Summary: Semantic Analysis

Transcription

1 Summary: Smantic Analysis Chck rrors not dtctd by lxical or syntax analysis Intrmdiat Cod Scop rrors: Variabls not dfind Multipl dclarations Typ rrors: Assignmnt of valus of diffrnt typs Invocation of functions with diffrnt numbr of paramtrs or paramtrs of incorrct typ Incorrct us of rturn statmnts 1 CS 412/413 Spring 2008 Introduction to Compilrs 2 Smantic Analysis Typ chcking Us typ chcking ruls Static smantics = formal framwork to spcify typchcking ruls Thr ar also control flow rrors: Must vrify that a brak or continu statmnt is always nclosd by a whil (or for) statmnt Java: must vrify that a brak X statmnt is nclosd by a for loop with labl X Can asily chck control-flow rrors by rcursivly travrsing th AST Sourc cod (charactr stram) Tokn stram Abstract syntax tr Abstract syntax tr + symbol tabls, typs Whr W Ar Lxical analysis Syntactic Analysis Smantic Analysis Intrmdiat Cod Gnration rgular xprssions grammars static smantics Intrmdiat Cod CS 412/413 Spring 2008 Introduction to Compilrs 3 CS 412/413 Spring 2008 Introduction to Compilrs 4

2 Usually two IRs: High-lvl IR Languag-indpndnt (but closr to languag) C Fortran Pascal Intrmdiat Cod HIR Low-lvl IR Machin indpndnt (but closr to machin) LIR Pntium Java bytcod PowrPC High-lvl IR Tr nod structur, ssntially ASTs High-lvl constructs common to many languags Exprssion nods Statmnt nods Exprssion nods for: Intgrs and program variabls Binary oprations: 1 OP 2 Arithmtic oprations Logic oprations Comparisons Unary oprations: OP Array accsss: 1[2] CS 412/413 Spring 2008 Introduction to Compilrs 5 CS 412/413 Spring 2008 Introduction to Compilrs 6 High-lvl IR Statmnt nods: Block statmnts (statmnt squncs): (s1,, sn) Variabl assignmnts: v = Array assignmnts: 1[2] = 3 If-thn-ls statmnts: if c thn s1 ls s2 If-thn statmnts: if c thn s Whil loops: whil (c) s Function call statmnts: f(1,, N) Rturn statmnts: rturn or rturn May also contain: For loop statmnts: for(v = 1 to 2) s Brak and continu statmnts Switch statmnts: switch() { v1: s1,, vn: sn } Low-Lvl IR Low-lvl rprsntation is ssntially an instruction st for an abstract machin Altrnativs for low-lvl IR: Thr-addrss cod or quadrupls (Dragon Book): a = b OP c Tr rprsntation (Tigr Book) Stack machin (lik Java bytcod) CS 412/413 Spring 2008 Introduction to Compilrs 7 CS 412/413 Spring 2008 Introduction to Compilrs 8

3 Thr-Addrss Cod In this class: thr-addrss cod a = b OP c Has at most thr addrsss (may hav fwr) Also namd quadrupls bcaus can b rprsntd as: (a, b, c, OP) Exampl: a = (b+c)*(-); t1 = b + c t2 = a = t1 * t2 Low IR Instructions Assignmnt instructions: Binary oprations: a = b OP c arithmtic: ADD, SUB, MUL, DIV, MOD logic: AND, OR, XOR comparisons: EQ, NEQ, LT, GT, LEQ, GEQ Unary opration a = OP b Arithmtic MINUS or logic NEG Copy instruction: a = b Load /stor: a = *b, *a = b Othr data movmnt instructions CS 412/413 Spring 2008 Introduction to Compilrs 9 CS 412/413 Spring 2008 Introduction to Compilrs 10 Low IR Instructions, cont. Flow of control instructions: labl L: labl instruction jump L: Unconditional jump cjump a L: conditional jump Function call call f(a 1,., a n ) a = call f(a 1,., a n ) Is an xtnsion to quads IR dscribs th Instruction St of an abstract machin m = 0; if (c == 0) { m = m+ n*n; } ls { m = m + n; } Exampl m = 0 t1 = (c == 0) fjump t1 falsb t2 = n * n m = m + t2 jump nd labl falsb m = m+n labl nd CS 412/413 Spring 2008 Introduction to Compilrs 11 CS 412/413 Spring 2008 Introduction to Compilrs 12

4 How To Translat? May hav nstd languag constructs Nstd if and whil statmnts Nd an algorithmic way to translat Solution: Start from th AST rprsntation Dfin translation for ach nod in th AST in trms of a (rcursiv) translation of its constitunts Notation Us th notation T[] = low-lvl IR of high-lvl IR construct T[] is squnc of low-lvl IR instructions If is xprssion (or statmnt xprssion), T[] rprsnts a valu Dnot by t = T[] th low-lvl IR of, whos rsult valu is stord in t For variabl v, dfin T[v] to b v, i.., t = T[v] is copy instruction t = v CS 412/413 Spring 2008 Introduction to Compilrs 13 CS 412/413 Spring 2008 Introduction to Compilrs 14 Translating Exprssions Binary oprations: t = T[ 1 OP 2 ] (arithmtic oprations and comparisons) t1 = T[ 1 ] t2 = T[ 2 ] t = t1 OP t2 Unary oprations: t = T[ OP ] t1 = T[ ] t = OP t1 1 OP OP 2 Translating Boolan Exprssions t = T[ 1 OR 2 ] t1 = T[ 1 ] t2 = T[ 2 ] t = t1 OR t2 but how about short-circuit OR, for which w should comput 2 only if 1 valuats to fals 1 OR 2 CS 412/413 Spring 2008 Introduction to Compilrs 15 CS 412/413 Spring 2008 Introduction to Compilrs 16

5 Translating Short-Circuit OR Short-circuit OR: t = T[ 1 SC-OR 2 ] t = T[ 1 ] tjump t Lnd t = T[ 2 ] SC-OR 1 2 Translating Short-Circuit AND Short-circuit AND: t = T[ 1 SC-AND 2 ] t = T[ 1 ] fjump t Lnd t = T[ 2 ] SC-AND 1 2 how about short-circuit AND? CS 412/413 Spring 2008 Introduction to Compilrs 17 CS 412/413 Spring 2008 Introduction to Compilrs 18 Array and Fild Accsss Nstd Exprssions Array accss: t = T[ v[] ] t1 = T[ ] t = v[t1] Fild accss: t = T[ 1.f ] t1 = T[ 1 ] t = t1.f array v. 1 2 In ths translations, xprssions may b nstd; Translation rcurss on th xprssion structur Exampl: t = T[ (a - b) * (c + d) ] t1 = a t2 = b T[ (a - b) ] t3 = t1 t2 t4 = b t5 = c T[ (c + d) ] t5 = t4 + t5 t = t3 * t5 T[ (a - b) * (c + d) ] CS 412/413 Spring 2008 Introduction to Compilrs 19 CS 412/413 Spring 2008 Introduction to Compilrs 20

6 Translating Statmnts Statmnt squnc: T[ s1; s2; ; sn ] T[ s1 ] T[ s2 ] T[ sn ] sq s1 s2 sn IR instructions of a statmnt squnc = concatnation of IR instructions of statmnts Assignmnt Statmnts Variabl assignmnt: T[ v = ] t = T[ ] v = t [altrnativly] v = T[ ] var-assign Array assignmnt: T[ v[1] = 2 ] t1 = T[ 1 ] array-assign t2 = T[ 2 ] v[t1] = t2 v 1 2 v CS 412/413 Spring 2008 Introduction to Compilrs 21 CS 412/413 Spring 2008 Introduction to Compilrs 22 Translating If-Thn-Els Translating If-Thn T[ if () thn s1 ls s2 ] T[ if () thn s ] t1 = T[ ] fjump t1 Lfals T[ s1 ] jump Lnd labl Lfals T[ s2 ] if-thn-ls s1 s2 t1 = T[ ] fjump t1 Lnd T[ s ] if-thn s CS 412/413 Spring 2008 Introduction to Compilrs 23 CS 412/413 Spring 2008 Introduction to Compilrs 24

7 Whil Statmnts Switch Statmnts T[ whil () { s } ] labl Ltst t1 = T[ ] fjump t1 Lnd T[ s ] jump Ltst whil s T[ switch () { cas v1: s1,, cas vn: sn } ] t = T[ ] c = t!= v1 tjump c L2 T[ s1 ] jump Lnd labl L2 c = t!= v2 tjump c L3 T[ s2 ] jump Lnd labl LN c = t!= vn tjump c Lnd T[ sn ] v1 switch s1 vn sn CS 412/413 Spring 2008 Introduction to Compilrs 25 CS 412/413 Spring 2008 Introduction to Compilrs 26 Call and Rturn Statmnts Nstd Statmnts T[ call f(1, 2,, N) ] t1 = T[ 1 ] t2 = T[ 2 ] tn = T[ N ] call f(t1, t2,, tn) T[ rturn ] t = T[ ] rturn t call f 1 2 N rturn Sam for statmnts as xprssions: rcursiv translation Exampl: T[ if c thn if d thn a = b ] t1 = c fjump t1 Lnd1 t2 = d fjump t2 Lnd2 t3 = b T[ a = b ] a = t3 2 1 T[ if d ] T[ if c thn ] CS 412/413 Spring 2008 Introduction to Compilrs 27 CS 412/413 Spring 2008 Introduction to Compilrs 28

8 t1 = c fjump t1 Lnd1 t2 = d fjump t2 Lnd2 t3 = b a = t3 2 1 IR Lowring Efficincy c if-thn if-thn d a = b fjump c Lnd fjump d Lnd a = b Labl Lnd CS 412/413 Spring 2008 Introduction to Compilrs 29