使用__m256d寄存器 [英] Using __m256d registers

问题描述

您如何使用__m256d?

说我想使用英特尔AVX指令

Say I want to use the Intel AVX instruction _mm256_add_pd on a simple Vector3 class with 3-64 bit double precision components (x, y, and z). What is the correct way to use this?

由于x，y和z是Vector3类的成员，因此_我可以在union中使用__m256d变量声明它们吗?

Since x, y and z are members of the Vector3 class, _can I declare them in union with an __m256d variable?

union Vector3
{
  struct { double x,y,z ; } ;
  __m256d _register ;  // the Intel register?
} ;

那我可以去

Vector3 add( const Vector3& o )
{
  Vector3 result;
  result._register = _mm256_add_pd( _register, o._register ) ; // add 'em
  return result; 
}

这行得通吗?还是我需要宣布临时人员，

Is that going to work? Or do I need to declare temporaries,

Vector3 add( const Vector3& o )
{
  __m256d d1 = *(__m256d*)(&x) ; // ? Cast to __m256d?
  __m256d d2 = *(__m256d*)(&o.x) ; // ? Cast to __m256d?
  __m256d result = _mm256_add_pd( d1, d2 ) ; // add 'em
  return Vector3( result ) ; // make a ctor that accepts __m256d?
}

编辑

我想出了这个例子，

Edit

I came up with this example,

#include <stdio.h>
#include <intrin.h>

int main()
{
  __m256d a, b, res;

  for( int i = 0; i < sizeof(__m256d)/sizeof(double); i++ )
  {
    a.m256d_f64[i] = i ;
    b.m256d_f64[i] = 2*i ;
  }

  // Perform __4__ adds.
  res = _mm256_add_pd(a, b);

  for( int i = 0; i < sizeof(__m256d)/sizeof(double); i++ )
  {
    printf("%f + %f = %f\n", a.m256d_f64[i], b.m256d_f64[i], res.m256d_f64[i]);
  }
  puts("");
}

我想现在的问题是，做 _mm256_add_pd做 load 个操作自动，如果我不这样做会搞乱什么吗?要将我的__m256d寄存器声明为接近使用它们的本地变量? (我担心酒店房间/办公桌抽屉类型问题)

I guess the question is now, does _mm256_add_pd do load operations automatically, or will something get messed up if I don't declare my __m256d registers as locals close to where they are used? (I'm afraid of a hotel room / deskdrawer type problem)

我尝试在我的大型项目中添加一个__m256寄存器，然后得到了一大堆

I tried adding an __m256 register to my rather large project, and I got a whole bunch of

错误C2719:'值':带有__declspec(align('32'))的形式参数无法对齐

error C2719: 'value': formal parameter with __declspec(align('32')) won't be aligned

错误，这使我相信您不能将__m256寄存器保留在一个类中，而是应将它们声明为本地变量?

Errors, it leads me to believe that you can't keep __m256 registers inside a class, instead they should be declared as locals?

推荐答案

首先，我想澄清一些混乱之处. __m256d不是寄存器的类型，它是可以将 加载到AVX寄存器中的数据类型. __m256d不再是寄存器，而int只是寄存器.有几种方法可以将数据输入__m256d(或任何其他向量类型):

First I'd like to clear up a little confusion. __m256d isn't a type of register, it's a data type that can be loaded into an AVX register. A __m256d is no more a register than an int is a register. There are a few ways to get data in and out of an __m256d (or any other vector type):

使用union:是的，union技巧可行.由于联合通常具有正确的对齐方式(尽管malloc可能不正确，请使用posix_memalign或_aligned_malloc)，因此效果很好.

Using a union: Yes, the union trick works. It works very well, since the union will generally have the correct alignment (although malloc might not, use posix_memalign or _aligned_malloc).

class Vector3 {
public:
    Vector3(double xx, double yy, double zz);
    Vector3(__m256d vvec);


    Vector3 operator+(const Vector3 &other) const
    {
        return Vector3(_mm256_add_pd(vec, other.vec));
    }

    union {
        struct {
            double x, y, z;
        };
        __m256d vec; // a data field, maybe a register, maybe not
    };
};

使用内在函数:在函数内部，通常更容易使用内在函数来获取向量类型中的数据.

Using intrinsics: Inside a function, it's usually easier to use intrinsics to get data in and out of a vector type.

__m256d vec = ...;
double x, y, z;
vec = _mm256_add_pd(vec, _mm256_set_pd(x, y, z, 0.0));

使用指针强制转换:由于多种原因，强制转换指针是最后的选择.

Using pointer casts: Casting pointers is the last resort for a couple of reasons.

1. 指针可能未正确对齐.

1. The pointer might not be aligned correctly.

投射指针有时可能会干扰编译器的别名分析.

Casting pointers can sometimes mess with the compiler's aliasing analysis.

指针转换绕过了许多安全保证.

Pointer casting bypasses a number of safety guarantees.

因此，我只使用指针转换来浏览大量数据.

So I'd only use pointer casting to plow through a big array of data.

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