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Intermediate Code Generation

Intermediate Code Generation. Chapter 6. Bac k -end and Fron t -end of A Com piler. Bac k -end and Fron t -end of A Com piler. Cl ose t o sou r c e lan g uage (e . g. s y n t a x t r e e ). Cl ose t o machine l an g uage.

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Intermediate Code Generation

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  1. Intermediate Code Generation Chapter 6

  2. Back-endandFront-end ofACompiler

  3. Back-endandFront-end ofACompiler Closetosource language (e.g.syntaxtree) Closetomachinelanguage mxncompilerscanbebuiltby writingjust mfrontends andnback ends

  4. Back-endandFront-end ofACompiler • Includes: • Typechecking • Any syntactic checksthat remain afterparsing • (e.g. ensurebreak statementisenclosedwithin while-,for-,orswitchstatements).

  5. Component-BasedApproachtoBuildingCompilers Sourceprogram inLanguage-1 Sourceprogram inLanguage-2 Non-optimizedIntermediateCode OptimizedIntermediate Code Target-2 machinecode Target-1 machinecode

  6. IntermediateRepresentation(IR) Akind ofabstractmachinelanguagethat canexpress thetargetmachine operations withoutcommitting totoo much machine details. :MWhyIR?

  7. WithoutIR C Pascal FORTRAN C++ SPARC HPPA x86 IBM PPC

  8. WithIR C SPARC Pascal HPPA IR FORTRAN x86 IBM PPC C++

  9. WithIR C ? Pascal CommonBackend IR FORTRAN C++

  10. Advantagesof UsinganIntermediateLanguage Retargeting-Builda compilerforanewmachine by attaching anew codegeneratorto an existing front-end. Optimization-reuse intermediate code optimizers in compilers fordifferentlanguages and different machines. Note:the terms “intermediate code”,“intermediate language”, and “intermediate representation”are all used interchangeably.

  11. IssuesinDesigninganIR • Whether touseanexisting IR • if targetmachine architecture is similar • if the new language is similar • Whether theIRis appropriate for the kind ofoptimizations tobeperformed • e.g. speculationandpredication • sometransformations maytake muchlongerthanthey wouldona different IR

  12. IssuesinDesigninganIR • Designing anewIRneeds to consider • Level(how machine dependent it is) • Structure • Expressiveness • Appropriateness for general andspecial optimizations • Appropriateness forcodegeneration • Whethermultiple IRs should be used

  13. Multiple-Level IR Target code High-level IR Low-level IR SourceProgram … Semantic Check High-level Optimization Low-level Optimization

  14. UsingMultiple-levelIR • Translating from onelevelto another inthe compilationprocess • Preserving anexisting technology investment • Some representationsmaybe more appropriate foraparticular task.

  15. Commonly UsedIR • :MPossibleIR forms • Graphical representations:such as syntaxtrees,AST (Abstract SyntaxTrees),DAG • Postfixnotation • Threeaddresscode • SSA(StaticSingle Assignment) form • :MIR should haveindividual components thatdescribe simplethings

  16. SyntaxTree

  17. SyntaxTree + + * a * - b c d a - b c

  18. SyntaxTree + + * a * - b c d a - b c

  19. SyntaxTree + + * a * - b c d a - b c Nodecan have morethan one parent • DirectedAcyclicGraph (DAG): • Morecompactrepresentation • Givescluesregardinggenerationofefficient code

  20. Example Construct the DAG for:

  21. HowtoGenerateDAGfrom Syntax-DirectedDefinition? Allwhat is neededis that functionssuchasNode andLeaf abovecheck whethera nodealready exists.Ifsucha nodeexists, a pointer isreturnedtothatnode.

  22. HowtoGenerateDAGfrom Syntax-DirectedDefinition?

  23. DataStructure:Array

  24. DataStructure:Array Leaves Leftandrightchildren ofanintermediatenode Operationcode Scanningthearray eachtimea newnodeis needed,is not anefficientthingtodo.

  25. DataStructure:HashTable Hashfunction=h(op,L,R)

  26. Three-AddressCode • Anotheroptionforintermediatepresentation • Builtfromtwoconcepts: • –addressesandinstructions • Atmostoneoperator

  27. Address • Canbeone ofthe following: • Aname: source program name • Aconstant • Compiler-generatedtemporary

  28. Instructions Procedure callsuch as p(x1, x2,…,xn)is implementedas:

  29. Example OR

  30. Choiceof OperatorSet • Rich enoughto implementthe • operationsofthe sourcelanguage • Closeenough to machine instructionsto • simplifycodegeneration

  31. DataStructure • Howto present these instructionsin a • datastructure? • Quadruples • Triples • Indirecttriples

  32. DataStructure: Quadruples • Hasfourfields:op, arg1, arg2,result • Exceptions: • Unaryoperators:noarg2 • Operatorslikeparam:noarg2,noresult • (Un)conditionaljumps: targetlabelis the • result

  33. op arg1 arg2 result tl= tz= b *tt t3= minusct4= b *t3 t6= tz+ t4 a = t5 minusc 0 1 2 3 4 5 = I tsI I

  34. DataStructure: Triples • Onlythree fields:noresultfield • Results referredtoby itsposition

  35. DataStructure: IndirectTriples • Wheninstructions are movingaround duringoptimizations: quadruples are betterthantriples. • Indirecttriplessolvethisproblem Optimizingcompliercanreorderinstruction list, insteadofaffectingthetriplesthemselves List ofpointerstotriples

  36. Single-Static-Assignment(SSA) • Isan intermediatepresentation • Facilitatescertaincode optimizations • Allassignmentsare tovariableswith distinct names

  37. Single-Static-Assignment(SSA) Example: Ifwe usedifferent names for Xintruepartand false part,thenwhichname shall weuseinthe assignmentof y = x* a? Theanswer is:Ø-function Returnsthevalueofits argument that correspondstothe control-flow paththat was takentoget totheassignment statementcontainingtheØ-function

  38. Example

  39. TypesandDeclarations • Typechecking:to ensurethat typesof operands matchthe type expectedby operator • Determinethe storage needed • Calculate the address ofanarray reference • Insertexplicittypeconversion • Choose the writeversionofanoperator • …

  40. StorageLayout • From thetype,we candetermine • amount of storage at runtime. • Atcompiletime, we will use thisamount • to assign its namea relativeaddress. • Typeandrelativeaddressare saved in • thesymboltable entry of thename. • Datawith lengthdeterminedonly at run timesavesa pointer inthe symboltable.

  41. StorageLayout • Multibyteobjectsarestoredin consecutivebytesandgiven the address of thefirstbyte • Storageforaggregates(e.g. arrays and classes)isallocatedinonecontiguous block ofbytes.

  42. B

  43. offset = offset+T.width; } D€

  44. Tokeeptrack ofthenext availablerelative address Create asymbol tableentry

  45. TranslationsofStatements andExpressions Syntax-DirectedDefinition (SDD) Syntax-DirectedTranslation (SDT)

  46. Addressholdingvalue of E (e.g.tmp variable,name,constant) Three-address codeofE Buildan instruction Getatemporary variable Current symbol table

  47. a=b+-c • t1 = minusc t2;b+tl a = t2

  48. Generating three-address codeincrementally toavoidlongstrings manipulations • gen() does twothings: • generate threeaddressinstruction • •appenditto the sequenceof • instructionsgeneratedsofar

  49. Arrays • Elementsof thesametime • Storedconsecutively inmemory • Inlanguageslike C or Java elements are:0, 1, …, n-1 • Insomeotherlanguages: • low, low+1, …, high

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