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POC# vs. C++ performance -- why doesn't .NET perform the most basic optimizations (like dead code elimination)?
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2013-12-05T07:38:06.117
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2015-01-31T06:27:00.140
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2013-12-05T11:17:29.510
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Body
I'm seriously doubting if the C# or .NET JIT compilers perform any useful optimizations, much less if they're actually competitive with the most basic ones in C++ compilers. Consider this extremely simple program, which I conveniently made to be valid in both C++ and C#: <pre><code>#if __cplusplus #else static class Program { #endif static void Rem() { for (int i = 0; i < 1 << 30; i++) ; } #if __cplusplus int main() #else static void Main() #endif { for (int i = 0; i < 1 << 30; i++) Rem(); } #if __cplusplus #else } #endif </code></pre> When I compile and run it in the newest version of C# (VS 2013) in release mode, it doesn't terminate in any reasonable amount of time. Edit: Here's another example: <pre><code>static class Program { private static void Test2() { } private static void Test1() { #if TEST Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); Test2(); #else Test2(); #endif } static void Main() { for (int i = 0; i < 0x7FFFFFFF; i++) Test1(); } } </code></pre> When I run this one, it takes a lot longer if <code>TEST</code> is defined, even though everything is a no-op and <code>Test2</code> should be inlined. Even the the most ancient C++ compilers I can get my hands on, however, optimize everything away, making the programs return immediately. <h3>What prevents the .NET JIT optimizer from being able to make such simple optimizations? Why?</h3>
Tags
<c#><c++><.net><performance><optimization>
Title
C# vs. C++ performance -- why doesn't .NET perform the most basic optimizations (like dead code elimination)?
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