/openmp (启用 OpenMP 支持)/openmp (Enable OpenMP Support)

04/15/2019

本文内容

导致编译器处理 #pragma omp OpenMP 支持的指令。Causes the compiler to process #pragma omp directives in support of OpenMP.

语法Syntax

/openmp [：实验 性]/openmp[:experimental]

/openmp/openmp

备注Remarks

#pragma omp 用于指定 指令 和 子句。#pragma omp is used to specify Directives and Clauses. 如果编译中未指定 /openmp ，则编译器将忽略 openmp 子句和指令。If /openmp isn't specified in a compilation, the compiler ignores OpenMP clauses and directives. 即使未指定 /openmp ，编译器也会处理 OpenMP 函数调用。OpenMP Function calls are processed by the compiler even if /openmp isn't specified.

C + + 编译器当前支持 OpenMP 2.0 标准。The C++ compiler currently supports the OpenMP 2.0 standard. 但是，Visual Studio 2019 现在还提供 SIMD 功能。However, Visual Studio 2019 also now offers SIMD functionality. 若要使用 SIMD，请使用 /openmp：实验 性选项编译。To use SIMD, compile by using the /openmp:experimental option. 使用 /openmp 开关时，此选项将启用常见 openmp 功能和其他 openmp SIMD 功能，这些功能不可用。This option enables both the usual OpenMP features, and additional OpenMP SIMD features not available when using the /openmp switch.

使用 /openmp 和 /clr 编译的应用程序只能在单个应用程序域进程中运行。Applications compiled by using both /openmp and /clr can only be run in a single application domain process. 不支持多个应用程序域。Multiple application domains aren't supported. 也就是说，当模块构造函数 (.cctor) 运行时，它将检测是否使用 /openmp 编译了该进程，以及该应用程序是否已加载到非默认运行时。That is, when the module constructor (.cctor) is run, it detects if the process is compiled using /openmp, and if the app is loaded into a non-default runtime.

如果尝试将使用 /openmp 和 /clr 编译的应用加载到非默认应用程序域中，则会在 TypeInitializationException 调试器外部引发异常，并且 OpenMPWithMultipleAppdomainsException 调试器中会引发异常。If you attempt to load an app compiled using both /openmp and /clr into a non-default application domain, a TypeInitializationException exception is thrown outside the debugger, and a OpenMPWithMultipleAppdomainsException exception is thrown in the debugger.

在下列情况下，也可能会引发这些异常：These exceptions can also be raised in the following situations:

如果你的应用程序是使用 /clr 编译的，而不是使用 /openmp 编译的，并且加载到非默认应用程序域中，则该过程包含使用 /openmp 编译的应用。If your application is compiled using /clr but not /openmp, and is loaded into a non-default application domain, where the process includes an app compiled using /openmp.

如果将 /clr 应用传递到实用程序(如 regasm.exe)，则会将其目标程序集加载到非默认应用程序域中。If you pass your /clr app to a utility, such as regasm.exe, which loads its target assemblies into a non-default application domain.

公共语言运行时的代码访问安全性不适用于 OpenMP 区域。The common language runtime's code access security doesn't work in OpenMP regions. 如果在并行区域外部应用 CLR 代码访问安全性属性，则该属性在并行区域中不起作用。If you apply a CLR code access security attribute outside a parallel region, it won't be in effect in the parallel region.

Microsoft 不建议你编写允许部分受信任的调用方的 /openmp 应用程序。Microsoft doesn't recommend that you write /openmp apps that allow partially trusted callers. Don't use AllowPartiallyTrustedCallersAttribute, or any CLR code access security attributes.

在 Visual Studio 开发环境中设置此编译器选项To set this compiler option in the Visual Studio development environment

打开项目的“属性页” 对话框。Open the project's Property Pages dialog box.

展开 "配置属性" " > c/c + + > 语言" 属性页。Expand the Configuration Properties > C/C++ > Language property page.

修改 OpenMP 支持 属性。Modify the OpenMP Support property.

以编程方式设置此编译器选项To set this compiler option programmatically

示例Example

下面的示例显示了在启动线程池后使用线程池的某些影响。The following sample shows some of the effects of thread pool startup versus using the thread pool after it has started. 假设有一个 x64、单核、双处理器，线程池需要大约16毫秒的时间启动。Assuming an x64, single core, dual processor, the thread pool takes about 16 ms to start up. 在此之后，线程池的开销较低。After that, there's little extra cost for the thread pool.

使用 /openmp 进行编译时，对 test2 的第二次调用的运行时间不会超过使用 /openmp- 进行编译的时间，因为没有线程池启动。When you compile using /openmp, the second call to test2 never runs any longer than if you compile using /openmp-, as there's no thread pool startup. 在一百万次迭代中， /openmp 版本比 /openmp- 版本更快，后者的第二次调用 test2。At a million iterations, the /openmp version is faster than the /openmp- version for the second call to test2. 25次迭代时， /openmp- 和 /openmp 版本都注册小于时钟粒度。At 25 iterations, both /openmp- and /openmp versions register less than the clock granularity.

如果你的应用程序中仅有一个循环，并且该循环在不到15毫秒的时间内运行 (则) 计算机上的大约系统开销进行调整， /openmp 可能不适合。If you have only one loop in your application and it runs in less than 15 ms (adjusted for the approximate overhead on your machine), /openmp may not be appropriate. 如果更高，你可能需要考虑使用 /openmp。If it's higher, you may want to consider using /openmp.

// cpp_compiler_options_openmp.cpp

#include

#include

#include

#include

volatile DWORD dwStart;

volatile int global = 0;

double test2(int num_steps) {

int i;

global++;

double x, pi, sum = 0.0, step;

step = 1.0 / (double) num_steps;

#pragma omp parallel for reduction(+:sum) private(x)

for (i = 1; i <= num_steps; i++) {

x = (i - 0.5) * step;

sum = sum + 4.0 / (1.0 + x*x);

}

pi = step * sum;

return pi;

}

int main(int argc, char* argv[]) {

double d;

int n = 1000000;

if (argc > 1)

n = atoi(argv[1]);

dwStart = GetTickCount();

d = test2(n);

printf_s("For %d steps, pi = %.15f, %d milliseconds\n", n, d, GetTickCount() - dwStart);

dwStart = GetTickCount();

d = test2(n);

printf_s("For %d steps, pi = %.15f, %d milliseconds\n", n, d, GetTickCount() - dwStart);

}

请参阅See also