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Destination Operands

The base instruction class includes a field for the destination operand, even though not all instructions use it. The dst_op method returns the value of this operand. If an instruction does not produce a result, the destination operand should always be a null operand. Moreover, the destination may be null even if an instruction does produce a result. In that case, the result value is computed and then discarded.

If the destination operand is a variable, its type must be compatible with the result type of the instruction.

If the result of an instruction is a temporary value, the destination operand is a pointer to the instruction where the value is used. Note that this is different than an instruction pointer in a source operand. The source operands of an instruction point to the children in an expression tree, while the destination operand points to the parent instruction.

The set_dst method may be used to set the destination operand for an instruction. However, there are some important restrictions on using this method:

• For derived classes with instructions that never produce result values, it is illegal to call set_dst. This allows the library to ensure that the destination will always be a null operand (3). Trying to use set_dst for one of these instructions will cause an error.
• You may not set the destination operand to point to another instruction; trying to do so will cause an error. This is to prevent inconsistent pointers. The destination should only point to another instruction if that other instruction has a source operand that points back to it. Thus, the library automatically sets the destination operand when an instruction is used as a source operand.
• If the destination operand already contains a pointer to another instruction, you cannot use set_dst to overwrite it. This also causes a run-time error. As in the previous case, this is to prevent inconsistent pointers. Instead, you must first call the instruction's remove method to clear the destination operand. See section Source Operands.

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