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Published byLiana Knapper Modified over 3 years ago

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Backpatching: The syntax directed definition we discussed before can be implemented in two or more passes (we have both synthesized attributes and inheritent attributes). –Build the tree first –Walk the tree in the depth first order. The main difficulty with code generation in one pass is that we may not know the target of a branch when we generate code for flow-of –control statements Backpatching is the technique to get around this problem. –Generate branch instructions with empty targets –When the target is known, fill in the label of the branch instructions (backpatching).

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Example: generate intermediate code for the following program segment If (a < b) then I := I+1 else j:= I+1 100: if a < b then goto ??? 101: goto ??? 102: t1 = I+1 103: I = t1 104: goto ??? 105: t1 = I+1 106: j = t1 107:

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Boolean expressions E->E1 or M E2 E->E1 and M E2 E-> not E1 E-> ( E1 ) E->id1 relop id2 E->true E->false M-> E has two attributes truelist and falselist to store the list of goto instructions with empty destinations. –Truelist: goto TRUELABEL –Falselist: goto FALSELABLE M.quad: the number for current instruction Makelist(quad): create a list.

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E->E1 or M E2 { backpatch(E1.falselist, M.qual); E.truelist = merge(E1.truelist, E2.truelist); E.falselist = E2.falselist;} E->E1 and M E2 {backpatch(E1.truelist, M.qual); E.truelist = E2.truelist; E.falselist = merge(E1.truelist, E2.truelist);} E-> not E1 {E.truelist = E1.falselist, E.falselist = E1.truelist;} E-> ( E1 ) {E.truelist = E1.truelist; E.falselist = E1.falselist;} E->id1 relop id2 {E.truelist = makelist(nextquad); E.falselist = makelist(nextquad+1); emit(‘if’ id1.place relop.op id2.place ‘goto ???’); emit(‘goto ???’); } E->true {E.truelist = makelist(nextquad); emit(‘goto ???’);} E->false {E.falselist = makelist(nextquad);emit(‘goto ???’);} M-> {M.quad = nextquad;}

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Example: generating code using one pass LR parser a < b or (c < d and e < f) 100: If (a < b) goto ??? 101: goto ??? 102: if c < d goto ??? 103: goto ??? 104: If e < f goto ??? 105: goto ???

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Flow of control statements S -> if E then S1 S -> if E then S1 else S2 S -> while E do S1 S-> begin L end S-> A L->L; S L->S Attributes: E.truelist, E.falselist S.nextlist: goto statements to the next instruction after this statement.

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S -> if E then M S1 { backpatch(E.truelist, M.quad); S.nextlist = merge(E.falselist, S1.nextlist);} S-> if E then M1 S1 N else M2 S2 { backpatch(E.truelist, M1.quad); backpatch(E.falselist, M2.quad); S.nextlist := merge(S1.nextlist, N.nextlist, S2.nextlist);} S -> while M1 E do M2 S1 {backpatch(E.truelist, M2.quad); Backpatch(S1.nextlist, M1.quad);S.nextlist = E.falselist; Emit(‘goto ‘ M1.quad);} S-> begin L end {S.nextlist = L.nextlist} S-> A {S.nextlist = null;} L->L; M S {backpatch(L1.nextlist, M.quad);L.nextlist = S.nextlist;} L->S {L.nextlist = S.nextlist;} N-> {N.nextlist = makelist(nextquad); emit(‘goto ???’);} M-> {M.quad = nextquad;}

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While (a < b or (c < d and e < f)) do if c < d then x:= y+z else x:= y-z

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–Procedure calls: Must generate the calling sequence and returning sequence. Can be done either at the calling or at the called procedure – depending on machine and OS. –A simplified example: S->call id (Elist) {for each item p on queue do emit(‘param’ p); emit(‘call’ id.place);} Elist->Elist1, E {append E.place to the end of the queue;} Elist->E {initialize queue to contain only E.place;}

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