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    <p>There's a little more to this than just saying: "You didn't turn on the optimizer."</p> <p>At least at a guess, you didn't (initially) turn on the optimizer in <em>either</em> case. Despite this, the C# version <em>without</em> optimization turned on ran almost as fast as the D version <em>with</em> optimization. Why would that be?</p> <p>The answer stems from the difference in compilation models. D does static compilation, so the source is translated to an executable containing machine code, which then executes. The only optimization that happens is whatever is done during that static compilation.</p> <p>C#, by contrast, translates from source code to MSIL, an intermediate language (i.e., basically a bytecode). That is then translated to machine language by the JIT compiler built into the CLR (common language runtime--Microsoft's virtual machine for MSIL). You can specify optimization when you run the C# compiler. That only controls optimization when doing the initial compilation from source to byte code. When you run the code, the JIT compiler does its thing--and it does its optimization whether you specify optimization in the initial translation from source to byte code or not. That's why you get much faster results with C# than with D when you didn't specify optimization with either one.</p> <p>I feel obliged to add, however, that <em>both</em> results you got (7 and 8 seconds for D and C# respectively) are really pretty lousy. A decent optimizer <em>should</em> recognize that the final output didn't depend on the loop at all, and based on that it should eliminate the loop completely. Just for comparison, I did (about) the most straightforward C++ translation I could:</p> <pre><code>#include &lt;iostream&gt; #include &lt;time.h&gt; int main() { long summer = 0; auto start = clock(); for (long i = 0; i &lt; 10000000000; i++) summer++; std::cout &lt;&lt; double(clock() - start) / CLOCKS_PER_SEC; } </code></pre> <p>Compiled with VC++ using <code>cl /O2b2 /GL</code>, this consistently shows a time of 0.</p>
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    1. POWhy are for loops in D so much slower than for loops in C#?
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    1. USJerry Coffin
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    1. POWhy are for loops in D so much slower than for loops in C#?
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    1. COThank you for taking the time to answer with such depth. Grade A answer.
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      1. USDan Doe
    2. COSomeone file a bug. This is old hat for optimizer and with all the CTFE stuff in D it should be rather trivial to do: just keep track of what is deterministic and CTFE (almost) anything who's inputs are so marked.
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    3. COLDC also has 0 seconds. Which simply confirms widely spread opinion that one should not use DMD if optimizations are really important.
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      1. USMihails Strasuns
 

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