What is the fastest way to set a single memory cell to zero in x86? Typically the way I do it is this:
C745D800000000 MOV [ebp-28], 0
As you can see this has a pretty chunky encoding since it is using all 4 bytes for the constant. With a plain register I can use MVZE which is more compact, but MVZE does not work with memory.
I was thinking maybe clear a register, then MOV the register value to the memory. Then, it would be two instructions, but only 5 bytes total instead of the one 7-byte instruction above. Following the rule "if its shorter, its usually faster", this might be preferable.
Unfortunately, what you have written here is the only way to "directly" zero out a memory cell. Of course, XORing out a register and then moving it to some memory location would also work, but I don't know if that would be any faster.
If you happen to have a register whose value is zero and you're sure of it, then by all means use it. Otherwise, just stick with the mov [ebp-28], 0. Keep in mind that mem, imm operands are known to be one of the slowest : if you profile your code and find out that this is a bottleneck, try initializing a register to zero at the beginning of your function (or whatever) and then using it throughout the code, as a sort of a predefined constant.