Relative performance of x86 inc vs. add instruction

Question

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.

Answer 1

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).