title: C Loop Optimizations with Conditionals on Looping Variable
body: <p>Apologies if this is asked in the archives.  I found some similar questions, but none that seemed exactly what I wanted.</p>
  
  <p>The distilled version of the problem I'm working on is as follows. I have a series of calculations to perform that will store values in 4 (very large) arrays: A,B,C and D.  These calculations are interdependent, for example calculating b[i] could require using a[i-1].  I am capable of expressing everything in a single loop, but that results in edge cases where for certain values of i, only some of the calculations should be performed.  For example:</p>
  
  <pre><code>for(i=0;i&lt;end;i++)
  {
      if(i == 0)
          //calculate A[i+1] and B[i+1]
      else if (i == end-1)
          //calculate C[i-1] and D[i-1]
      else
          //calculate A[i+1], B[i+1], C[i-1], D[i-1]
  }
  </code></pre>
  
  <p>For issues of performance, I would like to avoid having conditionals within my loop.  Evaluating a conditional would be cheap compared to the calculations, but possibly not negligible.  My question is if a compiler might reliably expand that to</p>
  
  <pre><code>//calculate A[1] and B[1]
  for(i=1;i&lt;end-1;i++)
  {
      //calculate A[i+1], B[i+1], C[i-1], D[i-1]
  }
  //calculate C[end-2] and D[end-2]
  </code></pre>
  
  <p>I gather from the archives that the compiler would break apart my loop if the conditional expressions were constant, but here they depend on i, which in principle <em>could</em> be changed by some of my calculations.  Will it detect that I'm not tampering with the iteration variable and thus break it apart in the sensible way?</p>
  
  <p>Extra information, in case you decide to answer the question by suggesting a better way to do things:</p>
  
  <p>Originally the code was written with 4 loops, to calculate elements for each of the arrays.  This was the most intuitive way to write the code, but it was inefficient.  Since calculating elements in one array depended on elements in the other arrays, this meant I had to read in all 4 arrays from memory during each of the 4 loops.  Since these arrays do not fit in cache, this is not optimal and I needed code that would loop through my arrays only once.</p>
  
  <p>I'm also aware that I can break my loop apart by hand, and indeed that is how things are currently done.  However these calculations involve nontrivial formulas (and I can't afford the performance hit of calling a function during every iteration of this loop), so breaking apart the code caused code duplication that is not only very hard to read, but almost unmaintainable the next time my formulas get tweaked (which they will...)</p>
  
  <p>Thanks in advance!</p>