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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 added an "Auto-Parallelizer" (parallel optimizer) feature to the compiler that automatically parallelizes loops in your code, at all times maintaining functional correctness. These /Qpar-report switches enable the reporting feature of the auto-parallelizer and control the diagnostic information reported by the auto-parallelizer. Qpar-report has the following reporting levels:

/Qpar-report:1, Outputs an informational message for loops that are parallelized.

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

Additional Comments:

When the compiler automatically parallelizes loops in code, it spreads computation across multiple processor cores, thereby improving performance. A loop is parallelized only if the compiler determines that it is legal to do so and that parallelization would improve performance. Enabling such reports will help developers analyze why an optimization wasn't triggered in areas where they expected to see it. The reports help you spot any loops that you want and expect to be auto-parallelized. However, remember that the compiler cannot always tell which loops would benefit from parallelization: Only if the loop count is large, and the loop body contains significant computation, will it be worthwhile to spread its work across multiple cores and processors. Consider helping the compiler by including a #pragma loop(hint_parallel(n)) just before the loop of interest. The auto-parallelizer is not enabled by default, so use /Qpar.

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