c - 信息 C5012 : loop not parallelized due to reason '1008'

Question

我正在 x86_64 上试用 Visual Studio 2013 的自动矢量化器模式，我对以下内容感到有些惊讶。考虑原始代码:

static void rescale( double * __restrict out, const int * __restrict in, long n, const double intercept, const double slope )
{
    for( long i = 0; i < n; ++i )
        out[i] = slope * in[i] + intercept;
}

Visual Studio 返回它在这样一个天真的例子中失败了:

--- Analyzing function: rescale
c:\users\malat\autovec\vec.c(13) : info C5012: loop not parallelized due to reason '1008'

编译行在哪里(我现在只对SSE2感兴趣):

cl vec.c /O2 /Qpar /Qpar-report:2

查看文档:

• https://msdn.microsoft.com/en-us/library/hh872235(v=vs.120).aspx

导致:

• https://msdn.microsoft.com/en-us/library/jj658585(v=vs.120).aspx#BKMK_ReasonCode100x

内容如下:

The compiler detected that this loop does not perform enough work to warrant auto-parallelization.

有没有办法重写这个循环，以便正确触发自动矢量化器模式？

我未能使用简单的方法重写代码:

static void rescale( double * __restrict out, const double * __restrict in, long n, const double intercept, const double slope )
{
    for( long i = 0; i < n; ++i )
        out[i] = slope * in[i] + intercept;
}

在上述情况下，Visual Studio 仍然报告:

--- Analyzing function: rescale
c:\users\malat\autovec\vec.c(13) : info C5012: loop not parallelized due to reason '1008'

我应该如何重写我的初始代码以取悦 Visual Studio 2013 的自动矢量化器模式？我想用 64 位 double vector 做 a * b + c:SSE2

Answer 1

MSDN link you posted 底部附近的示例代码建议使用 hint_parallel pragma:

void code_1008()
{
    // Code 1008 is emitted when the compiler detects that
    // this loop does not perform enough work to warrant 
    // auto-parallelization.

    // You can resolve this by specifying the hint_parallel
    // pragma. CAUTION -- if the loop does not perform
    // enough work, parallelizing might cause a potentially 
    // large performance penalty.

    // #pragma loop(hint_parallel(0)) //  hint_parallel will force this through
    for (int i=0; i<1000; ++i)
    {
        A[i] = A[i] + 1;
    }
}