南开大学编译原理第四章课件PPT.ppt

类型转换例子 x := y + i * jx、y——实数i、j——整数 t1 := i int* j t3 := inttoreal t1 t2 := y real+ t3 x := t2 8.3.6 记录（结构）域的访问 保存每个域的类型和相对地址?符号表 lookup可用于域名字 独立符号表 t：符号表指针，record(t)?T.type 翻译p^.info + 1 p的类型pointer(record(t)) p^的类型record(t) 得到t?查找info域 8.4 布尔表达式的翻译 E→E or E | E and E | not E | (E) | id relop id | true | false 8.4.1 两种翻译方式 数值编码：0－false，非0－ture 控制流语句属性true——布尔表达式为真时跳转到的程序位置属性false——布尔表达式为假时跳转到的程序位置 短路求值 E1 or E2，E1=false才对E2求值 注意副作用问题：函数调用 8.4.2 用数值表示布尔值 a or b and not c ? t1 := not c t2 := b and t1 t3 := a or t2 a b ? if a b then 1 else 0 100: if a b goto 103 101: t := 0 102: goto 104 103: t := 1 104: 翻译模式 E→E1 or E2 { E.place = newtemp; emit(E.place ‘:=’ E1.place ‘or’ E2.place); } E→E1 and E2 { E.place = newtemp; emit(E.place ‘:=’ E1.place ‘and’ E2.place); } E→not E1 { E.place = newtemp; emit(E.place ‘:=’ ‘not’ E1.place); } E→( E1 ) { E.place = E1.place; } E→id1 relop id2 { E.place = newtemp; emit(‘if’ id1.place relop.op id2.place ‘goto’ nextstat + 3); emit(E.place ‘:=’ ‘0’); emit(‘goto’ nextstat + 2); emit(E.place ‘:=’ ‘1’); } E→true { E.place = newtemp; emit(E.place ‘:=’ ‘1’); } E→false { E.place = newtemp; emit(E.place ‘:=’ ‘0’); } 例8.3 翻译a b or c d and e f 100: if a b goto 103 107: t2 := 1 101: t1 := 0 108: if e f goto 111 102: goto 104 109: t3 := 0 103: t1 := 1 110: goto 112 104: if c d goto 107 111: t3 := 1 105: t2 := 0 112: t4 := t2 and t3 106: goto 108 113: t5 := t1 or t4 8.4.3 控制流语句的翻译 S→if E then S1 | if E then S1 else S2 | while E do S1 语法制导定义－if S→ if E then S1 E.true = newlabel; E.false = S.next; S1.next = S.next; S.code = E.code || gen(E.true ‘:’) || S1.code 语法制导定义－if-else S→if E then S1 else S2 E.true = newlabel; E.false = newlabel; S1.next = S.next; S2.next = S.next; S.code = E.code || gen(E.true ‘:’) || S1.code || gen(‘goto’ S.next) || gen(E.false ‘:’) ||S2.code 语法制导定义 S→while E do S1 S.begin = newlabel; E.true = newlabel; E.false = S.next;