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<td class="content">在翻译if-then语句S→<b>if</b> E <b>then</b> S<span class="down">1</span>时，我们建立了一个新的标号E.true，并且用它来标识关于S<span
class="down">1</span>的代码的第一条指令，如图7.13（a）所示。表7.4给出了一个语法制导定义。在E的代码中将有这样的转移指令：若E为真则转移到E.true，并且若E为假则转移到S.next因此我们置E.false为S.next.
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表7.4 控制流语句的语法制导定义<br>
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<td width="60%" align="center">语义规则</td>
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<p>S -><b>if</b> E <b>then</b> S<sub>1</sub></p>
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<p>S -><b>if</b> E <b>then</b> S<sub>1</sub><br>
<b>else</b> S<sub>2</sub></p>
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<p>S -><b>while</b> E <b>do</b> S<sub>1</sub></td>
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E.true := newlabel;<br>
E.false := S.next;<br>
S<sub>1</sub>.next := S.next<br>
S.code := E.code ||<br>
gen(E.true ':') || S<sub>1</sub>.code<br>
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E.true := newlabel;<br>
E.false := newlabel;<br>
S<sub>1</sub>.next := S.next<br>
S<sub>2</sub>.next := S.next;<br>
S.code := E.code ||<br>
gen(E.true ':') || S<sub>1</sub>.code
||gen('goto' S.next) ||<br>
gen(E.false ':') || S<sub>2</sub>.code
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S.begin := newlabel;<br>
E.true := newlabel;<br>
E.false := S.next<br>
S<sub>1</sub>.next := S.begin<br>
S.code := gen(S.begin ':') <br>
|| E.code ||gen(S.true ':') <br>
|| S<sub>1</sub>.code <br>
||gen('goto' S.begin) <br>
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