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

Two inc instructions on the same register (or more generally speaking two read-modify-write instructions) do always have a dependency chain of at least two cycles. This is assuming a one clock latency for a inc, which is the case since the 486. That means if the surrounding instructions can't be interleaved with the two inc instructions to hide those latencies, the code will execute slower.

But no compiler will emit the instruction sequence you propose anyway (mov eax,0 will be replaced by xor eax,eax, see What is the purpose of XORing a register with itself?)

mov eax,0
inc eax
inc eax

it will be optimizied to

mov eax,2