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path. Whenentered,thecodeinabridgeisexecutedonlyonce,thenitjumpstothe
originalloop.
 Exit Bridge,drawningrey: : likeaninner r bridge,butinsteadofjumpingbackto
theoriginalloop,it jumpstoanother one. . Exit t bridgessharecharacteristics with
bothinnerandentrybridges:likeinnerbridgestheystartsfromaguardfailure,and
likeentrybridgestheyjumpstoadierentloopattheend.
a = = 0
i = = 0
N = = 100
while i < N:
if not i % 2:
a += 1
else:
a
*
= 2
i += 1
Figure 6.8: : RPython n loop
withtwoequallyhotcodepaths
Aloopislinearas longas allitsguardsarecold. . Whena
guardbecomeshot,aninnerbridgeisgeneratedfromthere
andattachedtotheguarditself: theideaisthat t whenthe
guardfails,theexecutionisdirectlytransferredtothisnew
trace,withoutanyneedofexitingtheruntimephase.Thus,
weeectivelygetatreeoftraces,asdescribedinsection5.6.
Asanexample,takethecodelistedinFigure6.8: itshows
theverysamealgorithmasinFigure5.5writteninRPython.
Sinceitis writteninRPythonitisaninterpreterloop,fol-
lowingthedenitiongiveninSection6.3: itcanbethought
astheequivalentofaninterpretermainloop.Forclarity,the
hints
jit_merge_point
and
can_enter_jit
havebeenomit-
ted. There e are twodierent code pathsintheloop,which
arebothfrequentlyexecutedand,hence,hot.
Supposethatthehotloopthreshold issetto3. . Thetracingphasestartsatthebeginning
ofthefourthiteration,withiequalto3: thus,thecodewillfollowthatpathinsidethe
elsebranch,producingtheloopshowninFigure6.9.Thereareseveralinterestingdetails
tonoteaboutthatloop:
 twoguards s areproduced: : therstonecorrespondstotheconditionofthe
if
,the
secondone to thecondition ofthe
while
loop. The e conditions are in that order
becausetracingstartsjustaftertheloophasbeenentered,sotherststatementit
executesisthe
if
 thelooptakestwoparametersasinput,calledinputarguments.Notethatthejump
attheendloopsbackpassingtheupdatedvaluesforianda
Notealsothattheloopcontainsthecodetohandleonlytheelsebranchofthe
if
: when
atruntimetheconditionis false,the guard
true(t
1
) (highlightedinblue)fails
3
,andthe
executionisswitchedbacktotheinterpretationphase.Eventuallytheguardbecomeshot,
sowestarttracingfromthereandattachthenewlycreatedbridgetotheexistingloop,
52
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loop(i
0
,a
0
)
t
0
= int
mod(i
0
,2)
t
1
=int
is
true(t
0
)
guard
true(t
1
)
a
1
=int
mul(a
0
,2)
i
1
= int
add(i
0
,1)
t
2
=int
lt(i
1
,100)
guard
true(t
2
)
jump(i
1
,a
1
)
Figure 6.9: : The e trace produced
bythecodeinFigure6.8
loop(i
0
,a
0
)
t
0
=int
mod(i
0
,2)
t
1
= int
is
true(t
0
)
guard
true(t
1
)
a
1
= int
mul(a
0
,2)
i
1
= int
add(i
0
,1)
t
2
=int
lt(i
1
,100)
guard
true(t
2
)
jump(i
1
,a
1
)
a
3
=int
add(a
2
,1)
i
3
=int
add(i
2
,1)
t
3
= int
lt(i
3
,100)
guard
true(t
3
)
jump(i
3
,a
3
)
i
2
i
0
a
2
a
0
Figure6.10: Thetracetreeproducedwhentheguard
becomeshot
gettingtheresultshowninFigure6.10.Thefactthatonebranchisinthemainloopand
theotherisintheinnerbridgeisonlyduetothethresholdbeingodd.Ititweresettoan
evennumber,thetwotraceswouldbeswapped.
Itisimportanttonotethatthebridgeneedstobecompiledandattachedtothemainloop
afterithasalreadybeencompiledandexecutedforwhile:thusthemachinecodemustbe
generatedinawaythatcanbedynamicallyextendedasneeded.AswewillseeinChapter
7,thisrequirementposessomeproblemsfortheCLIJITbackend.
6.5 Escapeanalysisandtraceoptimization
Oncethetracehasbeencreated,itispassedtothetrace optimizer,whichdoesconstant
folding,removesunneededoperations,andmoregenerallyproducessmallerandmoree-
cienttraces. Forthemostpart,itispossibletoexploitallthewellknownandstandard
techniquesthathavebeendevelopedduringmanyyearsofresearchincompilers(seee.g.
3
Theoriginalconditionis not i % 2butthelogicalnegationdoesnotappearsinthetrace: : thisis
duetoanoptimizationthatreplacespairsofbool_not/guard_falsewithguard_true.
53
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[BGS94]forasurveyonthem).
6.5.1 Escapeanalysis
class W_Int:
def __init__(self, val):
self.val = val
def getval(self):
return self.val
def f(n):
obj = W_Int(n)
while obj.getval() > 0:
nextval = obj.getval() - 1
obj = W_Int(nextval)
Figure 6.11: : Exampleof f boxedarithmetic
Oneofthemostimportantoptimizations
isobtainedby performing escape analy-
sis[Bla99] [dCGS
+
99] to remove unnec-
essary allocations s of objects. . For r ex-
ample,considerthecodeinFigure6.11,
which shows a typicalpattern that ap-
pearsindynamiclanguages. The
W_Int
class represents boxed integers: : because
ofthearithmeticoperationperformedin-
sidetheloop,anewtemporary
W_Int
in-
stance is created d at t each iteration n (see
alsoSection2.1.2).
The unoptimized d loop produced during
tracingisshowninFigure6.12.Theloop
takesobj
0
,oftype
W_Int
,asaninputar-
gument. Intherstblock,theclassofobj
0
isguardedinordertosafelyinlinethebody
of
getval
(asexplainedinSection5.4),thenthevalue for
nextval
iscomputed. Then,
thesecondblockallocatesanewobjectoftype
W_Int
andinlinesthebodyof
__init__
,
initializingtheeld
val
. Finally,thelastblockinlinesagainthebodyof
getval
,thenit
evaluatestheconditionoftheloop.
Asimplestaticanalysisofthetraceshowsthattheobjectallocatedinsidetheloopnever
escapesoutside.Thus,wecanavoidtoallocateit,andexplode itseldsintosimplelocal
variables.Thus,each
setfield_gc
totheobjectbecomesanassignmenttothecorrespond-
ingvariable,andaccordinglyfor
getfield_gc
.Moreover,operationslike
guard_class
can
beremovedastheyarestaticallyknowntobetrue.
TheresultingoptimizedloopisshowninFigure6.13:notethatinsteadofallocatinganew
objectinsidetheloop,wereservethespaceforallitseldaslocalvariables(inthiscase
onlyone,obj
val,whichgetsrenamedbecauseoftheSSI form). Accordingly,insteadof
havingoneinputargumentoftype
W_Int
,wehaveasmanyinputargumentsasitselds:
inthiscase,obj
val
0
isoftype
int
.
Obviously, such h an optimized loop works s only if
obj
is already y of type
W_Int
before
enteringtheloop. Thus,wesay y thattheloopisspecialized d forthattype. . Iflaterinthe
executionoftheprogramwetrytoenteragaintheloopbut
obj
isofadierenttypethan
W_Int
,westarttracingagainandproduceanotherspecializedversionoftheloop.
54
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loop(obj
0
)
guard
class(obj
0
,W
Int)
val
0
= geteld
gc(obj
0
,val)
nextval
0
= int
sub(val
0
,1)
obj
1
= new
with
vtable(W
Int)
seteld
gc(obj
1
,nextval
0
,val)
guard
class(obj
1
,W
Int)
val
1
= geteld
gc(obj
1
,val)
t
0
= int
gt(val
1
,0)
guard
true(t
0
)
jump(obj
1
)
Figure6.12: Unoptimizedtrace: : objisallo-
cated
loop(obj
val
0
)
obj
val
1
=int
sub(obj
val
0
,1)
t
1
=int
gt(obj
val
1
,0)
guard
true(t
1
)
jump(obj
val
1
)
Figure 6.13: : Optimizedtrace: : objis s virtu-
alized
Theobjectsforwhichweavoidthe allocationare calledvirtual instances s andthe e same
techniqueworksalsoforarrayswhosesizeisstaticallyknownattracingtime,whichare
calledvirtualarrays. Generically,wereferstoeithervirtualinstancesorarrays s withthe
termvirtuals.Toourknowledge,theJITcompilersgeneratedbyPyPyaretheonlytracing
JITsthatexploitescapeanalysistooptimizethegeneratedcode.
6.5.2 Reenteringaspecializedloop
Once the specialized d loop has s beencompiled, we e need a way y to o enter r it. . This s is not
straightforward,as nowthereisamismatchbetweenthe layer ofthe interpreter,which
givesusareal
W_Int
instance,andthelayeroftheloop,whichneedsanunboxedone.
Enteringtheloopforthersttimeitiseasy:remindthatduringtracingwehaveactually
executed oneiterationoftheloopshowninFigure6.12,soweknowthecurrentvalueof
obj
1
:nextval,whichcorrespondstoobj
val
0
inFigure6.13:thereforewecanjustenterthe
specializedlooppassingit that value. . Theloopwillrununtiloneguard d fails,thenthe
executionoftheprogramcontinuesasusual.
55
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However what if f we e want t to re-enter r the specialized d loop p a a second time, , later r in the
executionoftheprogram? Westillhavethemismatchbetweenthe
W_Int
instancegiven
bytheinterpreterandthevirtualizedoneexpectedbytheloop,butthistimewedonot
knowwhichvaluetopassit.Oneeasysolutionistoenteragainthetracingphaseinorder
todoone(unoptimized)iterationoftheloopandcomputethevalueforobj
1
:nextval,then
discardthetraceandenterthecompiledloop.
entry
bridge(obj
0
)
guard
class(obj
0
,W
Int)
val
0
=geteld
gc(obj
0
,val)
nextval
0
= int
sub(val
0
,1)
t
0
=int
gt(nextval
0
,0)
guard
true(t
0
)
jump(nextval
0
)
loop(obj
val
0
)
obj
val
1
= int
sub(obj
val
0
,1)
t
1
= int
gt(obj
val
1
,0)
guard
true(t
1
)
jump(obj
val
1
)
Figure6.14: The e entrybridgethatjumpstotheoptimizedloop
Clearlycomputinganddiscardinganewtraceeverytimealoopisenteredisnotecient.
Thereforeinsteadofbeingthrownaway,itis compiledintoanentrybridge e (seeSection
6.4)thatthenjumpsdirectlytothespecializedloop,asshowninFigure6.14. Onceithas
beencompiled,theentrybridgewillbereusedallthetimesitisnecessarytore-enterthe
loop.
6.5.3 Forcingvirtuals
Instancesandarraysthatareallocatedinsidetheloopcanbevirtualizedonlyiftheydon’t
escapetheloopitself.But,whathappensiflaterweaddaninnerbridgetotheloopwhich
containssomeoperationsthatmaketheobjectescaping?
ConsiderforexamplethecodeinFigure6.15,where
external_func
isanarbitraryfunction
that for some reason cannot be traced, , and thus inlined, by the JIT.Calling external
functionsisoneoftheoperationsthatmakeanobjectescaping.
56
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def f(n):
obj = = W_Int(n)
while obj.getval() > 0:
if obj.getval() < 10:
external_func(obj)
nextval = obj.getval() - 1
obj = W_Int(nextval)
Figure6.15: Exampleofvirtualwhichisforced
Ifwecall
f
withalarge
n
,fortherstit-
erationstheconditionofthe
if
willnever
betrue,thustheJITseesatracewhich
isverysimilartotheoneinFigure6.12
andvirtualizes
obj
.
Figure 6.16 shows the resulting g loop:
the yellow blocks s represents s the e main
loop, while e the green blocks repre-
sents the e bridge which is s attached d to
guard
false(t
0
),wheniteventuallystarts
tofail.
Aslongwestayinthemainloop,
obj
isvirtual.However,whenweenterthebridgeweneed
arealobjectoftype
W_Int
topassto
external_func
:thus,anewobjectisallocatedand
initializedusingthevaluesofitsvirtualelds,inthiscaseobj
val
0
.Thisprocessiscalled
forcingofvirtuals.Oncethevirtualhasbeenforced,wecancallthefunctionandcontinue
theexecutionasnormal. Thistechniqueisveryimportanttogetgoodperformance,asit
allocatesobjectslazilyonlywhenitisreallynecessary.
57
loop(obj
val
0
)
t
0
=int
lt(obj
val
0
,10)
guard
false(t
0
)
obj
val
1
=int
sub(obj
val
0
,1)
t
1
=int
gt(obj
val
1
,0)
guard
true(t
1
)
jump(obj
val
1
)
obj
0
= new
with
vtable(W
Int)
seteld
gc(obj
0
,obj
val
2
,val)
call(external
func,obj
0
)
obj
val
3
= geteld
gc(obj
0
,val)
obj
val
4
=int
sub(obj
val
3
,1)
t
2
=int
gt(obj
val
4
,0)
guard
true(t
2
)
jump(obj
val
4
)
obj
val
2
obj
val
0
Figure6.16: Loopthatshowshowvirtualsare forced
58
59
Chapter7
TheCLIJITbackend
7.1 TeachingtheJITfrontendObjectOrientation
AsexplainedbySection3.2,theTranslationToolchainuseseitherlltype orootype forits
internalrepresentation.Originally,thePyPyJITcompilergeneratorhasbeendesignedfor
lltype,inparticularforbeingtranslatedwiththeCbackendandtoemitmachinecodefora
CPUlikethex86 instructionset.BeforewritingthebackendfortheCLIvirtualmachine,
wehadtoportboththecodewriterandtheJITfrontendtoootype.InthespiritofChapter
4,theportinghasbeendonewithboththeCLIandtheJVMinmind,withtheideathat
writingaJITbackendfortheJVMshouldbeveryeasyoncethewholeinfrastructureis
ready.
Inparticular, the e most important dierence between lltype e and d ootype is howto inline
method calls. . As s we sawinSection6.2, , to inline e amethod callthetracer mustknow
whichjitcode itcorrespondsto. . Ingeneral,toknowwhichimplementationofthemethod
willbecalledonaspecicobject,wemustknowitsclass:thus,duringthetracingphase,
thetracerputa
guard_class
infrontofeachmethodinvocation,whichfromthatpoint
oncanbeassumedtobeknown.
Inlltypeamethodimplementationisrepresentedbyafunctionpointerstoredinthevtable
1
oftheclass,calledusingthe
indirect_call
operation.Sincetheclass,andhenceitsvtable,
isknown,wecaneasilydiscoverthememoryaddressofthetargetmethod.Then,wecan
lookupatable,called
indirectcall_dict
,whichmapsmemoryaddressestojitcodesto
ndthedesiredone.
1
TheVirtualMethodTableorvtableisthestandardmechanismtoimplementlatebindinginstatically
typedobjectorientedlanguages. Thevtableisaper-classtableoffunctionpointerspointingtothecorrect
implementationofeachmethodforthegivenclass.
60
class A:
def f(self):
return 1
def g(self):
return 2
class B(A):
def f(self):
return 3
Figure 7.1: : RPythonhierarchyofclasses
Methodf
Class
Jitcode
A
A.f
B
B.f
Methodg
Class
Jitcode
A
A.g
Figure7.2: Methodtablesforthe e jitcodes
Inootype,classesandmethodsarerstclasscitizens:methodsareinvokedusingthe
oosend
operation,whichtakescareofdispatchingtothecorrectimplementationandistranslated
intoavirtualmethodcallonthetargetVM
2
.Thus,thevirtualmethoddispatchingmech-
anismishiddenfromusandneitherthevtable northememoryaddressesofthemethod
implementationscanbeaccessed.
Therefore,insteadofusingtheabovementioned
indirectcall_dict
,webuildadierent
tablefor eachpossible methodwhichmapstheexact classoftheobjecttothejitcode
that corresponds tothe right methodimplementation. . Then, , when the e tracer needs to
inlineamethodcall,itcanjustlookuptheclassoftheobject,oroneofitssuperclasses,
inthetableforthemethoditisconsidering.
Figure 7.1shows anexample in RPythonwith one class
A
which denes two methods,
and its subclass
B
which overrides one. . Figure e 7.2 shows the method tables computed
forbothmethods. BeingRPython,thesetofmethodsforeachclassdoesnotchangeat
runtime(seeSection3.2),thusthemethodtablescanbecomputedattranslationtimeby
thecodewriter,whilethefrontendusesthematcompiletime.
7.2 TheinterfacebetweentheJITfrontendandback-
end
ThissectiondescribeshowtheJITfrontendinteractswiththebackend: evenifmostof
thejobofthebackendisdoneduringthecompilation phase(seeFigure5.1),wewillsee
thatitplaysanimportantrolealsofortheotherphasesofthetracingJIT.Thefrontend
isdesignedtobeasgenericaspossible,buttherearetasksforwhichitisindispensableto
2
callvirtontheCLIandinvokevirtualontheJVM
61
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