Design and Implementation of an Ahead-of-Time Compiler for PHP Paul Biggar

220 Pages·2010·1.48 MB·English

Design and Implementation of an
Ahead-of-Time Compiler for PHP
by
Paul Biggar
Dissertation
Presented in fulfillment
of the requirements
for the Degree of
Doctor of Philosophy
TRINITY COLLEGE DUBLIN
APRIL, 2010
Declaration
I, the undersigned, declare that this work has not previously been submitted as an
exerciseforadegreeatthis,oranyotherUniversity,andthatunlessotherwisestated,
ismyownwork.
PaulBiggar
14thApril,2010
ii
/
Permission to Lend and or Copy
I,theundersigned,agreethatTrinityCollegeLibrarymaylendorcopythisdisserta-
tionuponrequest.
PaulBiggar
14thApril,2010
iii

Abstract
Inrecentyearstheimportanceofdynamicscriptinglanguages—suchasPHP,Python,
RubyandJavascript—hasgrownastheyareusedforanincreasingamountofsoftware
development. Scriptinglanguagesprovidehigh-levellanguagefeatures,afastcompile-
modify-testenvironmentforrapidprototyping,strongintegrationwithdatabaseand
webdevelopmentsystems,andextensivestandardlibraries. PHPpowersmanyofthe
mostpopularwebapplicationssuchasFacebook,WikipediaandYahoo. Ingeneral,
thereisatrendtowardswritinganincreasingamountofanapplicationinascripting
language rather than in a traditional programming language, not least to avoid the
complexityofcrossingbetweenlanguages.
Despitetheirincreasingpopularity,mostscriptinglanguageimplementationsremain
interpreted. Typically,theseimplementationsareslow,betweenoneandtwoorders
ofmagnitudeslowerthanC. Improvingtheperformanceofscriptinglanguageimple-
mentationswouldbeofsignificantbenefit,howevermanyofthefeaturesofscripting
languagesmakethemdifficulttocompileaheadoftime. Inthisdissertationweargue
thatahead-of-timecompilationofscriptinglanguagesisbothpossibleandvaluable.
We present phc, an ahead-of-time compiler for the PHP language. We describe the
design and implementation of the compiler, and identify specific challenges in the
designofacompilerforadynamicscriptinglanguage.
Wedeterminethattherearethreeimportantfeaturesofscriptinglanguagesthatare
difficulttocompileorreimplement. Sincescriptinglanguagesaredefinedprimarily
throughthesemanticsoftheiroriginalimplementations,theyoftenchangesemantics
betweenreleases. TheyprovideCAPIs,usedbothforforeign-functioninterfacesand
towritethird-partyextensions. TheseAPIstypicallyhavetightintegrationwiththe
originalimplementation,andareusedtoprovidelargestandardlibraries,whichare
difficult to re-use, and costly to reimplement. Finally, they support run-time code
generation. Thesefeaturesmaketheimportantgoalofcorrectnessdifficulttoachieve
forcompilersandreimplementations.
We present a technique to support these features in ourahead-of-time compilerfor
PHP. Our technique uses the original PHP implementation through the provided
PHP C API, both in our compiler, and in our generated code. We support all of
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theseimportantscriptinglanguagefeatures,particularlyfocusingonthecorrectness
ofcompiledprograms. Additionally,ourapproachallowsustoautomaticallysupport
limitedfuturelanguagechanges. Wepresentadiscussionandperformanceevaluation
ofthistechnique,whichweshowincreasestheexecutionspeedofPHPprogramsby
1.55xinourbenchmarks. Inordertoimprovethisperformance,staticanalysisand
optimizationarerequired.
However, static analysis of scripting languages such as PHP is difficult due to the
featuresfoundintheselanguages. Thesefeaturesincludedynamictypingwithimplicit
typeconversions,dynamicaliasing,implicitobjectandarraycreation,andoverloading
ofsimpleoperators. Wefindthatasaresult,simpleanalysistechniquessuchasSSA
and def-use chains are not straightforward to use, and that a single unconstrained
variablecanruinouranalysis. WedescribethechallengingsemanticsofPHP,anda
staticanalysertomodelthem. Ouranalysiscombinesaliasanalysis,type-inference
and constant-propagation for PHP, computing results that are essential for other
analysesandoptimizations. Ourempiricalresultsshowthatouranalysisiscapableof
determiningthatalmostallprogramvariablesareunaliased,andthatdynamictypes
forover60%ofvariablescanbestaticallydeterminedbyouranalysis.
Acknowledgements
Morethananyone,I’dliketothankDavidGreggforhissupervision,helpandadvice
throughout the last six and a half years. He guided me through my undergraduate
research,throughmyPhDapplications,andthroughoutmyPhD. Ithinkitisfairto
saythatnearlyeverythingIknowaboutresearchcomesfromDavid.
Many supervisors are hands-off, David is certainly not. He has read everything I
produced,hemetmefrequentlytoguidemyresearchandhisdoorisalwaysopen. His
expertiseonallmanneroftopicsrelatedtocompilersandresearchhasbeeninvaluable.
ItistherareproblemthatDavidcouldnothelpmeon. Davidmadeithisbusinessto
understandeverythingIdid. I’mnotsureheknewanythingaboutscriptinglanguages
whenImethim,buthespentaconsiderableamountoftimeandefforttounderstand
myresearchthoroughly.
FromthemomentIstartedthisPhD,Davidhasbeenthereforme. Anytimethere
wasaproblem,hehadtimetohelpwithit. Inparticular,atthestartof2007Iwent
through a particularly bad time. My then-current research wasn’t going anywhere,
and a change was needed. At the time, neither of us was really sure I’d make it to
finishmyPhD. Duringthattimeinparticular,hishelpandguidancewasverymuch
appreciated.
Manyofmycolleaguesmeritthanks,inparticularourresearchgroup: KevinWilliams,
Nicholas Nash, Robert (Bobb) Crosbie, Jason McCandless and (Raymond) Scott
Manley. They listened to my ideas and critiqued my work. Kevin also worked on
scriptinglanguages,andIlearnedalotfromchattingtohim. Nickdidanamazingjob
inourfirstpaper,andI’mveryproudoftheresult.
JohnGilbertandEdskodeVrieswerecollaboratorsonphc. Isatintheirofficeforthe
firstyearofmyresearch,andIthinkIlearnedmoreaboutcompilersfromthemthan
fromanyothersource. Ofcourse,theyalsostartedphc,andinvitedmetoworkonit,
towhichIowethemagreatdebt. Idon’tthinkthecompilerIwouldhaveotherwise
workedonwouldhavebeenanythingassuccessful.
IwanttothankEdskoinparticular. WorkingwithEdskoisalessoninhowtocode
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well,andIlearnedagreatdealfromworkingonphcwithhim. Hereadalotofmy
code,mycommitlogs,andallmypapers. ArguingwithEdskohelpedmefleshouta
greatmanyideas;nounclearideagetspasthim. Healsotookontheherculeaneffort
ofreadingmywholePhDinonenight.
I also want to thank Kevin, Nick, Bobb, Jason, Scott, John and Edsko for their
friendship. Itwouldhavebeenalongfouryearswithoutit.
Jimmy Cleary interned withme during the summerof2009. He workedon many
aspectsofphc,writingfeatures,fixingbugs,andrunningamassiveamountofexper-
iments. His output far outstripped the time it took to train him in, and I honestly
believeIwouldnothavethisPhDfinishedwithouthishelp.
Manypeopletookthetimetoreadmypapersanddissertation,andoffercomments,
includingDavid,Kevin,Nick,Bobb,Jason,Scott,Edsko,JohnandJimmy. Thanks
alsotoNunoLopes,GrahamKelly,ChristopherJones,AlexGaynor,JamesStanier,
CorneliusRiemenschneider,andEtienneKneuss,whoallprovidedvaluablefeedback.
PeterHayestaughtmethetrickofreadingmypapersoutloud. Ibelievethismadea
considerabledifferencetothequalityofmywriting.
ItravelledconsiderablythroughoutmyPhD. Ilearnedagreatdealfromtravellingto
PLDI,ASPLOSandHOPL,fromspeakingtotoomanypeopletolisthere. Seminars
andsummerschoolsalsotaughtmeagreatdeal. ThankstoMarkusSchordan,Sebas-
tianHackandFabriceRastello,andKoendeBosschereforacceptingmetoDagstuhl,
theSSAseminarandtheACACESsummerschool,respectively. Ilearnedagreatdeal,
and met a lot of interesting people. Lectures from Mike Hind, Kathryn McKinley
andBarbaraRyderwereparticularlyinterestingandinspiring.
I had many opportunities thanks to the kind help of others. I want to thank Dan
QuinlanforhisinvitationtoLivermore,toIanTaylorforarrangingtheGoogleTech
Talk,toBrianShireforhavingmetoFacebook,andtoDanielBerlinformentoring
meongccduringtheGoogleSummerofCode. Theseseeminglysmallopportunities
havebenefitedmemuchmorethanyoumightknow.
My work was funded by the Irish Research Council for Science, Engineering and
TechnologyfundedbytheNationalDevelopmentPlan. Iwouldhavebeencompletely
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unable to work on a PhD without this funding. The Dun Laoghaire Rathdown
CountyCouncilpaidmyfees,andgavemeasmallstipend. Iwanttothankthemfor
theirhelp.
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MyfamilyandfriendshavebeenverysupportivethroughoutthisPhD. Noamount
ofthankscanexpresshowmuchIvaluetheirfriendshipandlove.
IcouldneverhavegottentowhereIamwithoutmyfiancée,OrlaMcHenry. Thelast
twoyearshavebeenalotofworkandstress,andwithouthertowelcomemehome,I
would have struggled to make it through. She brought me cookies when I worked
late,cheeredmeupwhenIwasdown,andtoldmeitwouldbefinewhenIworried.
She was always there when I needed her. Knowing how proud she is of me always
makesmesmile. OfallofthethingsI’vegainedinthelastfouryears,nothingmakes
mehappierthanOrla. I’veneverfeltluckierthanIamtohaveher.
Thankyouall.