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POWriting performance critical C# code in C++
 

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    <p>I'm currently working on some performance critical code, and I have a particular situation where I'd love to write the whole application in C#, but performance reasons mean C++ ends up being FAR faster. </p> <p>I did some benchmarking on two different implementations of some code (One in C#, another in C++) and the timings showed that the C++ version was 8 times faster, both versions in release mode and with all optimizations enabled. (Actually, the C# had the advantage of being compiled as 64-bit. I forgot to enable this in the C++ timing)</p> <p>So I figure, I can write the majority of the code base in C# (Which C# makes very easy to write), and then write native versions of things where the performance is critical. The particular code piece I tested in C# and C++ was one of the critical areas where > 95% of processing time was spent.</p> <p>What's the recommended wisdom on writing native code here though? I've never written a C# application that calls native C++, so I have no idea what to do. I want to do this in a way that minimizes the cost of having to do the native calls as much as possible.</p> <p>Thanks!</p> <p>Edit: Below is most of the code that I'm actually trying to work on. It's for a n-body simulation. 95-99% of the CPU time will be spent in Body.Pairwise().</p> <pre><code>class Body { public double Mass; public Vector Position; public Vector Velocity; public Vector Acceleration; // snip public void Pairwise(Body b) { Vector dr = b.Position - this.Position; double r2 = dr.LengthSq(); double r3i = 1 / (r2 * Math.Sqrt(r2)); Vector da = r3i * dr; this.Acceleration += (b.Mass * da); b.Acceleration -= (this.Mass * da); } public void Predict(double dt) { Velocity += (0.5 * dt) * Acceleration; Position += dt * Velocity; } public void Correct(double dt) { Velocity += (0.5 * dt) * Acceleration; Acceleration.Clear(); } } </code></pre> <p>I also have a class that just drives the simulation with the following methods:</p> <pre><code> public static void Pairwise(Body[] b, int n) { for (int i = 0; i &lt; n; i++) for (int j = i + 1; j &lt; n; j++) b[i].Pairwise(b[j]); } public static void Predict(Body[] b, int n, double dt) { for (int i = 0; i &lt; n; i++) b[i].Predict(dt); } public static void Correct(Body[] b, int n, double dt) { for (int i = 0; i &lt; n; i++) b[i].Correct(dt); } </code></pre> <p>The main loop looks just like:</p> <pre><code>for (int s = 0; s &lt; steps; s++) { Predict(bodies, n, dt); Pairwise(bodies, n); Correct(bodies, n, dt); } </code></pre> <p>The above is just the bare minimum of a larger application I'm actually working on. There's some more things going on, but the most performance critical things occur in these three functions. I know the pairwise function is slow (It's n^2), and I do have other methods that are faster (Barnes-hutt for one, which is n log n) but that's beyond the scope of what I'm asking for in this question.</p> <p>The C++ code is nearly identical:</p> <pre><code>struct Body { public: double Mass; Vector Position; Vector Velocity; Vector Acceleration; void Pairwise(Body &amp;b) { Vector dr = b.Position - this-&gt;Position; double r2 = dr.LengthSq(); double r3i = 1 / (r2 * sqrt(r2)); Vector da = r3i * dr; this-&gt;Acceleration += (b.Mass * da); b.Acceleration -= (this-&gt;Mass * da); } void Predict(double dt) { Velocity += (0.5 * dt) * Acceleration; Position += dt * Velocity; } void Correct(double dt) { Velocity += (0.5 * dt) * Acceleration; Acceleration.Clear(); } }; void Pairwise(Body *b, int n) { for (int i = 0; i &lt; n; i++) for (int j = i + 1; j &lt; n; j++) b[i].Pairwise(b[j]); } void Predict(Body *b, int n, double dt) { for (int i = 0; i &lt; n; i++) b[i].Predict(dt); } void Correct(Body *b, int n, double dt) { for (int i = 0; i &lt; n; i++) b[i].Correct(dt); } </code></pre> <p>Main loop:</p> <pre><code>for (int s = 0; s &lt; steps; s++) { Predict(bodies, n, dt); Pairwise(bodies, n); Correct(bodies, n, dt); } </code></pre> <p>There also exists a Vector class, that works just like a regular mathematical vector, which I'm not including for brevity.</p>
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    1. COC# shouldn't be slower. you can achieve faster code using unchecked blocks to avoid overflow checking and other stuff.
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      1. USYochai Timmer
    2. COhttp://stackoverflow.com/questions/5326269/is-c-really-slower-than-say-c
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      1. USYochai Timmer
    3. CO@Yochai: I already tried that, by wrapping all my arithmetic in unsafe blocks. It boils down to a few functions which do lots of floating point math, but I'm doing -a lot- of computation each second. Also, I'd like to know how to call C++ from C#, as I already do have some existing code that's written in C++. Some of the newer code I'd like to write isn't computation related and is easier to write in C# though.
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