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<P><font class="example"><img border="0" src="images/dingyi.gif" width="32" height="31">例8.4</font> 赋值语句d：＝（a－b）＋（a－c）＋（a－c）可以翻译成如下的三地址语句序列：<br><br>
t：＝a-b<br>
u：＝a-c<br>
v：＝t+u<br>
d：＝v＋u<br>
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其中d在基本块的出口，假定它是活跃的。上述代码生成算法对此子地址语句序列产生的目标代码如表8．3所示，表中给出了在代码生成过程中寄存器描述器和地址描述器的值，因为a,b,c一直在存储器中，所以在地址描述器中没有显示它们。我们还假定作为临时变量的t，u和v不在存储器中，除非用MOV指令明显地将它们的值存放到存储器之中。</P>
<P>getreg（t：＝a－b）返回R0作为存放t的当前值的寄存器。由于a不在R0中,生成指令MOV a，R0和SUB
b,R0，然后更新寄存器描述器以记录R0包含t。</P>
<P>代码生成以此种方式继续进行，直到最后的三地址语句d：＝u＋v处理完为止，注意，因为这儿的u没有下次引用，R1将变为空。最后，在基本块的出口处生成指令MOV
R0，d,存储活跃变量d的值。</P>
<P align=center>表8．3  代码序列 </P>
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<TD width="21%">
<DIV align=center>语句</DIV></TD>
<TD width="23%">
<DIV align=center>生成的代码</DIV></TD>
<TD width="23%">
<DIV align=center>寄存器描述</DIV></TD>
<TD width="33%">
<DIV align=center>地址描述器</DIV></TD></TR>
<TR>
<TD height=21 width="21%">
<DIV align=center>t：＝a－b</DIV></TD>
<TD height=21 width="23%">
<DIV align=center>MOV a,R0</DIV></TD>
<TD height=19 rowSpan=2 width="23%">
<DIV align=center>空寄存器</DIV>
<DIV align=center>R0包含t</DIV></TD>
<TD height=21 rowSpan=2 width="33%">
<DIV align=center>t在R0中</DIV>
<DIV align=center></DIV></TD></TR>
<TR>
<TD width="21%">
<DIV align=center> </DIV></TD>
<TD width="23%">
<DIV align=center>SUB b,R0</DIV></TD></TR>
<TR>
<TD width="21%">
<DIV align=center></DIV></TD>
<TD width="23%">
<DIV align=center></DIV></TD>
<TD width="23%">
<DIV align=center></DIV></TD>
<TD width="33%">
<DIV align=center></DIV></TD></TR>
<TR>
<TD width="21%">
<DIV align=center>u：＝a－c</DIV></TD>
<TD width="23%">
<DIV align=center>MOV a,R1</DIV></TD>
<TD width="23%">
<DIV align=center>R0包含t</DIV></TD>
<TD rowSpan=2 width="33%">
<DIV align=center>t在R0中</DIV>
<DIV align=center>u在R1中</DIV></TD></TR>
<TR>
<TD width="21%">
<DIV align=center> </DIV></TD>
<TD width="23%">
<DIV align=center>SUB c,R1</DIV></TD>
<TD width="23%">
<DIV align=center>R1包含u</DIV></TD></TR>
<TR>
<TD width="21%">
<DIV align=center></DIV></TD>
<TD width="23%">
<DIV align=center></DIV></TD>
<TD width="23%">
<DIV align=center></DIV></TD>
<TD width="33%">
<DIV align=center></DIV></TD></TR>
<TR>
<TD height=23 width="21%">
<DIV align=center>v：＝t＋u</DIV></TD>
<TD height=23 width="23%">
<DIV align=center>ADD R1,R0</DIV></TD>
<TD height=23 width="23%">
<DIV align=center>R0包含v</DIV></TD>
<TD rowSpan=2 width="33%">
<DIV align=center>
<DIV align=center>u在R1中</DIV>v在R0中</DIV></TD></TR>
<TR>
<TD height=2 width="21%">
<DIV align=center> </DIV></TD>
<TD height=2 width="23%">
<DIV align=center> </DIV></TD>
<TD height=2 width="23%">
<DIV align=center>R1包含u</DIV></TD></TR>
<TR>
<TD width="21%">
<DIV align=center></DIV></TD>
<TD width="23%">
<DIV align=center></DIV></TD>
<TD width="23%">
<DIV align=center></DIV></TD>
<TD width="33%">
<DIV align=center></DIV></TD></TR>
<TR>
<TD width="21%">
<DIV align=center>d：＝v＋u</DIV></TD>
<TD width="23%">
<DIV align=center>ADD R1,R0</DIV></TD>
<TD width="23%">
<DIV align=center>R0包含d</DIV></TD>
<TD width="33%">
<DIV align=center>d在R0中</DIV></TD></TR>
<TR>
<TD height=22 width="21%">
<DIV align=center> </DIV></TD>
<TD height=22 width="23%">
<DIV align=center>MOV R0,d</DIV></TD>
<TD height=22 width="23%">
<DIV align=center> </DIV></TD>
<TD height=22 width="33%">
<DIV align=center>d在R0和存储器中</DIV></TD></TR></TBODY></TABLE>
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