Symbol Table management

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1 TDDD Compilers nd interpreters TDDB44 Compiler Construction Symol Tles Symol Tles in the Compiler Symol Tle mngement source progrm Leicl nlysis Syntctic nlysis Semntic nlysis nd Intermedite code gen Code optimiztion Code genertion sequence of chrs: IF sum=5 THEN.. sequence of tokens: IF sum = 5 prse tree, derivtion tree internl form, intermedite code internl form Error Mngement Peter Fritzson, Christoph Kessler, IDA, Linköpings universitet, 009. oject progrm 4. Symol Tle Functionlity Function: Gther informtion out nmes which re in progrm. A symol is dt structure, where informtion out progrm ojects is gthered. Is used in oth the nlysis nd synthesis phses. The symol is uilt up during the leicl nd syntctic nlysis. Provides help for other phses during compiltion: Semntic nlysis: type conflict? Code genertion: how much nd wht type of runtime spce is to e llocted? Error hndling: Hs the error messge "Vrile A undefined" lredy een issued? The symol phse or symol mngement refer to the symol s storge structure, its construction in the nlysis phse nd its use during the whole compiltion. Requirements nd Concepts Requirements for symol mngement quick insertion of n identifier quick serch for n identifier efficient insertion of informtion (ttriutes) out n id quick ccess to informtion out certin id Spce nd time efficiency Importnt concepts Identifiers, nmes Lvlues nd rvlues Environments nd indings Opertors nd vrious nottions Leicl nd dynmic scope structures Identifiers nd Nmes Identifiers Nmes An identifier is string, e.g. ABC. A nme denotes spce in memory, i.e., it hs vlue nd vrious ttriutes, e.g. type, scope. Emple: procedure A; vr : ; procedure B; vr : ; sme identifier ut different nmes A nme cn e denoted y severl identifiers, soclled lising. {(,C),(y,C), y ddress: C 5 Lvlue nd Rvlue There is difference etween wht is ment y the right nd the left side of n ssignment. Emple: Lvlue := * c; i := i + ; Rvlue Certin epressions hve either l or rvlue, while some hve oth lvlue nd rvlue. Epression hs l vlue i+ no > [i] no hs r vlue

2 Binding: <nmes, ttriutes> Sttic nd Dynmic Lnguge Concepts Nmes Come from the leicl nlysis nd some dditionl nlysis. ttriutes Come from the syntctic nlysis, semntic nlysis nd code genertion phse. Binding is ssociting n ttriute with nme, e.g. procedure foo; vr k: chr; { Bind k to chr procedure fie; vr k: integer; { Bind k to integer Sttic Concepts Definition of suprogrm Declrtion of nme Scope of declrtion Dynmic Counterprts Cll y suprogrm Binding of nme Lifetime of inding Environments nd Bindings Different environments re creted during eecution, e.g. when clling suprogrm An environment consists of numer of nme indings Distinguish etween environment nd stte, e.g. the ssignment A := B; chnges the current stte, ut not the environment. environment stte nme memory vlue Env: nme memory Stte: memory vlue Emple Env = {(,C),(y,C),(z,C), Stte = {(C,),(C,5),(C,9), In the environment Env, inds to memory cell C, nd memory cell C hs the vlue, A nme is ound to memory cell, storge loction, which cn contin vlue. A nme cn hve severl different indings in different environments, e.g. if procedure clls itself recursively. Scope. Leicl Scope How do we find the oject which is referenced y nonlocl nmes? Two different methods re used: Leicl nd dynmic scope. Leicl or sttic scope progrm foo; vr ; sttic procedure fie(); vr y egin y := ; end; end. The oject is determined y investigting the progrm tet, stticlly, t compiletime The oject with the sme nme in the nerest enclosing scope ccording to the tet of the progrm Is used in the lnguges Pscl, Algol, C, C++, Jv, Modelic, etc Dynmic Scope The oject is determined during runtime y investigting the current cll chin, to find the most recent in the chin. Is used in the lnguges LISP, APL, Mthemtic (hs oth). Emple: Dynmicscope p vr ; p vr ; p; p; y:= ; Which is referenced in the ssignment sttement p? It depends on whether p is clled from p or p. p Leicl or Dynmic Scope Which is referenced in procedure fie in the progrm elow if leicl/sttic scoping pplies? dynmic scoping pplies? sttic min fum fie progrm foo; vr ; sttic procedure fie(); vr y egin y := ; (* which? *) end; dynmic procedure fum(); vr ; egin := 5; fie(); dynmic end; egin := 0; fum(); end

3 Structures Algol, Pscl, Simul, Ad re typicl lockstructured lnguges. s cn e nested ut my not overlp Sttic scoping pplies for these lnguges: A nme is visile (ville) in the lock the nme is declred in. If lock B is nested in B, then nme ville in B is lso ville in B if the nme hs not een redefined in B. B B Sttic nd Dynmic Chrcteristics in Lnguge Constructs Sttic chrcteristics Chrcteristics which re determined during compiltion. Emples: A Psclvrile type Nme of Pscl procedure Scope of vriles in Pscl Dimension of Psclrry The vlue of Pscl constnt Memory ssignment for n integer vrile in Pscl Dynmic chrcteristics Chrcteristics tht cn not e determined during compiltion, ut cn only e determined during runtime. Emples The vlue of Pscl vrile Memory ssignment for dynmic vriles in Pscl (ccessile vi pointer vriles) Advntges nd Disdvntges Sttic constructs Reduced freedom for the progrmmer + Allows type checking during compiltion + Compiltion is esier + More efficient eecution Dynmic constructs Less efficient eecution ecuse of dynmic type checking + Allows more fleile lnguge constructions (e.g. dynmic rrys) More out this will e included in the lecture on memory mngement. 4.5 Symol Tle Design (decisions tht must e mde) Structuring of vrious types of informtion (ttriutes) for ech nme: string spce for nmes informtion for procedures, vriles, rrys, ccess s (opertions) on the symol scope, for lockstructured lnguges. Choosing dt structures for the symol which enle efficient storge nd retrievl of informtion. Three different dt structures will e emined: Liner lists Trees s Design choices: One or more s Direct informtion or pointers (or indees) 4. Structuring Prolems for Symol Dt String Spce for Identifiers When nme is declred, the symol is filled with vrious its of informtion out the nme: 0 m n Normlly the symol inde is used insted of the ctul nme. For emple, the prse tree for the sttement <ssignment> m <ssop> true (or inde for ":=") Method : Fied spce of m epected chrcters FORTRAN4: chrcters, Hedrick Pscl: 0 chrcters KALLE SUM ttriutes ttriutes Method : <length, pointer> (e.g. Sun Pscl: 04 chrcters 5 ttri utes KALLE SUM This is oth time nd spceefficient. How cn the string which represents the nme e stored? Net come two different wys. Method : without specifying length: $KALLE$SUM$ where $ denotes end of string. The nme nd informtion must remin in the symol s long s reference cn occur. For lockstructured lnguges the spce cn e reused

4 String Spce for Identifiers Method, cont. Identifiers cn vry in length Must e stored in token Nme field of symol just points to first chrcter Symol To e kept s long s references cn occur funct \0 s u m \0 f o o r \0 nme ttr link doule doule Usully, full nmes kept only during compiltion Eception: Added to the progrm s constnt pool in the.dt segment if symolic deugging or reflection should e enled (e.g., gcc g file.c to prepre for symolic deugging) Informtion in the Symol Tle nme ttriute type (integer, oolen, rry, procedure, ) length, precision, pcking density ddress (lock, offset) declred or not, used or not.. int vlue $i$ You cn directly llocte spce in the symol for ttriutes whose size is known, e.g. type nd vlue of simple vrile Compiler representtion of nmes A unique nd compct internl representtion for nme is the inde (ddress in compiler ddress spce) of its symol entry. Used insted of full nme (string) in the internl representtion of progrm Time nd spce efficient Emple: Prsetree for epression <epression> cd <= yefgh; Informtion in the Symol Tle for Arrys Fied Alloction Fied lloction (BASIC, FORTRAN4) The numer of dimensions is known t compiltion. FORTRAN4: m dimensions, integer inde. KALLE Arry L U L U L U INTEGER Fied in dvnce Dim. limits lower/upper ound Element type <identifier> <lteq_token> 4. <identifier> Symol nme ttr link cd doule yefgh doule 4. Informtion in the Symol Tle for Arrys Fleile Alloction Fleile lloction (Pscl, Simul, ADA, Jv) Aritrry numer of dimensions, elements of ritrry type. v Pscl: vr v: rry[..0,.. z ] of integer rry type rry type You cn ccess n element v[i,j] in the ove rry y clculting its ddress: dr = BAS + k*((i)*r)+j) where r= numer of elements/rows, integer nd k= numer of memory cells/elements (ytes, words) '' 0 'z' integer integer Symol Tle Dt nd Opertions Set of symol items serchle y nme + scope Dt stored for ech entry: nme ttriutes type (int, ool, rry, ptr, ) ddress (lock, offset) declred or not, used or not Opertions lookup ( nme ) insert ( nme ) put ( nme, ttriute, vlue ) get ( nme, ttriute) enterscope () eitscope() ADT Dictionry + Scoping Control

5 Dt Structures for Symol Tles Liner lists For flt symol s: (one lock of scope) Liner lists s (see dt structures for ADT Dictionry) For nested scopes: Trees of flt symol s Liner lists with scope control Only for psscompilers s with scope control (see following slides) Only for psscompilers ST nme ttr nme ttr nme ttr Unsorted liner lists Esy to implement Spce efficient Insertion itself is fst ut needs lookup to check if the nme ws lredy in Lookup is slow Inserting n identifiers nd doing m lookups requires O(n(n+m)) string comprisons Tle with Chining () Tle with Chining () nme foo c Symol entries nme lock link foo nme foo c Symol entries nme lock link foo c void foo ( void ) { void foo( void ) { 4.7 Much fster lookup on verge Degenertes towrds liner list for d hsh s 4.8 Tle with Chining () Serch the nme in hsh, h(symol) [0, k] where k = size If the entry is occupied, follow the link field. Insertion Serch + simple insertion t the end of the symol (use the sympos pointer). Efficiency Serch proportionl to n/k nd the numer of comprisons is (m + n) n / k for n insertions nd m serches. k cn e chosen ritrrily lrge. Positive Very quick serch Negtive Reltively complicted Etr spce required, k words for the hsh. More difficult to introduce scoping. 4.9 Hierrchicl Symol Tles For nested scope locks 4.0 5

6 Treesed Symol Tle clss Br { int ; void foo( ) { void foo( ) { int inner( ) { flot ; int inner( ) { doule, y; foo( ); enterscope(), eitscope() insert(), lookup() 4. File/module scope: Glol symol nme ttr link Br Symol for Br nme ttr link int foo funct foo funct Symol for foo nme ttr link Symol for inner nme ttr link flot Symol for foo nme ttr link inner funct inner funct Symol for inner nme ttr link doule y doule For OnePss Compilers? clss Br { int ; void foo( ) { void foo( ) { int inner( ) { flot ; int inner( ) { doule, y; foo( ); enterscope(), eitscope() insert(), lookup() Symol for Br nme ttr link int foo funct foo funct Symol for foo nme ttr link Symol for inner nme ttr link 4. File/module scope: Glol symol nme ttr link Br After code ws emitted for foo resp. for inner, could relese its symol Symol for foo nme ttr link inner funct inner funct Symol for inner nme ttr link doule y doule s with chining + scoping (For OnePss Compilers Only) Current scope lock: 0 Symol entries nme lock link 0 s with chining + scoping Current scope lock: Symol entries nme lock link nme prog void p() { insert p nd enter new scope lock () void p() { insert prog nd enter new scope lock () s with chining + scoping s with chining + scoping Current scope lock: Symol entries nme lock link Current scope lock: Symol entries nme lock link void p() { void p() {

7 s with chining + scoping s with chining + scoping Current scope lock: Symol entries nme lock link Current scope lock: > Symol entries nme lock link c c p c p void p() { void p() { nd c hsh to the sme hsh vlue () use chining insert p nd enter new scope lock () s with chining + scoping s with chining + scoping Current scope lock: Symol entries nme lock link Current scope lock: Symol entries nme lock link void p() { c p c void p() { c p c mke hsh point to (stticlly) closest will lter find this one first in chin s with chining + scoping Opertions on Tle with Chining nd Scope () Informtion Current scope lock: void p() { = ; Symol entries nme lock link c p c lookup(): follow chin links Declring Serch long the chin for s hsh vlue. When nme (ny nme) in nother lock is found, is not douledefined. Insert t the eginning of the hsh chin. Referencing Serch long the chin for s hsh vlue. The first to e found is the right one. If is not found, is undefined. A new lock is strted Insert lock pointer in BLOCKTAB. End of the lock Move the lock down in BLOCKTAB. Move the lock down in SYMTAB. Move the hsh pointer to point t the previous lock