That paper uses no actual benchmarks, but rather grabbed a single system utility and then hand-optimized it; SPEC and geekbench show x86-64 comes in well over 4 bytes on average.
Sure, I never claimed it to be the be-all-end-all, just the only real source I could find. Adding "SPEC" or "geekbench" didn't really help.
Doing a little more digging, I have also found this [1], which claims "the results show that the average instruction length is about 2 to 3 bytes". On the other hand, this [2] finds that the average instruction length is 4.25 bytes.
Bytes per instruction doesn't really say anything useful for code density when talking about RISC vs. CISC though, since (arguably) the whole idea is that individual CISC instructions are supposed to do more than individual RISC instructions. A three instruction CISC routine at five bytes each is still a win over a four instruction RISC routine at four bytes each. Overall code size is what actually matters.
Doing a little more digging, I have also found this [1], which claims "the results show that the average instruction length is about 2 to 3 bytes". On the other hand, this [2] finds that the average instruction length is 4.25 bytes.
Bytes per instruction doesn't really say anything useful for code density when talking about RISC vs. CISC though, since (arguably) the whole idea is that individual CISC instructions are supposed to do more than individual RISC instructions. A three instruction CISC routine at five bytes each is still a win over a four instruction RISC routine at four bytes each. Overall code size is what actually matters.
[1] https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.40...
[2] http://www.cs.unc.edu/~porter/pubs/instrpop-systor19.pdf