Speed difference between If-Else and Ternary operator in C...?

Question

So at the suggestion of a colleague, I just tested the speed difference between the ternary operator and the equivalent If-Else block... and it seems that the ternary operator yields code that is between 1x and 2x faster than If-Else. My code is:

  gettimeofday(&tv3, 0);
  for(i = 0; i < N; i++)
  {
     a = i & 1;
     if(a) a = b; else a = c;
  }
  gettimeofday(&tv4, 0);


  gettimeofday(&tv1, 0);
  for(i = 0; i < N; i++)
  {
     a = i & 1;
     a = a ? b : c;
  }
  gettimeofday(&tv2, 0);

(Sorry for using gettimeofday and not clock_gettime... I will endeavor to better myself.)

I tried changing the order in which I timed the blocks, but the results seem to persist. What gives? Also, the If-Else shows much more variability in terms of execution speed. Should I be examining the assembly that gcc generates?

By the way, this is all at optimization level zero (-O0).

Am I imagining this, or is there something I'm not taking into account, or is this a machine-dependent thing, or what? Any help is appreciated.

Answer 1

You could also go completely branchless and measure if it makes any difference:

int m = -(i & 1);
a = (b & m) | (c & ~m);

On today's architectures, this style of programming has grown a bit out of fashion.

Answer 2

There's a good chance that the ternary operator gets compiled into a cmov while the if/else results in a cmp+jmp. Just take a look at the assembly (using -S) to be sure. With optimizations enabled, it won't matter any more anyway, as any good compiler should produce the same code in both cases.