Eigen库对齐问题:declspec(align('16')) 的形参将不被对齐









一:错误提示:error C2719: '_Val': formal parameter with __declspec(align('16')) won't be aligned



英文提示:error C2719: 'p': formal parameter with __declspec(align('16')) won't be aligned

中文提示:error C2719: “p”: 具有 __declspec(align('16')) 的形参将不被对齐

导致整个现象的主要原因是使用了Eigen库,Eigen为了使用SSE加速,所以内存分配上使用了128位的指针。

更加准确的说法:

“First, "fixed-size" should be clear: an Eigen object has fixed size if its number of rows and its number of columns are fixed at compile-time. So for example Matrix3f has fixed size, but MatrixXf doesn't (the opposite of fixed-size is dynamic-size).

The array of coefficients of a fixed-size Eigen object is a plain "static array", it is not dynamically allocated. For example, the data behind a Matrix4f is just a "float array[16]".

Fixed-size objects are typically very small, which means that we want to handle them with zero runtime overhead -- both in terms of memory usage and of speed.

Now, vectorization (both SSE and AltiVec) works with 128-bit packets. Moreover, for performance reasons, these packets need to be have 128-bit alignment.

So it turns out that the only way that fixed-size Eigen objects can be vectorized, is if their size is a multiple of 128 bits, or 16 bytes. Eigen will then request 16-byte alignment for these objects, and henceforth rely on these objects being aligned so no runtime check for alignment is performed.”


二:解决方案:

分为四种情况:

  • Cause 1: Structures having Eigen objects as members
  • Cause 2: STL Containers
  • Cause 3: Passing Eigen objects by value
  • Cause 4: Compiler making a wrong assumption on stack alignment (for instance GCC on Windows)

每种情况可以对照官方的说法。可以参考如下链接

http://eigen.tuxfamily.org/dox/TopicUnalignedArrayAssert.html

在此不再重复表述。



三:我的解决方法:eigen 3.1.2有 bug,  重装Eigen;

                关闭使用 预编译头;

                关闭编译选项里面的16位对齐;

                把Eigen::Vector4f    传值改为 传引用 Eigen::Vector4f &;

                把结构体 传值 全部转换为 传引用。躲过对齐!!!


编译器选项:C/C++ ---代码生成---结构图成员对齐;

可恶:忘记了当初是怎么解决的!!!!!



四:对stl vector 进行修改:http://blog.csdn.net/pkueecser/article/details/8602656


产生问题的resize()方法,改为:

     void  resize(size_type _Newsize,  const _Ty & _Val )
    { 
//  determine new length, padding with _Val elements as needed
         if  (size()  <  _Newsize)
            _Insert_n(end(), _Newsize 
-  size(), _Val);
        
else   if  (_Newsize  <  size())
            erase(begin() 
+  _Newsize, end());
    }

  注意其中红色部分,将传值改为传参,避免在参数栈上创建被对齐的结构的对象。然后,在我们使用std::vector< Foo<5, float> >之前包含我们的foo_vector.hpp头文件,就可以正常使用了。  注意红色部分.....

是微软坑呢?还是Eigen坑?个人感觉微软更坑!!!

后记:

       添加 修改为 869行为引用之后,引发2562行的错误,修改方式为 把第二个参数0去掉。

转载于:https://www.cnblogs.com/wishchin/p/9200373.html

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