, how long would laundry take? ABCD PM Midnight TaskOrderTaskOrder Time

EENG449b/Savvides Lec 4.5 1/25/05 Pipelined Laundry Start work ASAP Pipelined laundry takes 3.5 hours for 4 loads ABCD 6 PM Midnight TaskOrderTaskOrder Time

EENG449b/Savvides Lec 4.6 1/25/05 Pipelining Lessons Pipelining doesn’t help latency of single task, it helps throughput of entire workload Pipeline rate limited by slowest pipeline stage Multiple tasks operating simultaneously Potential speedup = Number pipe stages Unbalanced lengths of pipe stages reduces speedup Time to “fill” pipeline and time to “drain” it reduces speedup ABCD 6 PM 789 TaskOrderTaskOrder Time

EENG449b/Savvides Lec 4.7 1/25/05 Instruction Pipelining Execute billions of instructions, so throughput is what matters –except when? What is desirable in instruction sets for pipelining? –Variable length instructions vs. all instructions same length? –Memory operands part of any operation vs. memory operands only in loads or stores? –Register operand many places in instruction format vs. registers located in same place?

EENG449b/Savvides Lec 4.8 1/25/05 Requirements for Pipelining Goal: Start a new instruction at every cycle What are the hardware implications? Two different tasks should not attempt to use the same datapath resource on the same clock cycle. Instructions should not interfere with each other Need to have separate data and instruction memories Need increased memory bandwidth –A 5-stage pipeline operating at the same clock rate as pipelined version requires 5 times the bandwidth Need to introduce pipeline registers Register file used in two places in the ID and WB stages –Perform reads in the first half and writes in the second half.

EENG449b/Savvides Lec 4.9 1/25/05 Pipelining Limitations The CPU clock cannot run faster than the time needed for the slowest pipeline stage Imbalance of pipelining overhead Time needed to fill up the pipeline Pipelining speedup=avg. instruction time w/o pipelining/avg. instruction time w pipelining

EENG449b/Savvides Lec /25/05 Recap: 5 Steps of MIPS Datapath Memory Access Write Back Instruction Fetch Instr. Decode Reg. Fetch Execute Addr. Calc LMDLMD ALU MUX Memory Reg File MUX Data Memory MUX Sign Extend 4 Adder Zero? Next SEQ PC Address Next PC WB Data Inst RD RS1 RS2 Imm

EENG449b/Savvides Lec /25/05 Pipeline Requirements… Need separate instruction and Data memories: Structural Hazard Register file Read in the first half, write in the second half cycle

EENG449b/Savvides Lec /25/05 Add registers between pipeline stages Prevent interference between 2 instructions Carry data from one stage to the next Edge triggered

EENG449b/Savvides Lec /25/05 Pipelining Hazards Hazards: circumstances that would cause incorrect execution if next instruction where launched Structural Hazards:Attempting to use the same hardware to do two different things at the same time Data Hazards:Instruction depends on result of prior instruction still in the pipeline Control Hazards:Caused by delay between the fetching of instructions and decisions about changes in control flow (branches and jumps) Common Solution: “Stall” the pipeline, until the hazard is resolved by inserting one or more “bubbles” in the pipeline

EENG449b/Savvides Lec /25/05 Data Hazards Occurs when the relative timing of instructions is altered because of pipelining Consider the following code: DADD R1, R2, R3 DSUB R4, R1, R5 AND R6, R1, R7 OR R8, R1, R9 XOR R10, R1, R11 What is the problem here?

EENG449b/Savvides Lec /25/05 Data Hazard

EENG449b/Savvides Lec /25/05 Data Hazards: Data Forwarding

EENG449b/Savvides Lec /25/05 Data Hazards Requiring Stalls LD R1,0(R2) DSUB R4,R1,R5 AND R6,R1,R7 OR R8,R1,R9 HAVE to stall for 1 cycle…

EENG449b/Savvides Lec /25/05 Implementing Stalls for Data Hazard Pipeline Interlock –Hardware: detects a hazard and stall the pipeline until the hazard is detected Interlock function –Stall the pipeline at the instruction that wants to use the result of a hazard –Stall time: from beginning of instruction until the end of the stall

EENG449b/Savvides Lec /25/05 Stall Introduces Bubbles in the Pipeline

EENG449b/Savvides Lec /25/05 Branch Hazards A branch hazard is a control hazard Branch execution –PC=PC+4 (branch not taken) or Something else (branch taken) Simple solution –Redo an instruction fetch –BUT even a 1 cycle stall may cause 10 – 30% program execution overhead Some branch prediction can easily help with performance

EENG449b/Savvides Lec /25/05 Four Branch Hazard Alternatives #1: Stall until branch direction is clear #2: Predict Branch Not Taken –Execute successor instructions in sequence –“Squash” instructions in pipeline if branch actually taken –Advantage of late pipeline state update –47% MIPS branches not taken on average –PC+4 already calculated, so use it to get next instruction #3: Predict Branch Taken –53% MIPS branches taken on average –But haven’t calculated branch target address in MIPS »MIPS still incurs 1 cycle branch penalty »Other machines: branch target known before outcome

EENG449b/Savvides Lec /25/05 Four Branch Hazard Alternatives #4: Delayed Branch –Define branch to take place AFTER a following instruction branch instruction sequential successor 1 sequential successor sequential successor n branch target if taken –1 slot delay allows proper decision and branch target address in 5 stage pipeline –MIPS uses this Branch delay of length n

EENG449b/Savvides Lec /25/05 Delayed Branch Where to get instructions to fill branch delay slot? –Before branch instruction –From the target address: only valuable when branch taken –From fall through: only valuable when branch not taken –Canceling branches allow more slots to be filled Compiler effectiveness for single branch delay slot: –Fills about 60% of branch delay slots –About 80% of instructions executed in branch delay slots useful in computation –About 50% (60% x 80%) of slots usefully filled Delayed Branch downside: 7-8 stage pipelines, multiple instructions issued per clock (superscalar)

EENG449b/Savvides Lec /25/05 Scheduling Branch Delay Independent instruction Cannot be used Preferred when branch taken w/ high prob

EENG449b/Savvides Lec /25/05 Pipelining Performance Issues Consider an unpipelined processor 1ns/instruction cycle Frequency 4 cycles for ALU operations 40% 4 cycles for branches 20% 5 cycles for memory operations 40% Pipelining overhead 0.2ns For the unpipelined processor

EENG449b/Savvides Lec /25/05 Speedup from Pipelining Now if we had a pipelined processor, we assume that each instruction takes 1 cycle BUT we also have overhead so instructions take 1ns ns = 1.2ns

EENG449b/Savvides Lec /25/05 Considering the stall overhead

EENG449b/Savvides Lec /25/05 Performance of Branch Schemes Assuming an ideal CPI of 1:

EENG449b/Savvides Lec /25/05 Instruction Formats Review

EENG449b/Savvides Lec /25/05 Implementing a MIPS Pipeline We are developing a subset of the MIPS pipeline supporting –Load store word –Branch equal zero –Integer ALU Operations Remember MIPS has register-register ALU instructions (e.g Add R1, R2, R3) Attention: In the homework you will have to redesign the pipeline for register-memory instructions for ALU operations (e.g Add R1,R2,(R3)!!!

EENG449b/Savvides Lec /25/05 MIPS Datapath Review

EENG449b/Savvides Lec /25/05 MIPS Datapath Review

EENG449b/Savvides Lec /25/05 MIPS Datapath Review

EENG449b/Savvides Lec /25/05 MIPS Datapath Review

EENG449b/Savvides Lec /25/05 MIPS Basic Pipeline Data needs to be written in the registers at the end of each cycle Depend on instruction type Load or ALU operation LMD ALUOut

EENG449b/Savvides Lec /25/05 Events at every pipe stage

EENG449b/Savvides Lec /25/05 Events at every pipe stage

EENG449b/Savvides Lec /25/05 Controlling the Pipeline

EENG449b/Savvides Lec /25/05 Hazards Review From previous lecture we know the situations that would cause incorrect execution Structural Hazards - Data Hazards - Control Hazards -

EENG449b/Savvides Lec /25/05 Read After Write (RAW) Instr J tries to read operand before Instr I writes it Caused by a “Data Dependence” (in compiler nomenclature). This hazard results from an actual need for communication. Three Generic Data Hazards I: add r1,r2,r3 J: sub r4,r1,r3

EENG449b/Savvides Lec /25/05 Write After Read (WAR) Instr J writes operand before Instr I reads it Called an “anti-dependence” by compiler writers. This results from reuse of the name “r1”. Can’t happen in MIPS 5 stage pipeline because: – All instructions take 5 stages, and – Reads are always in stage 2, and – Writes are always in stage 5 I: sub r4,r1,r3 J: add r1,r2,r3 K: mul r6,r1,r7 Three Generic Data Hazards

EENG449b/Savvides Lec /25/05 Three Generic Data Hazards Write After Write (WAW) Instr J writes operand before Instr I writes it. Called an “output dependence” by compiler writers This also results from the reuse of name “r1”. Can’t happen in MIPS 5 stage pipeline because: – All instructions take 5 stages, and – Writes are always in stage 5 Will see WAR and WAW in later more complicated pipes I: sub r1,r4,r3 J: add r1,r2,r3 K: mul r6,r1,r7

EENG449b/Savvides Lec /25/05 MIPS Basic Pipeline Instruction issued IF ID EX IFWB Data Hazards can be detected here

EENG449b/Savvides Lec /25/05 Hardware Hazard Detection

EENG449b/Savvides Lec /25/05 Logic to Detect Load Interlocks

EENG449b/Savvides Lec /25/05 Forwarding of Results to the ALU Mem output ALU output

EENG449b/Savvides Lec /25/05 Control Hazards Revisited A branch causes a 3-cycle stall in the 5-stage pipeline Branch InstructionIF ID EX MEM WB Branch Successor+1 IF stall stall IF ID EX MEM WB Branch Successor+2 IF ID EX MEM WB Branch Successor+3 IF ID EX MEM WB Higher overhead than data hazards… Can HW changes improve that? YES! Try to make an early decision whether a branch is taken or not.

EENG449b/Savvides Lec /25/05 Improved Pipeline – Dealing with Branches Additional adder in ID stage Write the PC faster Can detect branch hazard 2 cycles earlier

EENG449b/Savvides Lec /25/05 Improved Pipeline – Dealing with Branches Additional adder in ID stage Write the PC faster Note change of order in text! Figure A.11 says a branch hazard would stall for 1 cycle. This is after the optimization in Figure A.24!!! Note the change of order…

EENG449b/Savvides Lec /25/05 Challenges in Pipeline Implementation Exceptions: Situations that can disrupt the in-order execution of instructions (interrupt, fault, exception) I/O device request Invoking an OS service from a user program Breakpoint Integer arithmetic overflow or FP arithmetic anomaly Page fault (not in main memory) Misaligned memory access Hardware failure Power loss etc…

EENG449b/Savvides Lec /25/05 Exceptions Requirements Synchronous vs. Asynchronous –Synchronous: Always at the same memory and time –What is asynchronous? User requested vs. coerced –User requested – not really exceptions –Coerced: not under the control of user program User maskable vs. user non-maskable With vs. between instructions Resume vs. terminate

EENG449b/Savvides Lec /25/05 Exception Types & Classification

EENG449b/Savvides Lec /25/05 Exception Challenges Exceptions happening within instructions Exceptions that need to be restarted – as in the case of a page fault Need to make sure that the occurrence of an exception does not influence the correct execution of a program Pipeline design should ensure that the processor is in a predictable state after the occurrence of an exception –Block writing to pipeline registers

EENG449b/Savvides Lec /25/05 Exceptions in MIPS Pipeline Pipeline StateProblem Exceptions IF Page fault on instruction fetch misaligned memory access memory protection violation ID Undefined or illegal opcode EX Arithmetic exception MEM Page fault on data fetch; misaligned memory access; memory protection violation WB None

EENG449b/Savvides Lec /25/05 Floating-Point Support in Pipelines Floating point operations will take more than 1 or 2 cycles to complete –Structural hazards –Data hazards Multiple functional units required –Loads, stores and integer ALUs –FP and integer multiplier –FP adder that handles FP add, subtract and conversion –FP and integer divider Initiation interval – number of cycles that must elapse before issuing two operations of a given type

EENG449b/Savvides Lec /25/05 Multiple FUs and Latencies Functional UnitLate ncy Initiation Interval Integer ALU01 Data memory (integer and FP Loads) 11 FP add31 FP Multiply61 FP Divide2425

EENG449b/Savvides Lec /25/05 Support for Multiple Outstanding Operations Additional pipeline registers needed

EENG449b/Savvides Lec /25/05 Hazards in Longer Pipelines 1.Divide unit is not fully pipelined - structural hazards can occur 2.Instructions have varying running times so the number of register writes required in a cycle can be larger than 1. 3.WAW hazards are possible, since instructions don’t reach WB in order 4.Instructions can complete in different order than the one they were issued causing problems with exceptions 5.Because of longer latency of operations, stalls for RAW hazards will be more frequent

EENG449b/Savvides Lec /25/05 FP Pipeline Hazards Example Figure A.34 Simultaneous writeback Stall an instruction in the ID stage Stall the instruction when it tries to enter WB

EENG449b/Savvides Lec /25/05 Checks for Detecting Hazards Three checks to be performed before a multicycle instruction can issue in the ID stage: Check for structural hazards –A structural unit is not busy and a write register port is available when needed Check for a RAW data hazard –Wait until the source registers are not listed as pending destinations Check for WAW data hazard –Determine an instruction that already issued has the same destination as this instruction. If so stall the instruction issue in ID.

EENG449b/Savvides Lec /25/05 MIPS R4000 Pipeline Decompose the 5-stage pipeline to a deeper 8-stage pipeline(superpipeline) –achieve higher clock rates => better performance Extra stages come from decomposing memory accesses Longer pipelines increase the amount of forwarding and branch delays

EENG449b/Savvides Lec /25/05 Branch Delay Cycles Branch outcome needs 3 cycles

EENG449b/Savvides Lec /25/05 Dynamic Scheduled Pipelines Simple pipelines result in hazards that require stalling. Static scheduling – compilers rearrange instructions to avoid stalls. Dynamic scheduling – processor executes instructions out-of-order to minimize stalls Dynamic scheduling requires splitting the ID stage into stages: –Issue – Decode instructions, check for structural hazards –Read operands – Wait until there are no data hazards, then read operands –Also need to know when each instruction begins and ends execution Requires a lot more bookkeeping! More when we discuss Tomasulo’s algorithm in chapter 3…

EENG449b/Savvides Lec /25/05 Scoreboarding Scoreboarding – a technique that allows out- of-order execution when resources are available and there are no data dependencies – originated in CDC6600 in the mid 60s. Scoreboard fully responsible for instruction execution and hazard detection –Requires changes in # of functional units and latency of operations –Needs to keep track of status of all instructions in execution

EENG449b/Savvides Lec /25/05 Scoreboarding II

EENG449b/Savvides Lec /25/05 More Hazards WAR and WAW hazards are now possible! DIV.D F0, F2, F4 ADD.D F10, F0, F8 SUB.D F8, F8, F14 DIV.D F0, F2, F4 ADD.D F10, F0, F8 SUB.D F10, F8, F14 WAR! If SUB.D Executes first WAW! If SUB.D Executes first

EENG449b/Savvides Lec /25/05 Refer to figures A.52 – A.54 for example scoreboard tables Scoreboarding is limited by: Amount of parallelism among instructions The number of scoreboard entries The number and types of functional units Presence of antidependencies and output dependencies