Probably this way they could reuse most of the instruction decoder between 32bit and 64bit mode, giving equal performance while saving transistors. I doubt it's

Probably this way they could reuse most of the instruction decoder between 32bit and 64bit mode, giving equal performance while saving transistors. I doubt it's related to compiling tools - instruction encoding dealt with by the assembler and those are easy to write. The actual changes that would have to be done to the compiler because of switching to 64bit are independent of the instruction encoding.