The C64 used the 6510 CPU which is a slightly different version of the 6502. One difference was that you had a few extra instructions. For example, I believe you could transfer the index registers so you wouldn't have to go to memory. TXY, TYX. These could be done in 1 clock cycle versus 4 clock cycles if you had to get a memory.
Well, I had a C64 and a C128 and I do remember using them. Am I misremembering?
[Update]
I guess there was an entire set of illegal opcodes on the 6510. Unfortunately, I longer have my source code; that shoebox is long gone.
The one thing I do remember is that it had something to do with registers because it gave my code a computer significant performance boost because I didn't have to go to memory so often.
I imagine emulators need to consider this.