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The Most Underused Compiler Switches in Visual C++

, March 11, 2014 Microsoft's Visual C++ team explains the nine most underused switches and why you should make sure to include them in your build.
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With Visual Studio 2012, we also introduced automatic vectorization feature in the compiler. The "auto-vectorizer" tries to make loops in your code run faster by automatically vectorizing your code — that's to say, using the SSE instructions available in all current mainline Intel and AMD chips. Auto-vectorization is enabled by default. You don't need to request this speedup. You don't need to throw a compiler switch. You don't need to set environment variables or registry entries. You don't need to change your C++ code. You don't need to insert #pragmas. The compiler just goes ahead and does it. It all comes for free.

So what does /Qvec-report bring to the table? In order to always maintain functional correctness, the compiler is sometimes conservative and fails automatic vectorization for a loop. The /Qvec-report provides diagnostic information about auto-vectorization, and is equipped with the following reporting levels:

/Qvec-report:1, Outputs an informational message for loops that are vectorized.

/Qvec-report:2, Outputs an informational message for loops that are vectorized and for loops that are not vectorized, together with a reason code.

Additional comments:

If you use C++, it is likely that you care a lot about runtime performance. Recompiling with VS2012 and above brings you that added performance with automatic vectorization. To fine-tune those hot-spots in your application, try out /Qvec-report. Sometimes, you can tweak the way you wrote a loop, allowing the compiler to then auto-vectorize it for you. For more information, see Jim Hogg's series of blogs on this topic.

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