I am interested knowing the best approach for bulk memory copies on an x86 architecture. I realize this depends on machine-specific characteristics. The main target is typical desktop machines made in the last 4-5 years.
I know that in the old days MOVSD with REPE was nominally the fastest approach because you could move 4 bytes at a time, but I have read that nowadays MOVSB is just as fast and is simpler to write, so you may as well do a byte move and just forget about the complexities of a 4-byte move.
A surrounding question is whether MOVxx instructions are worth it at all. If the CPU can run so much faster than the memory bus, then maybe it is pointless to use a CISC move and you may as well use a plain MOV. This would be most attractive because then I could use the same algorithms on other processor architectures like ARM. This brings up the analogous question of whether ARM’s specialized instructions for bulk memory moves (which are totally different than Intels) are worth it or not.
Note: I have read section 3.7.6 in the Intel Optimization Reference Manual so I am familiar with the basics. I am hoping someone can relate practical experience in the area beyond what is in this manual.
Modern Intel and AMD processors have optimisations on REP MOVSB that make it copy entire cache lines at a time if it can, making it the best (may not be fastest, but pretty close) method of copying bulk data.
As for ARM, it depends on the architecture version, but in general using an unrolled loop would be the most efficient.