第13讲 中间代码生成_3

控制流语句及其SDT哈尔滨工业大学陈鄞第六章中间代码生成P →S S →S 1S 2S →id =E ; | L = E ; S →if B then S 1| if B then S 1else S 2| while B do S 1控制流语句的基本文法例控制流语句的代码结构S if B then S 1else S 2布尔表达式B 被翻译成由跳转指令构成的跳转代码继承属性S.next ：是一个地址，该地址中存放了紧跟在S 代码之后的指令(S 的后继指令)的标号B.true ：是一个地址，该地址中存放了当B 为真时控制流转向的指令的标号B.false ：是一个地址，该地址中存放了当B 为假时控制流转向的指令的标号用指令的标号标识一条三地址指令S 1.next S 2.next S.nextif B.trueB.falsethen goto S.nextelse B.code S 1.codeS 2.codeS.codeP →{ S.next = newlabel (); }S { label (S.next );} S →{ S 1.next = newlabel (); } S 1{ label (S 1.next ); S 2.next = S.next ; } S 2S →id =E ; | L = E ; S →if B then S 1| if B then S 1else S 2| while B do S 1控制流语句的SDTlabel (L ): 将下一条三地址指令的标号赋给Lnewlabel ( ): 生成一个用于存放标号的新的临时变量L ，返回变量地址S →if B then S 1else S 2S →if {B.true =newlabel ();B.false =newlabel ();}Bthen {label (B.true );S 1.next =S.next ;}S 1{gen (‘goto ’S.next )}else {label (B.false );S 2.next =S.next ;}S 2if -then -else 语句的SDT if B.trueB.falseS.nextthen goto S.next else S 1.nextS 2.next B.code S 1.code S 2.codeS.codeif-then语句的SDTS→if B then S1S→if{B.true=newlabel();B.false=S.next;}Bthen{label(B.true);S1.next=S.next;}S1ifB.true B.falseS.next thenS1.next B.codeS1.codeS.codeS →while B do S 1S →while {S.begin =newlabel ();label (S.begin );B.true =newlabel ();B.false =S.next ;}Bdo {label (B.true );S 1.next =S.begin ;}S 1{gen (‘goto ’S.begin );}while -do 语句的SDTwhile B.true B.falseS.begin do goto S.beginS 1.nextB.code S 1.code S.next S.code控制流语句及其SDT哈尔滨工业大学陈鄞第六章中间代码生成布尔表达式及其SDT哈尔滨工业大学陈鄞第六章中间代码生成B → B or B | B and B | not B | (B )| E relop E | true | false布尔表达式的基本文法优先级：not > and >orrelop （关系运算符）：<, <=, >, >=，==, ! =关系表达式在跳转代码中，逻辑运算符&&、|| 和! 被翻译成跳转指令。

运算符本身不出现在代码中，布尔表达式的值是通过代码序列中的位置来表示的 例语句三地址代码if x <100 goto L2goto L 3L 3: if x >200 goto L 4goto L 1L 4: if x !=y goto L 2goto L 1L 2:x =0L 1:if ( x <100 || x >200 &&x !=y )x =0;B → E 1relop E 2{ gen (‘if ’ E 1.addr relop E 2.addr ‘goto ’ B.true )；gen (‘goto ’ B.false ); }B → true { gen (‘goto ’ B.true ); } B → false { gen (‘goto ’ B.false ); }B → ( {B 1.true =B.true ; B 1.false =B.false ; } B 1) B → not { B 1.true = B.false ; B 1.false = B.true ; } B 1布尔表达式的SDTB →B 1or B 2B → { B 1.true = B.true ; B 1.false = newlabel (); }B 1or { label (B 1.false ); B 2.true = B.true ; B 2.false = B.false ; }B 2B →B 1or B 2的SDTB.falseB 1.false B 2.false B 1.true B 2.true B.trueor B 1.codeB 2.codeB.codeB →B 1and B 2B → { B 1.true = newlabel (); B 1.false = B.false ; } B 1and { label (B 1.true ); B 2.true = B.true ; B 2.false = B.false ; }B 2B.falseB 1.false B 2.false B 1.true B 2.true B.trueand B 1.codeB 2.codeB.code B →B 1and B 2的SDT布尔表达式及其SDT哈尔滨工业大学陈鄞第六章中间代码生成控制流翻译的例子哈尔滨工业大学陈鄞第六章中间代码生成P →{a}S {a} S →{a}S 1{a}S 2S →id=E ;{a}| L =E ;{a}E →E 1+E 2{a}|-E 1{a}|(E 1){a}|id {a}| L {a} L →id[E ]{a}| L 1[E ]{a} S →if {a}B then {a}S 1| if {a}B then {a}S 1else {a}S 2| while {a}B do {a}S 1{a}B → {a}B or {a}B | {a}B and {a}B | not {a}B | ({a}B )| E relop E {a}| true {a}| false {a}控制流语句的SDT任何SDT 都可以通过下面的方法实现首先建立一棵语法分析树，然后按照从左到右的深度优先顺序来执行这些动作SDT 的通用实现方法P →{a}S {a} S →{a}S 1{a}S 2S →id=E ;{a}| L =E ;{a}E →E 1+E 2{a}|-E 1{a}|(E 1){a}|id {a}| L {a} L →id[E ]{a}| L 1[E ]{a} S →if {a}B then {a}S 1| if {a}B then {a}S 1else {a}S 2| while {a}B do {a}S 1{a}B → {a}B or {a}B | {a}B and {a}B | not {a}B | ({a}B )| E relop E {a}| true {a}| false {a}控制流语句的SDTwhile a < b do if c < d then x = y + z ;elsex = y –z ;S 1S 3S 2BB 1例do then S 3else S P B 1c<dS 1x =y +zifa <b B whileS 2x=y-zP { S.next = newlabel ( ); }S { label (S.next );}do then S 3else S B 1c<dS 1x =y +zP例S.n=L 1whileifa <b B S 2x=y-zS while {S.begin =newlabel ();label (S.begin );B.true =newlabel ();B.false =S.next ;}B do {label (B.true );S 1.next =S.begin ;}S 1{gen (‘goto ’S.begin );}S 3.n=L 2S.begin =L 2=1B → E 1relop E 2{ gen (‘if ’ E 1.addr relop E 2.addr ‘goto ’ B.true );gen (‘goto ’ B.false ); }dowhile then S 3else S B 1c<dS 1x =y +zP 例1:if a <b goto L 32: goto L 1goto L 2S.n=L 1S 2x=y-zif a <b B.t =L 3B.f =L 1=3BS if {B.true =newlabel ();B.false =newlabel ();}Bthen {label (B.true );S 1.next =S.next ;}S 1{gen (‘goto ’S.next );}else {label (B.false );S 2.next =S.next ;}S 2B 1.t =L 4B 1.f =L 5S 1.n=L 2S 2.n=L 2= 5= 8S 3.n=L 2S.n=L 1doif then S 3else SB a <b B 1c<dS 1x =y +z S2x=y-z P例1: if a < b goto L 32: goto L 13: if c < d goto L 44: goto L 55: t 1= y + z 6: x = t 17: goto L 28: t 2= y -z 9: x = t 210:11:goto L 2=11while B.t =L 3B.f =L 1=33115811S.begin =L 2=11: if a < b goto 32: goto 113: if c < d goto 54: goto 85: t 1= y + z6: x = t 17: goto 18: t 2= y -z9: x = t 210: goto 111:语句“while a <b do if c <d then x=y+z else x=y-z ”的三地址代码1: ( j <,a , b , 3)2:( j , -, -, 11 )3:( j <, c , d , 5 )4:( j , -, -, 8 ) 5:( + , y , z , t 1 ) 6:( = , t 1, -, x ) 7:( j , -, -, 1 )8:( -, y , z , t 2 ) 9:( = , t 2, -, x ) 10:( j , -, -, 1 )11:控制流翻译的例子哈尔滨工业大学陈鄞第六章中间代码生成。