com.goldencode.expr

Class CodeUnit.BranchData

java.lang.Object

com.goldencode.expr.CodeUnit.BranchData

Enclosing class:

CodeUnit

protected class CodeUnit.BranchData
extends java.lang.Object

This class encapsulates information necessary to branch execution of the JVM from one bytecode instruction to another in the bytecode array.

A BranchData object provides a bridge between the current code unit and a destination code unit. At the time the code unit containing this object is being assembled, the destination code unit typically has not yet been created. BranchData therefore stores an ExtraAst instance which represents the logical destination of the branch. This target AST acts as a placeholder for the destination code unit. After the assembly of all code units in a compiled expression's execute method has been completed, a fixup is performed to calculate the required offest necessary for this object's branch instruction to jump to the beginning of the destination code unit.

Field Summary

Fields

Modifier and Type	Field and Description
private Instruction	instruction Instruction which will cause the branch to occur.
private NumericArgument	offsetArg Offset to the destination address from the current instruction.
private ExtraAst	target Code unit which is the destination of the branch.

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	BranchData(boolean condition, ExtraAst target, int type, boolean binary) Constructor.

Method Summary

Modifier and Type	Method and Description
protected void	addBranchInstruction() This method returns the instruction which performs the branch.
protected void	fixupOffset() This method sets the offset value of the argument to the branch instruction.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

target

private ExtraAst target

Code unit which is the destination of the branch.

offsetArg

private NumericArgument offsetArg

Offset to the destination address from the current instruction.

instruction

private Instruction instruction

Instruction which will cause the branch to occur.

Constructor Detail

BranchData

protected BranchData(boolean condition,
                     ExtraAst target,
                     int type,
                     boolean binary)
              throws ExpressionException

Constructor. Selects an appropriate opcode for the operation, based on the value of condition, creates a branch instruction and its offset argument. The final value of the offset cannot be determined until the code array is laid out and all branch instructions are fixed up, so an offset of 0 is used as a temporary placeholder.

Parameters:

condition - Condition under which code unit branches to target's code unit. This may be ignored, depending upon the type specified. For instance, a type GOTO branch is unconditional, and therefore does not use condition as a branch trigger.

target - An AST which contains information about the destination of the branch instruction implied by type and condition. This AST will eventually have a CodeUnit object stored in its extra field, which represents the destination of the branch instruction.

type - Type of branch logic; indicates which bytecode instruction is used, depending also on condition.

binary - true if this code unit manages a binary operation (i.e., requires two operands from the runtime stack). If false, the branch will only consider a single stack operand (unary), or no operands (unconditional), depending upon the type parameter.

Throws:

ExpressionException - if an invalid branching condition is detected.

Method Detail

addBranchInstruction

protected void addBranchInstruction()

This method returns the instruction which performs the branch.

fixupOffset

protected void fixupOffset()

This method sets the offset value of the argument to the branch instruction. It is this offset, when added to the address of the current execution point, which determines the address of the destination instruction. This method is called after the final addresses have been assigned to each instruction in the bytecode array, so the branch offset is simply determined by subtracting the address of the branch instruction from the address of the destination instruction.