JVM Code Generation - Introduction to Compiler Construction in a Java World - page 180

5.3.2 Short-Circuited &&
Consider the logical && operator. In a logical expression like

arg 1 && arg 2

The semantics of Java, and so of j--, require that the evaluation of such an expression be
short-circuited. That is, if arg 1 is false , arg 2 is not evaluated; false is returned. If arg 1
is true , the value of the entire expression depends on arg 2 . This can be expressed using
the Java conditional expression, as arg 1 ? arg 2 :false . How do we generate code for this
operator? The code to be generated depends of whether the branch for the entire expression
is to be made on true , or on false :

Branchtotargetwhen Branchtotargetwhen
arg1&&arg2istrue:

arg1&&arg2isfalse:

branchtoskipif branchtotargetif
arg1isfalse arg1isfalse
branchtotargetwhen branchtotargetif
arg2istrue arg2isfalse

skip:...

For example, the code generated for

if(a>b&&b>c){
c=a;

}
else{
c=b;

}

would be

0:iload_1
1:iload_2
2:if_icmple15
5:iload_2
6:iload_3
7:if_icmple15
10:iload_1
11:istore_3
12:goto17
15:iload_2
16:istore_3
17:...

The codegen() method in JLogicalAndOp generates the code for the && operator:

publicvoidcodegen(CLEmitteroutput,StringtargetLabel,
booleanonTrue){
if(onTrue){
StringfalseLabel=output.createLabel();
lhs.codegen(output,falseLabel,false);
rhs.codegen(output,targetLabel,true);
output.addLabel(falseLabel);
}else{
lhs.codegen(output,targetLabel,false);
rhs.codegen(output,targetLabel,false);

}

}