You can get quite large code size savings by using the original 8 registers everywhere you can get away with 32bit instructions - which is quite often, since th

You can get quite large code size savings by using the original 8 registers everywhere you can get away with 32bit instructions - which is quite often, since they zero the upper 32 bits. e.g. XORL eax, eax will zero 64bit rax. Compilers don't seem very good at this, but I find I can usually save 10% code size when coding assembly routines just by paying attention to instruction width and which registers are used where. With 64bit instructions it doesn't matter which registers you use, the rex prefix is used anyway.