Exchanging physical to logical register mapping for obfuscation purpose when instruction of no operational impact is executed

A data processing apparatus comprising:

• processor means for performing data processing operations in response to data processing instructions, said data processing instructions referencing program model register means;
• physical register means for storing data values which are subjected to said data processing operations;
• tag storage means for storing for each physical register means a tag value indicative of one of said program model register means, said processor means referencing said tag storage means in order to perform said data processing operations and said tag storage means providing a one-to-one mapping between said program model register means and said physical register means;
• and
• tag value exchange means for performing a tag switch to exchange two tag values in said tag storage means,
• said tag value exchange means for performing said tag switch when said processor means executes a predetermined data processing instruction,
• wherein said predetermined data processing instruction references two program model registers and for which a choice of which two physical registers are mapped to which of said two program model registers will have no effect on an outcome of said data processing operations,
• and said tag value exchange means for performing said tag switch with respect to the tag values indicative of said two program model registers.

A method of data processing comprising:

• performing data processing operations in response to data processing instructions, said data processing instructions referencing program model registers;
• storing data values in a set of physical registers which are subjected to said data processing operations;
• in a tag storage unit, storing for each physical register a tag value indicative of one of said program model registers, and referencing said tag storage unit in order to perform said data processing operations and providing in said tag storage unit a one-to-one mapping between said program model registers and said physical registers;
• and
• performing a tag switch to exchange two tag values in said tag storage unit,
• wherein said tag switch is performed when a predetermined data processing instruction is executed,
• wherein said predetermined data processing instruction references two program model registers and for which a choice of which two physical registers are mapped to which of said two program model registers will have no effect on an outcome of said data processing operations,
• and said tag switch is performed with respect to the tag values indicative of said two program model registers.