alt Hacker NewsBecause "sub eax,eax" looks stupid. (and also clears the carry flag, unlike "xor eax, eax")
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HackerThemAll • today at 3:25 PM
If I remember correctly, sub used to be slower than xor on some ancient architectures.
Because "sub eax,eax" looks stupid. (and also clears the carry flag, unlike "xor eax, eax")
If I remember correctly, sub used to be slower than xor on some ancient architectures.
xor clears the carry as well? In fact, looks like xor and sub affect the same set of flags!
xor:
> The OF and CF flags are cleared; the SF, ZF, and PF flags are set according to the result. The state of the AF flag is undefined.
sub:
> The OF, SF, ZF, AF, PF, and CF flags are set according to the result.
(I don't have an x64 system handy, but hopefully the reference manual can be trusted. I dimly remembered this, or something like it, tripping me up after coming from programming for the 6502.)