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Spri
ng 2
005
CSE
P54
81
Inst
ruct
ion
-Lev
el P
aral
lelis
m (
ILP
)
Fin
e-g
rain
ed p
aral
lelis
m
Obt
aine
d by
:
•in
stru
ctio
n ov
erla
p in
a p
ipel
ine
•ex
ecut
ing
inst
ruct
ions
in p
aral
lel (
late
r, w
ith m
ultip
le in
stru
ctio
n is
sue)
In c
ontr
ast t
o:
•lo
op
-lev
elpa
ralle
lism
(m
ediu
m-g
rain
ed)
•p
roce
ss-l
evel
or t
ask-
leve
lor
thre
ad-l
evel
para
llelis
m (
coar
se-
grai
ned)
Spri
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005
CSE
P54
82
Inst
ruct
ion
-Lev
el P
aral
lelis
m (
ILP
)
Can
be
expl
oite
d w
hen
inst
ruct
ion
oper
ands
are
ind
epen
den
tof
eac
h ot
her,
for
exam
ple,
•tw
o in
stru
ctio
ns a
re in
depe
nden
t if t
heir
oper
ands
are
diff
eren
t
•an
exa
mpl
e of
inde
pend
ent i
nstr
uctio
ns
Eac
h th
read
(pr
ogra
m)
has
a fa
ir am
ount
of p
oten
tial I
LP
•ve
ry li
ttle
can
be e
xplo
ited
on to
day’
s co
mpu
ters
•re
sear
cher
s tr
ying
to in
crea
se it
ld R1, 0(R2)
or R7, R3, R8
Spri
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005
CSE
P54
83
Dep
end
ence
s
dat
a d
epen
den
ce: a
rises
from
the
flow
of v
alue
s th
roug
h pr
ogra
ms
•co
nsum
er in
stru
ctio
n ge
ts a
val
ue fr
om a
pro
duce
r in
stru
ctio
n•
dete
rmin
es th
e or
der
in w
hich
inst
ruct
ions
can
be
exec
uted
nam
e d
epen
den
ce: i
nstr
uctio
ns u
se th
e sa
me
regi
ster
but
no
flow
of d
ata
betw
een
them
•an
tid
epen
den
ce
•o
utp
ut
dep
end
ence
ld R1, 32(R3)
add R3, R1, R8
ld R1, 32(R3)
add R3, R1, R8
ld R1,
16 (
R3)
Spri
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005
CSE
P54
84
Dep
end
ence
s
con
tro
ldep
ende
nce
•ar
ises
from
the
flow
of c
ontr
ol
•in
stru
ctio
ns a
fter
a br
anch
dep
end
on th
e va
lue
of th
e br
anch
’s
cond
ition
var
iabl
e
Dep
ende
nces
inhi
bit I
LP
beqzR2, target
lwr1, 0(r3)
target:
addr1, ...
Spri
ng 2
005
CSE
P54
85
Pip
elin
ing
Impl
emen
tatio
n te
chni
que
(but
it is
vis
ible
to th
e ar
chite
ctur
e)
•ov
erla
ps e
xecu
tion
of d
iffer
ent i
nstr
uctio
ns
•ex
ecut
e al
l ste
ps in
the
exec
utio
n cy
cle
sim
ulta
neou
sly,
but
on
diffe
rent
inst
ruct
ions
Exp
loits
ILP
by
exec
utin
g se
vera
l ins
truc
tions
“in
par
alle
l”
Goa
l is
to in
crea
se in
stru
ctio
n th
roug
hput
Spri
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005
CSE
P54
86
Pip
elin
ing
Spri
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005
CSE
P54
87
Pip
elin
ing
No
t th
at s
imp
le!
•pi
pelin
e h
azar
ds
(str
uctu
ral,
data
, con
trol
)
•pl
ace
a so
ft “li
mit”
on
the
num
ber
of s
tage
s
•in
crea
se in
stru
ctio
n la
tenc
y (a
littl
e)
•w
rite
& r
ead
pipe
line
regi
ster
s fo
r da
ta th
at is
com
pute
d in
a
stag
e
•tim
e fo
r cl
ock
& c
ontr
ol li
nes
to r
each
all
stag
es
•al
l sta
ges
are
the
sam
e le
ngth
whi
ch is
det
erm
ined
by
the
long
est s
tage
•st
age
leng
th d
eter
min
es c
lock
cyc
le ti
me
IBM
Str
etch
(19
61):
the
first
gen
eral
-pur
pose
pip
elin
ed c
ompu
ter
Spri
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005
CSE
P54
88
Haz
ard
s
Str
uctu
ral h
azar
ds
Dat
a ha
zard
s
Con
trol
haz
ards
Wha
t hap
pens
on
a ha
zard
•in
stru
ctio
n th
at c
ause
d th
e ha
zard
& p
revi
ous
inst
ruct
ions
com
plet
e
•al
l sub
sequ
ent i
nstr
uctio
ns s
tall
until
the
haza
rd is
rem
oved
(in-o
rder
exe
cutio
n)
•on
ly in
stru
ctio
ns th
at d
epen
d on
that
inst
ruct
ion
stal
l(o
ut-o
f-or
der
exec
utio
n)
•ha
zard
rem
oved
•in
stru
ctio
ns c
ontin
ue e
xecu
tion
Spri
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005
CSE
P54
89
Str
uct
ura
l Haz
ard
s
Cau
se:
inst
ruct
ions
in d
iffer
ent s
tage
s w
ant t
o us
e th
e sa
me
reso
urce
in
the
sam
e cy
cle
e.g.
, 4 F
P in
stru
ctio
ns r
eady
to e
xecu
te &
onl
y 2
FP
uni
ts
So
luti
on
s:
•m
ore
hard
war
e (e
limin
ate
the
haza
rd)
•st
all (
tole
rate
the
haza
rd)
•le
ss h
ardw
are,
low
er p
erfo
rman
ce
•on
ly fo
r bi
g ha
rdw
are
com
pone
nts
Spri
ng 2
005
CSE
P54
810
Spri
ng 2
005
CSE
P54
811
Dat
a H
azar
ds
Cau
se:
•an
inst
ruct
ion
early
in th
e pi
pelin
e ne
eds
the
resu
lt pr
oduc
ed b
y an
in
stru
ctio
n fa
rthe
r do
wn
the
pipe
line
befo
re it
is w
ritte
n to
a r
egis
ter
•w
ould
not
hav
e oc
curr
ed if
the
impl
emen
tatio
n w
as n
ot p
ipel
ined
Typ
es RA
W (
data
: flo
w),
WA
R (
nam
e: a
ntid
epen
denc
e), W
AW
(na
me:
ou
tput
)
HW
so
luti
on
s
•fo
rwar
ding
har
dwar
e (e
limin
ate
the
haza
rd)
•st
all v
ia p
ipel
ined
inte
rlock
s
Co
mp
iler
solu
tio
n
•co
de s
ched
ulin
g (f
or lo
ads)
Spri
ng 2
005
CSE
P54
812
Dep
end
ence
s vs
. Haz
ard
s
Spri
ng 2
005
CSE
P54
813
Fo
rwar
din
g
Fo
rwar
din
g(a
lso
calle
d b
ypas
sin
g):
•ou
tput
of o
ne s
tage
(th
e re
sult
in th
at s
tage
’s p
ipel
ine
regi
ster
) is
bu
sed
(byp
asse
d) to
the
inpu
t of a
pre
viou
s st
age
•w
hy fo
rwar
ding
is p
ossi
ble
•re
sults
are
com
pute
d 1
or m
ore
stag
es b
efor
e th
ey a
re w
ritte
n to
a r
egis
ter
•at
the
end
of th
e E
X s
tage
for
com
puta
tiona
l ins
truc
tions
•at
the
end
of M
EM
for
a lo
ad
•re
sults
are
use
d 1
or m
ore
stag
es a
fter
regi
ster
s ar
e re
ad
•if
you
forw
ard
a re
sult
to a
n A
LU in
put a
s so
on a
s it
has
been
co
mpu
ted,
you
can
elim
inat
e th
e ha
zard
or
redu
ce s
talli
ng
Spri
ng 2
005
CSE
P54
814
Fo
rwar
din
g E
xam
ple
Spri
ng 2
005
CSE
P54
815
Fo
rwar
din
g Im
ple
men
tati
on
Fo
rwar
din
g u
nit
che
cks
whe
ther
forw
arde
d va
lues
sho
uld
be u
sed:
•be
twee
n in
stru
ctio
ns in
ID a
nd E
X
•co
mpa
re th
e R
-typ
e d
esti
nat
ion
reg
iste
r n
um
ber
in E
X/M
EM
pipe
line
regi
ster
to e
ach
sou
rce
reg
iste
r n
um
ber
in ID
/EX
•be
twee
n in
stru
ctio
ns in
ID a
nd M
EM
•co
mpa
re th
e R
-typ
e d
esti
nat
ion
reg
iste
r n
um
ber
in M
EM
/WB
to e
ach
sou
rce
reg
iste
r n
um
ber
in ID
/EX
If a
mat
ch, s
et M
UX
to c
hoos
e bu
ssed
val
ues
from
EX
/ME
Mor
ME
M/W
B
Spri
ng 2
005
CSE
P54
816
prod
ucer
prod
ucer
cons
umer
Spri
ng 2
005
CSE
P54
817
Fo
rwar
din
g H
ard
war
e
Har
dwar
e to
impl
emen
t for
war
ding
:
•de
stin
atio
n re
gist
er n
umbe
r in
pip
elin
e re
gist
ers
(but
nee
d it
anyw
ay b
ecau
se w
e ne
ed to
kno
w w
hich
reg
iste
r to
w
rite
whe
n st
orin
g an
ALU
or
load
res
ult)
•so
urce
reg
iste
r nu
mbe
rs(p
roba
bly
only
one
, e.g
., rs
on M
IPS
R2/
3000
)is
ext
ra)
•a
com
para
tor
for
each
sou
rce-
dest
inat
ion
regi
ster
pai
r
•bu
ses
to s
hip
data
and
reg
iste
r nu
mbe
rs −
the
BIG
cost
•la
rger
ALU
MU
Xes
for
2 by
pass
val
ues
Spri
ng 2
005
CSE
P54
818
Lo
ads
Lo
ads •
data
haz
ard
caus
ed b
y a
load
inst
ruct
ion
& a
n im
med
iate
use
of t
he
load
ed v
alue
•fo
rwar
ding
won
’t el
imin
ate
the
haza
rdw
hy?
data
not
bac
k fr
om m
emor
y un
til th
e en
d of
the
ME
M s
tage
•2
solu
tions
use
d to
geth
er
•st
allv
ia p
ipel
ined
inte
rlock
s
•sc
hed
ule
inde
pend
ent i
nstr
uctio
ns in
to th
e lo
add
elay
slo
t(a
pip
elin
e ha
zard
that
is e
xpos
ed to
the
com
pile
r) s
o th
at th
ere
will
be
no s
tall
Spri
ng 2
005
CSE
P54
819
Lo
ads
Spri
ng 2
005
CSE
P54
820
Imp
lem
enti
ng
Pip
elin
ed In
terl
ock
s
Det
ectin
g a
stal
l situ
atio
n
Haz
ard
det
ecti
on
un
it s
talls
the
use
afte
r a
load
•is
the
inst
ruct
ion
in E
X a
load
?
•do
es th
e de
stin
atio
n re
gist
er n
umbe
r of
the
load
= e
ither
sou
rce
regi
ster
num
ber
in th
e ne
xt in
stru
ctio
n?
•co
mpa
re th
e lo
ad w
rite
regi
ster
num
ber
in ID
/EX
to e
ach
read
re
gist
er n
umbe
r in
IF/ID
⇒ ⇒⇒⇒if
both
yes
, sta
ll th
e pi
pe 1
cyc
le
Spri
ng 2
005
CSE
P54
821
Imp
lem
enti
ng
Pip
elin
ed In
terl
ock
s
How
sta
lling
is im
plem
ente
d:
•n
ulli
fy t
he
inst
ruct
ion
in t
he
ID s
tag
e, th
e on
e th
at u
ses
the
load
ed v
alue
•ch
ange
EX
, ME
M, W
B c
ontr
ol s
igna
ls in
ID/E
X p
ipel
ine
regi
ster
to
0
•th
e in
stru
ctio
n in
the
ID s
tage
will
hav
e no
sid
e ef
fect
sas
it
pass
es d
own
the
pipe
line
•re
star
t th
e in
stru
ctio
ns
that
wer
e st
alle
d in
ID &
IF s
tag
es•
disa
ble
writ
ing
the
PC
---
the
sam
e in
stru
ctio
n w
ill b
e fe
tche
d ag
ain
•di
sabl
e w
ritin
g th
e IF
/ID p
ipel
ine
regi
ster
---
the
load
use
in
stru
ctio
n w
ill b
e de
code
d &
its
regi
ster
s re
ad a
gain
Spri
ng 2
005
CSE
P54
822
Lo
ads
haza
rd d
etec
tion
fetc
h ag
ain
deco
de a
gain
Spri
ng 2
005
CSE
P54
823
Imp
lem
enti
ng
Pip
elin
ed In
terl
ock
s
Har
dwar
e to
impl
emen
t sta
lling
:
•rt
reg
iste
r nu
mbe
r in
ID/E
X p
ipel
ine
regi
ster
(but
nee
d it
anyw
ay b
ecau
se w
e ne
ed to
kno
w w
hat r
egis
ter
to w
rite
whe
n st
orin
g lo
ad d
ata)
•bo
th s
ourc
e re
gist
er n
umbe
rs in
IF/ID
pip
elin
e re
gist
er(a
lread
y th
ere)
•a
com
para
tor
for
each
sou
rce-
dest
inat
ion
regi
ster
pai
r
•bu
ses
to s
hip
regi
ster
num
bers
•w
rite
enab
le/d
isab
le fo
r P
C
•w
rite
enab
le/d
isab
le fo
r th
e IF
/ID p
ipel
ine
regi
ster
•a
MU
X to
the
ID/E
X p
ipel
ine
regi
ster
(+
0s)
Triv
ial a
mou
nt o
f har
dwar
e &
nee
ded
for
cach
e m
isse
s an
yway
Spri
ng 2
005
CSE
P54
824
Co
ntr
ol H
azar
ds
Cau
se:
cond
ition
& ta
rget
det
erm
ined
afte
r th
e ne
xt fe
tch
has
alre
ady
been
do
ne
Ear
ly H
W s
olu
tio
ns
•st
all
•as
sum
e an
out
com
e &
flus
h pi
pelin
e if
wro
ng
•m
ove
bran
ch r
esol
utio
n ha
rdw
are
forw
ard
in th
e pi
pelin
e
Co
mp
iler
solu
tio
ns
•co
de s
ched
ulin
g
•st
atic
bra
nch
pred
ictio
n
To
day
’s H
W s
olu
tio
ns
•dy
nam
ic b
ranc
h pr
edic
tion
