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Quick question, assuming beforehand
mov eax, 0
which is more efficient?
inc eax
inc eax
or
add eax, 2
Also, in case the two incs are faster, do compilers (say, the GCC) commonly (i.e. w/o aggressive optimization flags) optimize var += 2 to it?
PS: Don't bother to answer with a variation of "don't prematurely optimize", this is merely academic interest.
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ADD is not always faster than INC , but it is almost always at least as fast (there are a few corner cases on certain older micro-architectures, but they are exceedingly rare), and sometimes significantly faster.
After all, both ADD and INC updates flag registers. The only difference is that INC doesn't update CF .
If you ever wanna know raw performance stats of x86 instructions, see Dr Agner Fogs listings (volume 4 to be exact). As for the part about compilers, thats dependent on the compiler's code generator, and not something you should rely on too much.
on a side note: I find it funny/ironic that in a question about performance, you used MOV EAX,0 to zero a register instead of XOR EAX,EAX :P (and if MOV EAX,0 was done beforehand, the fastest variant would be to remove the inc's and add's and just MOV EAX,2).
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