在MSVC中自动生成FMA指令

问题描述

MSVC多年来一直支持AVX / AVX2指令，根据，它可以自动生成说明。

但是以下两个函数都不能编译为FMA指令：

  float func1（float x，float y，float z）
 {
 return x * y + z; 
} 
 
 float func2（float x，float y，float z）
 {
 return std :: fma（x，y，z）更糟糕的是，std :: fma并不是作为一个单独的FMA指令实现的，而是一个单独的FMA指令，它执行得非常糟糕，慢于一个简单的 x * y + z （如果实现不依赖于FMA指令，std :: fma的性能会很差）。 
 
 
 
我使用 / arch：AVX2 / O2 / Qvec 标志编译。 
也尝试了 / fp：fast ，没有成功。
 
 
 
 MSVC被迫自动发出FMA指令？
 
 
 
  UPDATE  
 
 
 
 a href =https://msdn.microsoft.com/en-us/library/4f994tzs.aspx>  #pragma fp_contract（on | off）  
解决方案
我解决了这个长期存在的问题。
 
 
 
结果是，标志 / fp：fast ， / arch：AVX2  code> / O1 （或以上 / O1 ）不足以让Visual Studio 2015发出FMA指令。您还需要 打开 / GL 。
 
 
 
然后Visual Studio 2015将生成一个FMA指令 vfmadd213ss  for 
  float func1（float x，float y，float z）
 {
 return x * y + z; 
} 
  
 
 
 
 
 
 
 c> std :: fma ，我打开了一个。他们确认了 std :: fma 不会编译到FMA指令的行为，因为编译器不会将其视为内在。根据他们的反应，它将在未来的更新中得到修正，以获得最好的代码。
 
MSVC supports AVX/AVX2 instructions for years now and according to this msdn blog post, it can automatically generate fused-multiply-add (FMA) instructions.
Yet neither of the following functions compile to FMA instruction:
float func1(float x, float y, float z)
{
    return x * y + z;
}

float func2(float x, float y, float z)
{
     return std::fma(x,y,z);
}
Even worse, std::fma is not implemented as a single FMA instruction, it performs terribly, much slower than a plain x * y + z (the poor performance of std::fma is expected if the implementation doesn't rely on FMA instruction).
I compile with /arch:AVX2 /O2 /Qvec flags.Also tried it with /fp:fast, no success.
So the question is how can MSVC forced to automatically emit FMA instructions?
UPDATE
There is a #pragma fp_contract (on|off), which (looks like) does nothing.
 解决方案 
I solved this long-standing problem.
As it turns out, flags /fp:fast, /arch:AVX2 and /O1 (or above /O1) are not enough for Visual Studio 2015 to emit FMA instructions. You also need the "Whole Program Optimization" turned on with flag /GL.
Then Visual Studio 2015 will generate an FMA instruction vfmadd213ss for
float func1(float x, float y, float z)
{
    return x * y + z;
}
Regarding std::fma, I opened a bug at Microsoft Connect. They confirmed the behavior that std::fma doesn't compile to FMA instructions, because the compiler doesn't treat it as an intrinsic. According to their response it will be fixed in a future update to get the best codegen possible.
                        
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