JVM Code Generation - Introduction to Compiler Construction in a Java World - page 173

constructor <init> , <init> with descriptor ()V , of Square 's super class java/lang/Object ,
on the single argument this . After invoking the super constructor, the constructor simply
returns, obeying the return instruction at location 4.
The second, explicit method square() is even simpler. The stack frame information,
Stack=2,Locals=2,Args_size=2
says we need two locations for computation (the two operands to the multiplication op-
eration), two local variables allocated (for the implicit parameter this , and the explicit
parameter x ), and that there will be two actual arguments on the stack: this and the
argument for x . The code consists of four one-byte instructions:
0:iload_1
1:iload_1
2:imul
3:ireturn
The instruction at location 0 loads the value for x onto the stack; the instruction at location
1 does the same. The instruction at location 2 pops those two values off the stack and
performs an integer multiplication on them, leaving its result on the stack. The instruction
at location 3 returns the integer ( x*x ) on the stack as the result of the method invocation.
Of course, to emit these instructions, we first create a CLEmitter instance, which is an
abstraction of the class file we wish to build, and then call upon CLEmitter 's methods for
generating the necessary headers and instructions.
For example, to generate the class header,
publicclassSquareextendsjava.lang.Object
one would invoke the addClass() method on output , an instance of CLEmitter :
output.addClass(mods,"Square","java/lang/Object",false);
where
The mods denotes an ArrayList containing the single string \public",
Square is the class name,
java/lang/Object is the internal form for the fully qualified super class, and
false indicates that the class is not synthetic.
As a simpler example, the one-byte, no-argument instruction aload_1 may be generated
by
output.addNoArgInstruction(ALOAD_1);
To fully understand CLEmitter and all of its methods for generating code, one should
read through the CLEmitter.java source file, which is distributed as part of the j-- compiler.
One must sometimes be careful to pay attention to what the CLEmitter is expecting. For
example, sometimes a method requires a fully qualified name in Java form such as java.
lang.Object ; other times an internal form is required, that is, java/lang/Object .
For another, more involved example of code generation, we revisit the Factorial class
from Chapters 2 and 3. Recall the source code:
packagepass;
importjava.lang.System;