Performance Enhancement with Pipelining

Aneesh RaveendranCentre for Development of Advanced

Computing, INDIA

• What is pipelining ?• Pipeline Taxonomies• Instruction Pipelines• MIPS Instruction Pipeline• Pipeline Hazards• MIPS Pipelined Datapath• Load Word Instruction Example• Pipeline Datapath Example• Pipeline Control• Pipeline Instruction Example

• Pipeline Hazards• Control Hazards• Data Hazards• Detecting Data Hazards• Resolving Data Hazards• Forwarding Example• Stalling Example• Branch Hazards• Branching Example• Key terms

• There are two main ways to increase the performance of a processor through high-level system architecture• Increasing the memory access speed• Increasing the number of supported concurrent

operations• Pipelining !• Parallelism ?

• Pipelining is the process by which instructions are parallelized over several overlapping stages of execution, in order to maximize datapath efficiency

• Pipelining is analogous to many everyday scenarios

• Car manufacturing process• Batch laundry jobs• Basically, any assembly-line operation applies

• Two important concepts:• New inputs are accepted at one end before previously

accepted inputs appear as outputs at the other end;• The number of operations performed per second is

increased, even though the elapsed time needed to perform any one operation remains the same

Looking at the textbook’s example, we have a 4-stage pipeline of laundry tasks:

1. Place one dirty load of clothes into washer

2. Place the washed clothes into a dryer

3. Place a dry load on a table and fold

4. Put the clothes away

Graphically speaking:• Sequential (top) vs.• Pipelined (bottom)

execution

• There are two types of pipelines used in computer systems

• Arithmetic pipelines• Used to pipeline data intensive functionalities

• Instruction pipelines• Used to pipeline the basic instruction fetch and execute

sequence

• Other classifications include• Linear vs. nonlinear pipelines

• Presence (or lack) of feedforward and feedback paths between stages

• Static vs. dynamic pipelines• Dynamic pipelines are multifunctional, taking on a different form

depending on the function being executed

• Scalar vs. vector pipelines• Vector pipelines specifically target computations using vector

data

• Let us now introduce the pipeline we’re working with

• It’s a 5-stage instruction, linear, static and scalar pipeline, consisting of the following steps:

• Fetch instruction from Memory (IF)• Read registers while decoding the instruction (ID)• Execute the operation or calculate an address (EX)• Access an operand in data memory (MEM)• Write the result into a register (WB)

• Again, theoretically, pipeline speedup = number of stages in pipeline

Inst. Fetch (2ns), Reg. read/write (1ns), ALU op. (2ns), Data access (2ns)

Clk

Cycle 1

Multiple Cycle Implementation:

Ifetch Reg Exec Mem Wr

Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6 Cycle 7 Cycle 8 Cycle 9 Cycle 10

Load Ifetch Reg Exec Mem Wr

Ifetch Reg Exec Mem

Load Store

Pipeline Implementation:

Ifetch Reg Exec Mem WrStore

Clk

Single Cycle Implementation:

Load Store Waste

Ifetch

R-type

Ifetch Reg Exec Mem WrR-type

Cycle 1 Cycle 2

• Suppose • 100 instructions are executed• The single cycle machine has a cycle time of 45 ns• The multicycle and pipeline machines have cycle times of 10

ns• The multicycle machine has a CPI of 4.6

• Single Cycle Machine• 45 ns/cycle x 1 CPI x 100 inst = 4500 ns

• Multicycle Machine• 10 ns/cycle x 4.6 CPI x 100 inst = 4600 ns

• Ideal pipelined machine• 10 ns/cycle x (1 CPI x 100 inst + 4 cycle drain) = 1040 ns

• Ideal pipelined vs. single cycle speedup• 4500 ns / 1040 ns = 4.33

• What has not yet been considered?

• What makes it easy• all instructions are the same length• just a few instruction formats• memory operands appear only in loads and stores

• What makes it hard?• structural hazards: suppose we had only one memory• control hazards: need to worry about branch instructions• data hazards: an instruction depends on a previous instruction

• We’ll build a simple pipeline and look at these issues

• structural hazards: attempt to use the same resource two different ways at the same time• E.g., two instructions try to read the same memory at the

same time

• data hazards: attempt to use item before it is ready• instruction depends on result of prior instruction still in the

pipelineadd r1, r2, r3sub r4, r2, r1

• control hazards: attempt to make a decision before condition is evaulated• branch instructions

beq r1, loopadd r1, r2, r3

• Can always resolve hazards by waiting• pipeline control must detect the hazard• take action (or delay action) to resolve hazards

What do we need to split the datapath into stages ?

Pipeline registers (buffers) are similar to multicycle processor design

Instruction fetch stage

Instruction decode and register file read stage

Execute or address calculation stage

Memory access stage

Write back stage

Write register number comes from the MEM/WB pipeline register along with the data

Multiple-clock cycle (vs. single-clock cycle) pipelined diagrams

Single-cycle pipeline diagram with one instruction on the pipeline

Single-cycle pipeline diagram with two instructions on the pipeline

• What control signals are required ?• First, notice that the pipeline registers are written every

clock cycle, hence do not require explicit control signals, otherwise:

• Instruction fetch and PC increment• Again, asserted at every clock cycle

• Instruction decode and register file read• Again, asserted at every clock cycle

• Execution and address calculation• Need to select the result register, the ALU operation, and either

Read data 2 or the sign-extended immediate for the ALU

• Memory access• Need to read from memory, write to memory or complete

branch

• Write back• Need to send back either ALU result or memory value to the

register file

Execution/Address Calculation stage control

linesMemory access stage

control lines

Write-back stage control

lines

InstructionReg Dst

ALU Op1

ALU Op0

ALU Src

Branch

Mem Read

Mem Write

Reg write

Mem to Reg

R-format 1 1 0 0 0 0 0 1 0lw 0 0 0 1 0 1 0 1 1sw X 0 0 1 0 0 1 0 Xbeq X 0 1 0 1 0 0 0 X

• Structural hazard• Occurs when a combination of instructions is not supported by the

datapath• For example, a unified memory unit would need to be accessed in

stages 1 (IF) and 4 (MEM), which would cause a contention• Pipeline outright fails in the presence of structural hazards

• Control hazard• Occurs when a decision is made based on the results of one

instructions, while others are executing• For example, a branch instruction is either taken or not• Solutions that exist are stalling and predicting

• Data hazard• Occurs when an instruction depends on the results of an instruction

resident on the pipeline• For example, adding two register contents and storing their result into

a third register, then using that register’s contents for another operation

• Solutions that exist are based on forwarding

• Three major solutions• Stall• Predict• Delayed branch slot

• Stalling involves always waiting for the PC to be updated with the correct address before moving on

• A pipeline stall (or bubble) allows us to perform this wait• Quite costly, as we have to stall even if the branch fails

• Predicting involves guessing whether the branch is taken or not, and acting on that guess

• If correct, then proceed with normal pipeline execution• If incorrect, then stall pipeline execution

• Delayed branch involves executing the next sequential instruction with the branch taking place after that delayed branch slot

• The assembler automatically adjusts the instructions to make it transparent from the programmer

• The instruction has to be safe, as in it shouldn’t affect the branch

• Longer pipelines requires the use of more branch delay slots• Actual MIPS architecture solution

• Forwarding involves providing the inputs to a stage of one instruction before the completion of another instruction

• Valid if destination stage is later in time than the source stage

• Left diagram shows typical forwarding scenario (add then sub)

• Right diagram shows that we still need a stall in the case of a load-use data hazard (load then R-type)

sub $2, $1, $3and $12, $2, $5or $13, $6, $2add $14, $2, $2sw $14,

100($2)

• We could insert “no operation” (nop) instructions to delay the pipeline execution until the correct result is in the register file

sub $2, $1, $3nopnopand $12, $2, $5or $13, $6, $2add $14, $2, $2sw $14, 100($2)

• Too slow as it adds extra useless clock cycles• In reality, we try to find useful instructions to execute

between data-dependent instructions, but this happens too often to be efficient

• Let us try to formalize detecting a data hazard1. EX/MEM.RegisterRd = ID/EX.RegisterRs2. EX/MEM.RegisterRd = ID/EX.RegisterRt3. MEM/WB.RegisterRd = ID/EX.RegisterRs4. MEM/WB.RegisterRd = ID/EX.RegisterRt

sub $2, $1, $3and $12, $2, $5 Data hazard of type #1or $13, $6, $2 Data hazard of type #4add $14, $2, $2 No data hazard – register filesw $14, 100($2) No data hazard – correct operation

• Two modifications are in order• Firstly, we don’t have to forward all the time!

• Some instructions don’t write registers (e.g. beq)• Use RegWrite signal in WB control block to determine condition

• Secondly, the $0 register must always return 0• Can’t limit programmer of using it as a destination register• Use RegisterRd to determine if $0 is being used

1. If (EX/MEM.RegWrite & (EX/MEM.RegisterRd ≠ 0) & (EX/MEM.RegisterRd=ID/EX.RegisterRs)) ForwardA= 10

2. If (EX/MEM.RegWrite & (EX/MEM.RegisterRd ≠ 0) & (EX/MEM.RegisterRd=ID/EX.RegisterRt)) ForwardB= 10

3. If (MEM/WB.RegWrite & (MEM/WB.RegisterRd ≠ 0) & (MEM/WB.RegisterRd=ID/EX.RegisterRs)) ForwardA= 01

4. If (MEM/WB.RegWrite & (MEM/WB.RegisterRd ≠ 0) & (MEM/WB.RegisterRd=ID/EX.RegisterRt)) ForwardB= 01

• Let us examine the hardware changes to our datapath

• Remember that there is no hazard in the WB stage, because the register file is able to be written and read in the same stage

Mux control Source DescriptionForwardA = 00 ID/EX First ALU operand comes from RFForwardA = 01 EX/MEM First ALU operand forwarded from prior ALU resultForwardA = 10 MEM/WB First ALU operand forwarded from data memory or prior ALU resultForwardB = 00 ID/EX Second ALU operand comes from RFForwardB = 01 EX/MEM Second ALU operand forwarded from prior ALU resultForwardB = 10 MEM/WB Second ALU operand forwarded from data memory or prior ALU result

lw $2, 20($1)and $4, $2, $5or $8, $2, $6add $9, $4, $2slt $1, $6, $7

• Let us try to formalize detecting a stalling data hazard• If (ID/EX.MemRead & ((ID/EX.RegisterRt = IF/ID.RegisterRs) or (ID/EX.RegisterRt =

IF/ID/RegisterRt)))

• On the condition being true, we stall the pipeline!

• Other instructions are on the pipeline when we find out whether we take the branch or not!

• Two solutions• Assume branch is not taken• Dynamic branch prediction

• We’ve already discussed the first solution• Note that three instruction stages have to be flushed

when the branch is taken• Done similarly to a data hazard stall (control values set to

0s)

• We can increase branch performance by moving the branch decision to the ID stage (rather than the MEM stage)

• Branch target address calculated by moving adder into ID stage

• Branch decision done by comparing Rs and Rt• Flushing the IF stage instruction involves nop instructions

• Store, in a branch prediction buffer, the history of each branch instruction

• 1-bit requires one wrong prediction to update history table• 2-bits requires two wrong predictions to update history table

• Pipelining vs. Parallelism• Pipeline Stages• Pipeline Taxonomies• MIPS Instruction Pipeline• Structural Hazards• Control Hazards• Data Hazards• Pipeline Registers and Operation• Pipeline Control• Pipeline Throughput• Pipeline Efficiency

• Control Hazard Stalling• Control Hazard Predicting• Control Hazard Delayed Branch• Data Hazard Forwarding• Data Hazard Detection• Forwarding Unit• Data Hazard Stalling• Branch Prediction Buffer

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