Compiler construction in4303 lecture 4. Overview. Bottom-up (LR) parsing. LR(0) parsing. LR(0) parsing. LR(0) parsing. Compiler construction lecture 4

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1 Compler constructon lecture Compler constructon n lecture Bottom-up prsng Chpter.. Overvew synt nlyss: tokens S lnguge grmmr prser genertor ottom-up prsng push-down utomton CION/GOO tles LR(), SLR(), LR(), LLR() progrm tet lecl nlyss tokens synt nlyss S contet hndlng nnotted S Bottom-up (LR) prsng LR() prsng Left-to-rght prse, Rghtmost-dervton crete node when ll chldren re present hndle: nodes representng the rght-hnd sde of producton term IDN term IDN rest_epresson term IDN epresson rest_epr p ( noot mes ε ) runnng emple: epresson grmmr nput epresson OF epresson epresson term term term IDNIFIR ( epresson ) short-hnd notton LR() prsng LR() prsng keep trck of progress nsde potentl hndles when consumng nput tokens LR tems: N α β ntl set Z ( ) stck nput nput token () onto the stck compute new stte ε-closure: epnd dots n front of non-termnls

2 Compler constructon lecture LR() prsng LR() prsng stck S nput stck S nput reduce hndle on top of the stck compute new stte reduce hndle on top of the stck compute new stte LR() prsng LR() prsng stck S nput stck S S nput nput token on top of the stck compute new stte nput token on top of the stck compute new stte LR() prsng LR() prsng stck S S S nput stck S S S nput reduce hndle on top of the stck compute new stte reduce hndle on top of the stck compute new stte

3 Compler constructon lecture LR() prsng LR() prsng stck S nput stck S S nput nput token on top of the stck compute new stte reduce hndle on top of the stck compute new stte LR() prsng rnston dgrm stck Z ccept! nput S S Z ( ) Z S S S S ( ) ercse ( mn.) nswers complete the trnston dgrm for the LR() utomton cn you thnk of sngle nput epresson tht cuses ll sttes to e used? If yes, gve n emple. If no, epln.

4 Compler constructon lecture he LR tles LR() prsng concse notton stte GOO tle ( ) stte CION tle ( ) stck S S S S S S S S S S S S S S Z nput cton reduce y reduce y reduce y reduce y reduce y ccept he LR push-down utomton LR() conflcts SWICH cton_tle[top of stck]: CS : see ook; CS ( reduce, N α): POP the symols of α FROM the stck; S stte O top of stck; PUSH N ON the stck; S new stte O goto_tle[stte,n]; PUSH new stte ON the stck; CS empty: RROR; Shft-reduce conflct rry ndeng: [ ] [ ] () (reduce) ε-rule: Restpr ε pr erm Restpr () Restpr (reduce) LR() conflcts Hndlng LR() conflcts Reduce-reduce conflct ssgnment sttement: Z V := V (reduce) (reduce) typcl LR() tle contns mny conflcts soluton: use one-token look-hed two-dmensonl CION tle [stte,token] dfferent constructon of CION tle SLR() Smple LR LR() LLR() Look-hed LR

5 Compler constructon lecture SLR() prsng SLR() CION tle solves (some) -reduce conflcts stte look-hed token ( ) reduce N α ff token FOLLOW(N) FOLLOW() = {, ), } FOLLOW() = {, ), } FOLLOW(Z) = { } ( ) ( ) ( ) SLR() CION/GOO tle Unfortuntely... stte stck stck symol symol / look-hed / look-hed token token ( ) [ ] s s s s r r s r r r r r r r r r r r s s s s s : : : : : : [ ] SLR() leves mny -reduce conflcts unsolved prolem: FOLLOW(N) set s unon of ll contets n whch N my occur s s r r s s s r r s r r r s s r r r s s s s sn to stte n rn reduce rule n emple S r r r r r LR() utomton ercse ( mn.) S S S S S S S S S S S S : S : S : : derve the SLR() CION/GOO tle (wth -reduce conflct) for the grmmr: S

6 Compler constructon lecture nswers LR() prsng mntn follow set per tem LR() tem: N α β {σ} ε - closure for LR() tem sets: f set S contns n tem P α N β {σ} then forech producton rule N γ S must contn the tem N γ {τ} where τ = FIRS( β {σ} ) LR() utomton LR() utomton S S S S S S S {} S {} {} {} S S {} S {} {} {} S {} S {} {} {} S S S FOLLOW(S) = {} FOLLOW() = {,} S S {} {} S {} S {} S S S S S {} S {} S {} LLR() prsng LLR() utomton LR tles re g comne equl sets y mergng lookhed sets S {} S {} {} {} S S {} S {} {} {} S {} S {} {} {} S S {} {} S {} S {} S S {} S {} S {}

7 Compler constructon lecture LLR() utomton LLR() CION/GOO tle S S S {} {} S {} S S {} {,} {,} {} {} {,} {,} {} {} {,} S S S {} {,} {} {,} S S S {} {,} stte stck symol / look-hed token s s s s r r r s s s r s r r : S : S : : Mkng grmmrs LR() or not? Summry grmmrs re often mguous * hndle -reduce conflcts (defult) longest mtch: precedence drectves nput: * * * reduce * synt nlyss: tokens S ottom-up prsng push-down utomton CION/GOO tles LR() NO look-hed SLR() LR() LLR() one-token look-hed, FOLLOW sets to solve -reduce conflcts SLR(), ut FOLLOW set per tem LR(), ut equl sttes re merged