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copied!<ol> <li><p>Inline the calls to lengthsq().</p></li> <li><p>Change pow(r2,-1.5) to 1/(r2*sqrt(r2)) to lower the cost of the computing r^1.5</p></li> <li><p>Use scalars (p_i_acc, etc.) inside the innner most loop rather than p[i].acc to collect your result. The compiler may not know that p[i] isn't aliased with p[j], and that might force addressing of p[i] on each loop iteration unnecessarily.</p></li> </ol> <p>4a. Try replacing the if (...) tau_q = with </p> <pre><code> tau_q=minimum(...,...) </code></pre> <p>Many compilers recognize the mininum function as one they can do with predicated operations rather than real branches, avoiding pipeline flushes.</p> <p>4b. [EDIT to split 4a and 4b apart] You might consider storing tau_..q2 instead as tau_q, and comparing against r2/v2 rather than r2*r2/v2*v2. Then you avoid doing two multiplies for each iteration in the inner loop, in trade for a single squaring operation to compute tau..q2 at the end. To do this, collect minimums of tau_q1 and tau_q2 (not squared) separately, and take the minimum of those results in a single scalar operation on completion of the loop]</p> <ol start="5"> <li>[EDIT: I suggested the following, but in fact it isn't valid for the OP's code, because of the way he updates in the loop.] Fold the two loops together. With the two loops and large enough set of particles, you thrash the cache and force a refetch from non-cache of those initial values in the second loop. The fold is trivial to do.</li> </ol> <p>Beyond this you need to consider a) loop unrolling, b) vectorizing (using SIMD instructions; either hand coding assembler or using the Intel compiler, which is supposed to be pretty good at this [but I have no experience with it], and c) going multicore (using OpenMP).</p>

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