I've seen claims that it's different for different architectural registers, e.g. _lots_ of backing store for rax, less for rbx. It's likely to be significant for the vector registers too which could plausibly have features like one backing store for the various widths, in which case deliberately using the smaller vectors would sometimes win out. I'll never bother to write the asm by hand with that degree of attention but would like better cost models in the compiler backend.
adrian_b|13 days ago
Both register files have a few hundred of scalar, respectively vector registers.
Besides these 2 big register files, there are a few other registers for renaming some special registers, e.g. the flags register and the AVX-512 mask registers.
Between the general-purpose registers there are no renaming differences, any of the 16 registers can be mapped to any of the hundreds of hidden registers, regardless if the register name used in the program is RAX, RCX or whatever.
Some differences between apparently similar instructions may be caused not by the fact that they use RAX or another register, but by whether they affect the flags or not, because the number of renaming registers available for flags is much smaller than the hundreds available for GPRs.