Topic 5: semantic analysis. 5.5 Types of Semantic Actions

Transcription

1 Top 5: semant analyss 5.5 Types of Semant tons Semant analyss Other Semant tons Other Types of Semant tons revously, all semant atons were for alulatng attrbute values. In a real ompler, other types of atons are also useful: Reportng an error Changng the Symbol Table Generatng ode et. 2 1

2 Semant analyss Other Semant tons For example, we mght add an aton to modfy the symbol table when we have determned the type of an dentfer. Introdue a new funton to nsert an dentfyer nto the symbol table: : add_type(symbolstable* pt, Identfer, Type type) Where: pt s a ponter to the symbols table. T {.type:=t.type}, T nt {T.type:=nteger}, T real {T.type:=real}, d, { d.type:=.type,add_type(&pt,,.type)}, {add_type(&pt,,.type)} 3 n xtended xample, Semant analyss Other Semant tons T T nt T real 1, := * 2 ( 1 ) {.type:=t.type}, {T.type:=nteger}, {T.type:=real}, { 1.type:=.type, add_type(&pt,,.type)}, {add_type(&pt,,.type)}, { &pt.type=.type.type:=.type} {.value:= 1.value+ 2.value,.type:= 1.type}, {.value:= 1.value* 2.value,.type:= 1.type}, {.value:= 1.value,.type:= 1.type}, {.value:=.value,.type:=.type}, {.value:=.value,.type:=.type}}, 4 2

3 Syntax-dreted efntons xamples The followng s the analyses of nt var1, x, y x:=(3+4)*5 T nt var1, x, y x := ( ) * 5 5 Syntax-dreted efntons xamples T nt var1, x, y x := ( ) * 5 6 3

4 Syntax-dreted efntons xamples T nt var1, x, y x := ( ) * 5 7 Syntax-dreted efntons xamples T y(nt) nt var1, x, y x := ( ) * 5 8 4

5 Syntax-dreted efntons xamples T y(nt) x(nt) var1(nt) nt var1, x, y x := ( ) * 5 9 Syntax-dreted efntons xamples T y(nt) x(nt) var1(nt) nt var1, x, y x := ( ) *

6 Syntax-dreted efntons xamples T y(nt) x(nt) var1(nt) v:4 v:5 nt var1, x, y x := ( ) * 5 11 Syntax-dreted efntons xamples T y(nt) x(nt) var1(nt) v:4 v:4 v:5 nt var1, x, y x := ( ) *

7 Syntax-dreted efntons xamples T y(nt) x(nt) var1(nt) v:7 v:4 v:4 v:5 nt var1, x, y x := ( ) * 5 13 Syntax-dreted efntons xamples T y(nt) x(nt) v:7 v:5 var1(nt) v:7 v:4 v:4 v:5 nt var1, x, y x := ( ) *

8 Syntax-dreted efntons xamples T y(nt) x(nt) v:7 5 v:5 var1(nt) v:7 v:4 v:4 v:5 nt var1, x, y x := ( ) * 5 15 Syntax-dreted efntons xamples T y(nt) x(nt) v:7 5 v:5 var1(nt) v:7 v:4 v:4 v:5 nt var1, x, y x := ( ) *

9 Top 5: semant analyss 5.4 mbedded tons Semant analyss mbedded tons tons ntermxed wth Symbols Some reursve desent (top-down) systems depend on plang semant atons between eah symbol on the RHS, e.g., :- M 1 $aton1 M 2 $aton2 M 3 $aton3 For example: :- T {R. := T.val} R {.val := R.s} Often resented as: :- T {R. := T.val} R {.val := R.s} 18 9

10 Semant analyss mbedded tons Beneft of atons between symbols The beneft of ths approah n top down analyss s that: If semant atons only performed at the ompleton of a produton, then semant attrbutes for eah element wll only be avalable one all onsttuents reognsed. By plang atons between symbols, attrbutes an be alulated before we start analysng a onsttuent, and an thus be passed down to the onsttuent s hldren. :- T {R. := T.val} R {.val := R.s} T num val=4 val=4 =4 R 19 Semant analyss mbedded tons tons between symbols n Bottom-Up analyss In an R parser, a gven parsng state may be proessng several alternatve produtons, e.g., S 8 :- (. ) :- (. + T ) If the urent parse stak has symbols: Stak: ( then t s not lear f we are workng on :- (+T) or :- () Only when we redue s the deson made as to whh rule s ntended. THUS, we annot perform atons plaed between RHS symbols! 20 10

11 Semant analyss mbedded tons Rewrtng grammars to allow atons between symbols We an rewrte grammars wth atons between symbols to work wth R() parsers. Basally, for eah aton between symbols: We reate an artfal symbol We provde a lambda rule for that symbol wth the aton at ts end Old: :- T {R. := T.val} R {.val := R.s} New: :- T X R X :- λ {.val := R.s} {R. := T.val} Now, the aton s performed when the lambda rule s redued! 21 Semant analyss Rewrtng grammars to allow atons between symbols Note that the semant rule of X spefes symbols from the produton for. The ompler must keep trak of the symbol ontext for eah lambda rule. YCC does ths automatally. mbedded tons :- T X R X :- λ {.val := R.s} {R. := T.val} In some ases, ths s not needed: rog :- begn CST els Stmts end CST :- λ {learsymbtable()} 22 11

12 Top 5: semant analyss 5.5 Summary Semant analyss Summary Summary of Semant nalyss n ttrbute Grammar s an extenson of a CFG, wth: 1. ttrbutes assoated wth eah Symbol 2. Rules assoated wth eah CFG produton to move attrbutes about ttrbutes an be nherted or synthessed Semant Rules an aess global data strutures, suh as a symbol table

13 Semant analyss Summary Summary of Semant nalyss () In a multple-pass ompler, one an: 1. Buld the parse tree 2.Calulate dependenes between attrbutes 3.Fnd the topologal sort of the attrbutes 4.xeute the attrbute evaluaton rules n that order OR Use a reursve desent approah to resolvng attrbutes on parse tree nodes. 25 Semant analyss Summary Summary of Semant nalyss () sngle-pass ompler utlses restrted attrbute grammars to enable attrbute evaluaton whle onstrutng the parse tree S-attrbuted grammars sute bottom-up analyss -attrbuted grammars sute top-down parsng -attrbuted grammars an be used n bottom-up parsng wth more work 26 13

14 Top 5: semant analyss 5.6 Semant nalyss n relaton to programmng strutures Semant analyss V Strutures that appear n hgh level programmng languages elaratons Mathematal expressons ssgnment Condtonal Instrutons oops (for, whle, repeat) roedures ll of these aeas of the grammar wll need some form of semant aton

15 Semant analyss Varable elaratons 1. Consult the Symbol Table to verfy that the dentfer has not already been delared 2. Insert the new dentfer n the Symbol Table Make sure that type restrtons are satsfed. ssgn temporary varables to hold ntermedate results, e.g., = (b + 5) * Mathematal xpressons assgn a regster to hold result of b+5 29 Semant analyss ssgnment d := expreson Verfy that the dentfer has been prevously delared Verfy ompatblty of types between dentfer and expresson Condtonal xpressons and oops Generate ode for the labels of jump targets Funtons earaton Store n tyhe symbol table that the dentfer s a funton, and ts parameters Calls Chek that the funton s delared Chek number, type and order of the parameters 30 15