How many ways to set a register to zero?

Question

I\'m curious how many ways are there to set a register to zero in x86 assembly. Using one instruction. Someone told me that he managed to find at least 10 ways to do it.

Answer 1

A couple more possibilities:

sub ax, ax

movxz, eax, ah

Edit: I should note that the movzx doesn't zero all of eax -- it just zero's ah (plus the top 16 bits that aren't accessible as a register in themselves).

As for being the fastest, if memory serves the sub and xor are equivalent. They're faster than (most) others because they're common enough that the CPU designers added special optimization for them. Specifically, with a normal sub or xor the result depends on the previous value in the register. The CPU recognizes the xor-with-self and subtract-from-self specially so it knows the dependency chain is broken there. Any instructions after that won't depend on any previous value so it can execute previous and subsequent instructions in parallel using rename registers.

Especially on older processors, we expect the 'mov reg, 0' to be slower simply because it has an extra 16 bits of data, and most early processors (especially the 8088) were limited primarily by their ability to load the stream from memory -- in fact, on an 8088 you can estimate run time pretty accurately with any reference sheets at all, and just pay attention to the number of bytes involved. That does break down for the div and idiv instructions, but that's about it. OTOH, I should probably shut up, since the 8088 really is of little interest to much of anybody (for at least a decade now).