c++ - Is comparing to zero faster than comparing to any other number?

Question

Is

if(!test)

faster than

if(test==-1)

I can produce assembly but there is too much assembly produced and I can never locate the particulars I'm after. I was hoping someone just knows the answer. I would guess they are the same unless most CPU architectures have some sort of "compare to zero" short cut.

thanks for any help.

Answer 1

Typically, yes. In typical processors testing against zero, or testing sign (negative/positive) are simple condition code checks. This means that instructions can be re-ordered to omit a test instruction. In pseudo assembly, consider this:

Loop:
  LOADCC r1, test // load test into register 1, and set condition codes
  BCZS   Loop     // If zero was set, go to Loop

Now consider testing against 1:

Loop:
  LOAD   r1, test // load test into register 1
  SUBT   r1, 1    // Subtract Test instruction, with destination suppressed
  BCNE   Loop     // If not equal to 1, go to Loop

Now for the usual pre-optimization disclaimer: Is your program too slow? Don't optimize, profile it.