Efficiency of if-construct

[JEichhorn]

I was wondering, which of the following code snippets would usually perform faster.

First avoiding IF statements within the loop, at the cost of a few additional arithmetic operations:

   DO k = 2,n-2
   DO i = 2,l-2
      vvb          = sign(0.5,v(i,2,k))
      zwil1(i,1,k) = (0.5+vvb)*eps(i,1,k) - (vvb-0.5)*zwil1(i,2,k)
   END DO
   END DO

Second using IF blocks:

   DO k = 2,n-2
   DO i = 2,l-2
      IF (v(i,2,k) .LT. 0.0) THEN
         zwil1(i,1,k)   = zwil1(i,2,k)
      ELSE
         zwil1(i,1,k)   = eps(i,1,k)
      END IF
   END DO
   END DO

Of course i could just try out on one machine at least, but nevertheless any comments on this would be appreciated.

 

[xwb]

It depends on whether the implementation of SIGN has jumps in it. On some machine architectures, it is a built in instruction so there won't be any jumps. If SIGN has jumps, you're just trading one set of jumps for another.

 

[salgerman]

The way I see it, the second loop structure would be much faster...first of all, I thought IF evaluations were pretty fast; second, the second loop does not have multiplications or summations, it is just a straight out assignment.

 

[JEichhorn]

I have checked both versions on a quadcore i7 both under Windows 8.1 and Ubuntu 14.04 LTS, compiled with Absoft 14.0. For a problem consuming ~10 minutes of CPU time, the difference between both versions was less than a few seconds. So i will stay with the IF blocks with their superior readability.