Pipeline Hazards

5/18/2018 Pipeline Hazards

1/94Pipeline HazardsCSCE430/830

Pipeline: Hazards

CSCE430/830 Computer Architecture

Lecturer: Prof Hon! "ian!

Courtes# of Prof $ifen! %hu& ' of (aine

)all& *00+

Portions of these slides are deri,ed from:-a,e Patterson . 'C



Pipelinin! utline

1ntroduction -efinin! Pipelinin!

Pipelinin! 1nstructions

Hazards

Structural hazards -ata Hazards

Control Hazards

Performance

Controller implementation



Pipeline Hazards

2here one instruction cannot immediatel#follo another

#pes of hazards Structural hazards5 attempt to use the same resource 6#

to or more instructions Control hazards5 attempt to ma7e 6ranchin! decisions

6efore 6ranch condition is e,aluated

-ata hazards5 attempt to use data 6efore it is read#

Can ala#s resol,e hazards 6# aitin!



Structural Hazards

Attempt to use the same resource 6# to ormore instructions at the same time

Eample: Sin!le (emor# for instructions anddata

Accessed 6# 1) sta!e Accessed at same time 6# (E( sta!e

Solutions -ela# the second access 6# one cloc7 c#cle& 9

Pro,ide separate memories for instructions data

his is hat the 6oo7 does

his is called a ;Har,ard Architecture

L2

5

RD1

RD2

RN1

RN2

WN

WD

RegisterFile

ALU

EXTND

16 32

RD

WD

Data

Memory

ADDR

32

MUX



5

RD1

RD2

RN1

RN2

WN

WD

RegisterFile

ALU

EXTND

16 32

RD

WD

Data

Memory

ADDR

32

MUX


8/94

Pipeline HazardsCSCE430/830

5

RD1

RD2

RN1

RN2

WN

WD

RegisterFile

ALU

EXTND

16 32

RD

WD

Data

Memory

ADDR

32

MUX


9/94


5

RD1

RD2

RN1

RN2

WN

WD

RegisterFile

ALU

EXTND

16 32

RD

WD

Data

Memory

ADDR

32

MUX


10/94


Eecutin! (ultiple 1nstructionsCloc7 C#cle ?

5

RD1

RD2

RN1

RN2

WN

WD

RegisterFile

ALU

EXTND

16 32

RD

WD

Data

Memory

ADDR

32

MUX


11/94


Eecutin! (ultiple 1nstructionsCloc7 C#cle +

5

RD1

RD2

RN1

RN2

WN

WD

RegisterFile

ALU

EXTND

16 32

RD

WD

Data

Memory

ADDR

32

MUX


12/94


Eecutin! (ultiple 1nstructionsCloc7 C#cle @

5

RD1

RD2

RN1

RN2

WN

WD

RegisterFile

ALU

EXTND

16 32

RD

WD

Data

Memory

ADDR

32

MUX


13/94


Eecutin! (ultiple 1nstructionsCloc7 C#cle 8

5

RD1

RD2

RN1

RN2

WN

WD

RegisterFile

ALU

EXTND

16 32

RD

WD

Data

Memory

ADDR

32

MUX


14/94


Alternati,e ie 5 (ultic#cle -ia!ram

IM REG ALU DM REGlw $r0, 10($r1)

sw $r3, 20($r4)

add $r5, $r6, $r7

CC 1 CC 2 CC 3 CC 4 CC 5 CC 6 CC 7

IM REG ALU DM REG

IM REG ALU DM REG

sub $r8, $r9, $r10 IM REG ALU DM REG

CC 8


15/94


Alternati,e ie 5 (ultic#cle -ia!ram

IM REG ALU DM REGlw $r0, 10($r1)

sw $r3, 20($r4)

add $r5, $r6, $r7

CC 1 CC 2 CC 3 CC 4 CC 5 CC 6 CC 7

IM REG ALU DM REG

IM REG ALU DM REG

sub $r8, $r9, $r10 IM REG ALU DM REG

CC 8

(emor# Conflict


16/94


ne (emor# Port Structural Hazards

I

nstr.

Order

Time (clock cycles)

Load

Instr 1

Instr 2

Stall

Instr 3

Reg

ALU DMemIfetch Reg

Reg

ALU DMemIfetch Reg

Reg

ALU DMemIfetch Reg

Cycle 1 Cycle Cycle ! Cycle " Cycle # Cycle $Cycle %

Reg

ALU DMemIfetch Reg

&'le &'le &'le &'le&'le


17/94


Structural Hazards

Some common Structural Hazards: (emor#:

eB,e alread# mentioned this one

)loatin! point: Since man# floatin! point instructions re=uire man# c#cles& itBs eas# for

them to interfere ith each other Startin! up more of one t#pe of instruction than there are

resources )or instance& the PA58+00 can support to AL' to load/store

instructions per c#cle 5 thatBs ho much hardare it has a,aila6le


18/94


Structural Hazards

-ealin! ith Structural HazardsStall

low cost, simple

Increases CPI

use for rare case since stalling has performance effect

Pipeline hardare resource

useful for multi-cycle resources

good performance

sometimes complex e.g., R!

9eplicate resource good performance

increases cost "# may$e interconnect delay%

useful for cheap or di&isi$le resources


19/94


Structural Hazards

Structural hazards are reduced ith these rules: Each instruction uses a resource at most once

Ala#s use the resource in the same pipeline sta!e

'se the resource for one c#cle onl#

(an# 91SC 1SAs are desi!ned ith this in mind

Sometimes ,er# difficult to do this )or eample& memor# of necessit# is used in the 1) and (E(

sta!es


20/94


Structural Hazards

2e ant to compare the performance of to machines 2hich machine is fasterD

(achine A: -ual ported memor# 5 so there are no memor# stalls (achine : Sin!le ported memor#& 6ut its pipelined implementation has a cloc7

rate that is >0? times faster

Assume:

1deal CP1 > for 6oth

Loads are 40F of instructions eecuted


21/94


Speed 'p E=uations for Pipelinin!

ielined

d'nieline

TimeCycleTimeCycle

C*Ist+ll*ielineC*IIde+ldeth*ielineC*IIde+l,eed'

ielined

d'nieline

TimeCycle

TimeCycle

C*Ist+ll*ieline1

deth*ieline,eed'

Inst)ercycles,t+llA-er+geC*IIde+lC*I)i)elined

or simle RI,C ieline/ C*I 0 1


22/94


Structural Hazards

2e ant to compare the performance of to machines 2hich machine is fasterD

(achine A: -ual ported memor# 5 so there are no memor# stalls (achine : Sin!le ported memor#& 6ut its pipelined implementation has a >0?

times faster cloc7 rate

Assume:

1deal CP1 > for 6oth

Loads are 40F of instructions eecuted

Speedp!" #pelne %ept&'(1 0) (*lo*+unp$pe'*lo*+p$pe)

" #pelne %ept&

Speedp," #pelne %ept&'(1 0-4 1)

(*lo*+unp$pe'(*lo*+unp$pe ' 1-05)

" (#pelne %ept&'1-4) 1-05 " 0-75 #pelne %ept&

Speed'pA/ Speed'p/ Pipeline -epth / G0@? Pipeline -epth >33

(achine A is >33 times faster


23/94


Pipelinin! Summar#

Speed 'p I Pipeline -epthJ if ideal CP1 is >& then:

Hazards limit performance on computers: Structural: need more H2 resources

-ata G9A2&2A9&2A2

Control

Speedup Pipeline -epth

> Pipeline stall CP1

KCloc7 C#cle 'npipelined

Cloc7 C#cle Pipelined


24/94


9e,ie

Speedup Pipeline -epth> Pipeline stall CP1

K Cloc7 C#cle 'npipelinedCloc7 C#cle Pipelined

,eed' of ieline


25/94


Pipelinin! utline



Hazards


Control Hazards

Performance



26/94


Pipeline Hazards


#pes of hazards Structural hazards5 attempt to use same resource tice

Control hazards5 attempt to ma7e decision 6eforecondition is e,aluated




27/94


-ata Hazards

-ata hazards occur hen data is used 6efore

it is read#

I! Reg

I! Reg

CC ' CC ( CC ) CC * CC + CC

ime "in cloc cycles%

su$ /(, /', /)

Programexecutionorder"in instructions%

and /'(, /(, /+

I! Reg 0! Reg

I! 0! Reg

I! 0! Reg

CC 1 CC 2 CC 3

'4 '4 '4 '4 '45 (4 (4 (4 (4 (4

or /'), /, /(

add /'*, /(, /(

sw /'+, '44"/(%

6alue ofregister /(7

0! Reg

Reg

Reg

Reg

0!

he use of the result of the S' instruction in the net three instructions causes adata hazard& since the re!ister * is not ritten until after those instructions read it


28/94


-ata Hazards9ead After 2rite G9A2

1nstr"tries to read operand 6efore 1nstr1 rites it

Caused 6# a ;-ependence< Gin compiler nomenclature hishazard results from an actual need for communication

8xecution 9rder is71nstr1

1nstr"

I. add r1,r2,r3

/. sub r4,r1,r3


29/94


-ata Hazards2rite After 9ead G2A9

1nstr"tries to rite operand before1nstr1 reads i Mets ron! operand

Called an ;anti5dependence< 6# compiler ritershis results from reuse of the name ;r>


30/94


-ata Hazards2rite After 2rite G2A2

1nstr"tries to rite operand before1nstr1rites it Lea,es ron! result G 1nstr1not 1nstr"

Called an ;output dependence< 6# compiler ritershis also results from the reuse of name ;r>


31/94


-ata Hazard -etection in (1PS G>

I! Reg

I! Reg



su$ /(, /', /)


and /'(, /(, /+

I! Reg 0! Reg

I! 0! Reg

I! 0! Reg

CC 1 CC 2 CC 3

'4 '4 '4 '4 '45 (4 (4 (4 (4 (4

or /'), /, /(

add /'*, /(, /(

sw /'+, '44"/(%

6alue of

register /(7

0! Reg

Reg

Reg

Reg

0!

I2ID ID234 342M3M M3M25&

1+342M3M.RegisterRd0 ID234.RegisterRs

1342M3M.RegisterRd 0 ID234.RegisterRt

+M3M25&.RegisterRd 0 ID234.RegisterRsM3M25&.RegisterRd 0 ID234.RegisterRt

Re+d +fter 5rite

34 h+6+rd

M3M h+6+rd


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-ata Hazards

Solutions for -ata Hazards Stallin!

)orardin!:

connect ne ,alue directl# to net sta!e

9eorderin!


33/94


-ata Hazard 5 Stallin!

0 2 4 6 8 10 12

IF ID EX MEM

16

add$s0,$t0,$t1

STALL

18

sub $t2,$s0,$t3 IF EX MEM

STALL

BUBBLE BUBBLE BUBBLE BUBBLE

BUBBLEBUBBLE BUBBLE BUBBLE BUBBLE

$s0writtenhere

Ws0

WB

$s0 readhere

Rs0

BUBBLE


34/94


-ata Hazards 5 Stallin!

Simple Solution to 9A2

:ardware detects R; and stalls ssumes register written then read each cycle

# low cost to implement, simple-- reduces IPC

ry to minimi


35/94


-ata Hazards 5 )orardin!

Ne# idea: connect ne ,alue directl# to net sta!e

Still read s0& 6ut i!nore in fa,or of ne result

Pro6lem: hat a6out load instructionsD

ID

0 2 4 6 8 10 12

IF ID EX MEM

16

add$s0,$t0,$t1

18


Ws0

WBRs0

new valueof s0


36/94


-ata Hazards 5 )orardin!

SALL still re=uired for load 5 data a,ail after (E(

(1PS architecture calls this dela#ed load& initialimplementations re=uired compiler to deal ith this

ID

0 2 4 6 8 10 12

IF ID EX MEM

16

lw$s0,20($t1)

18


Ws0

WBRs0

new valueof s0

STALL

BUBBLE BUBBLE BUBBLE BUBBLE BUBBLE


37/94


-ata Hazardshis is anotherrepresentation

of the stall

L2 9>& 0G9* 1) 1- EK (E( 2

S' 94& 9>& 9? 1) 1- EK (E( 2

AO- 9+& 9>& 9@ 1) 1- EK (E( 2

9 98& 9>& 9 1) 1- EK (E( 2

L2 9>& 0G9* 1) 1- EK (E( 2

S' 94& 9>& 9? 1) 1- stall EK (E( 2

AO- 9+& 9>& 9@ 1) stall 1- EK (E( 2

9 98& 9>& 9 stall 1) 1- EK (E( 2


38/94


)orardin!

I! Reg

I! Reg


-ime "in cloc. cycles%

su$ /(, /', /)


and /'(, /(, /+

I! Reg 0! Reg

I! 0! Reg

I! 0! Reg

CC 1 CC 2 CC 3

'4 '4 '4 '4 '45 (4 (4 (4 (4 (4

or /'), /, /(

add /'*, /(, /(

sw /'+, '44"/(%


0! Reg

Reg

Reg

Reg

0!

I2ID ID234 342M3M M3M25&

7o8 8o'ld yo' design the for8+rding9

:ey ide+ connect d+t+ intern+lly efore it;s stored


39/94


Oo )orardin!


40/94


-ata Hazard Solution: )orardin!

Ne# idea: connect data internall# 6efore itQs

stored

I! Reg

I! Reg



su$ /(, /', /)

Programexecution order"in instructions%

and /'(, /(, /+

I! Reg 0! Reg

I! 0! Reg

I! 0! Reg

CC 1 CC 2 CC 3

'4 '4 '4 '4 '45 (4 (4 (4 (4 (4

or /'), /, /(

add /'*, /(, /(

sw /'+, '44"/(%

6alue of register /( 7

0! Reg

Reg

Reg

Reg

A A A (4 A A A A A6alue of 8A5!8! 7

A A A A (4 A A A A6alue of !8!5;= 7

0!

Assumption:he re!ister file forards ,alues that are read

and ritten durin! the same c#cle


41/94


-ata Hazard Summar#

hree t#pes of data hazards 9A2 G(1PS 2A2 Gnot in (1PS

2A9 Gnot in (1PS

Solution to 9A2 in (1PS Stall

)orardin!

-etection Control EK hazard

(E( hazard

A stall is needed if read a re!ister after a loadinstruction that rites the same re!ister

9eorderin!


42/94


9e,ie

Speedup Pipeline -epth> Pipeline stall CP1

K Cloc7 C#cle 'npipelinedCloc7 C#cle Pipelined

,eed' of ieline


43/94


Pipelinin! utline



Hazards


Control Hazards

Performance



44/94


-ata Hazard 9e,ie

hree t#pes of data hazards 9A2 Gin (1PS and all others 2A2 Gnot in (1PS 6ut man# others

2A9 Gnot in (1PS 6ut man# others

)orardin!

9 i - t H d ) di


45/94


9e,ie: -ata Hazards )orardin!

S' s0& t0& t> Js0 t0 5 t>

A-- t*& s0& t3 Jt* s0 t3

S'

A--

1) 1- EK (E( 2

1) 1- EK (E( 2

EK Hazard: S' result not ritten until its 2& read# at end

of its EK& needed at start of A--Bs EK

EK/(E( )orardin!: forard s0 from EK/(E( to AL' inputin A-- EK sta!e GCC4

Note: can occur in sequentialinstructions

1 ! " % #

9 i - t H d ) di


46/94



S' s0& t0& t> Js0 t0 5 t>

A-- t*& s0& t3 Jt* s0 t3

S'

A--

1) 1- EK (E( 2

1) 1- EK (E( 2

EK Hazard -etection 5 EK/(E( )orardin! Conditions:

If ((EX/MEM.RegWrite = 1) & (EX/MEM.RegRD= ID/EX.RegRS))

If ((EX/MEM.RegWrite = 1) & (EX/MEM.RegRD = ID/EX.RegRT))

Then forward EX/MEM resut to EX stage

Note: In PH3, also check that EX/MEM.RegRD 0

1 ! " % #

9 i - t H d ) di


47/94



S' s0& t4& s3 Js0 t4 s3

A-- t*& s>& t> Jt* s0 t>9 s*& t3& s0 Js* t3 9 s0

S'

A--

9

1) 1- EK (E( 2

1) 1- EK (E( 2

1) 1- EK (E( 2

(E( Hazard: S' result not ritten until its 2& stored in

(E(/2& needed at start of 9Bs EK

(E(/2 )orardin!: forard s0 from (E(/2 to AL'input in 9 EK sta!e GCC?

Note: can occur in instructions In& In+2

1 ! " % #

9 i - t H d ) di


48/94



S' s0& t4& s3 Js0 t4 s3

A-- t*& s>& t> Jt* s0 t>9 s*& t3& s0 Js* t3 9 s0

S'

A--

9

1) 1- EK (E( 2

1) 1- EK (E( 2

1) 1- EK (E( 2

(E( Hazard -etection 5 (E(/2 )orardin! Conditions:If ((MEM/W!.RegWrite = 1) & (MEM/W!.RegRD = ID/EX.RegRS))

If ((EX/MEM.RegWrite = 1) & (EX/MEM.RegRD = ID/EX.RegRT))

Then forward MEM/W! resut to EX stage

Note: In PH3, also check that MEM/WB.RegRD 0

1 ! " % #


49/94


-ata Hazard -etection in (1PS

I! Reg

I! Reg



su$ /(, /', /)

Programexecution

order"in instructions%

and /'(, /(, /+

I! Reg 0! Reg

I! 0! Reg

I! 0! Reg

CC 1 CC 2 CC 3

'4 '4 '4 '4 '45 (4 (4 (4 (4 (4

or /'), /, /(

add /'*, /(, /(

sw /'+, '44"/(%


0! Reg

Reg

Reg

Reg

0!

I2ID ID234 342M3M M3M25&

1+342M3M.RegisterRd 0 ID234.RegisterRs

1342M3M.RegisterRd 0 ID234.RegisterRt+M3M25&.RegisterRd 0 ID234.RegisterRsM3M25&.RegisterRd 0 ID234.RegisterRt

*rolem9

342M3M.Reg5rite m'st e +sserted00Gt0 Js0 memor# ,alue

A-- t*& s0& t3 Jt* s0 t3

L2

A--

1) 1- EK (E( 2

1) 1- EK (E( 2

L2 doesnBt rite s0 to 9e! )ileuntil the end of CC?& 6utA-- reads s0 from 9e! )ile in CC3

1 ! " % #

-ata Hazards Stalls


60/94


-ata Hazards Stalls

L2 s0& >00Gt0 Js0 memor# ,alueA-- t*& s0& t3 Jt* s0 t3

L2

A--

1) 1- EK (E( 2

1) 1- EK (E( 2

EK/(E( forardin! onBt or7& 6ecause the data isnBtloaded from memor# until CC4 Gso itBs not in EK/(E(re!ister

1 ! " % #

-ata Hazards Stalls


61/94


-ata Hazards Stalls


L2

A--

1) 1- EK (E( 2

1) 1- EK (E( 2

(E(/2 forardin! onBt or7 either& 6ecause A--eecutes in CC4

1 ! " % #

-ata Hazards Stalls: implementation


62/94




L2

A--

1) 1- EK (E( 2

1) 1- 1- EK (E( 2

2e must handle this hazard 6# ;stallin!< the pipelinefor > Cloc7 C#cle G6u66le

"u""

e

1 ! " % #



63/94




L2

A--

1) 1- EK (E( 2

1) 1- 1- EK (E( 2

2e can then use (E(/2 forardin!& 6ut of course thereis still a performance loss

"u""

e

1 ! " % #



64/94



Stall 1mplementation T>: Compiler detects hazard and

inserts a OP Gno re! chan!es GSLL 0& 0& 0

L2 s0& >00Gt0 Js0 memor# ,alue

OP Jdumm# instruction

A-- t*& s0& t3 Jt* s0 t3

L2

OP

A--

1) 1- EK (E( 2

1) 1- EK (E( 2

1) 1- EK (E( 2

"u""e

"u""e

"u""e

"u""e

"u""e

Pro6lem: e ha,e to rel# on the compiler

1 ! " % #



65/94



Stall 1mplementation T*: Add a ;hazard detection unit< tostall current instructionfor > CC if:

1-5Sta!e Hazard -etection and Stall Condition:

If ((ID/EX.Me#Read = 1) & $on% a W reads #e#

((ID/EX.RegRT = I'/ID.RegRS) $RS wi read oad dest (RT)(ID/EX.RegRT = I'/ID.RegRT))) $RT wi read oad dest

L2 s0& >00Gt0 Js0 memor# ,alue

A-- t*& s0& t3 Jt* s0 t3

L2

A--

1) 1- EK (E( 2

1) 1- EK (E( 2



66/94



he effect of this stall ill 6e to repeat the 1- Sta!e of thecurrent instruction hen e do the (E(/2 forardin! onthe net Cloc7 C#cle

L2

A--

1) 1- EK (E( 2

1) 1- 1- EK (E( 2

2e do this 6# preser,in! the current ,alues in 1)/1- for useon the net Cloc7 C#cle

-ata Hazards: A Classic Eample


67/94


-ata Hazards: A Classic Eample

1dentif# the data dependencies in thefolloin! code 2hich of them can 6eresol,ed throu!h forardin!D

S' *& >& 39 >*& *& ?

S2 >3& >00G*

A-- >4& *& *

L2 >?& >00G*

A-- 4& @& >?

-ata Hazards 5 9eorderin!


68/94


!1nstructions

Assumin! e ha,e data forardin!& hat arethe hazards in this codeD

lw $t0, 0($t1)lw $t2, 4($t1)sw $t2, 0($t1)sw $t0, 4($t1)

9eorder instructions to remo,e hazard:lw $t0, 0($t1)lw $t2, 4($t1)sw $t0, 4($t1)sw $t2, 0($t1)

-ata Hazard Summar#


69/94


-ata Hazard Summar#

hree t#pes of data hazards 9A2 G(1PS

2A2 Gnot in (1PS

2A9 Gnot in (1PS

Solution to 9A2 in (1PS

Stall )orardin!

-etection Control EK hazard

(E( hazard

A stall is needed if read a re!ister after a loadinstruction that rites the same re!ister

9eorderin!

Pipelinin! utline


70/94


p !Oet class



Hazards


Control Hazards

Performance


Pipeline Hazards


71/94


Pipeline Hazards


#pes of hazards Structural hazards5 attempt to use same resource tice

Control hazards5 attempt to ma7e decision 6eforecondition is e,aluated



Control Hazards


72/94


Control Hazards

A control hazard is hen e need to find the

destination of a 6ranch& and canBt fetch an# neinstructions until e 7no that destination

A 6ranch is either

a7en: PC I PC 4 1mmediate Oot a7en: PC I PC 4

Control Hazard on ranchesControl Hazards


73/94


Control Hazard on rancheshree Sta!e Stall

Control Hazards

>0: 6e=r>&r3&3+

>4: and r*&r3&r?

>8: or r+&r>&r@

**: add r8&r>&r

3+: or r>0&r>&r>>

9e!

AL' -(em1fetch 9e!

9e!

AL' -(em1fetch 9e!

9e!

AL' -(em1fetch 9e!

9e!

AL' -(em1fetch 9e!

9e!

AL' -(em1fetch 9e!

The en+lty 8hen r+nch t+ke is ! cycles

Pipeline Hazards

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