Working with DirectXMath and D3DXMath

Question

Back in D3DXMath we had ability to multiply, add or subtract even divide vector types, which were D3DXVECTOR2, D3DXVECTOR3, D3DXVECTOR4 structures..... Now in DirectXMath incarnation we have XMFLOAT2, XMFLOAT3, XMFLOAT4 and XMVECTOR. If i want to do any math operation i must do conversion from XMFLOAT to XMVECTOR either way Visual Studio is throwing an error "There is no user defined conversion". Why is that ? Actually it's a fact that in a new version(Windows 8.1, 10) of DirectX math library vector operation slightly has changed . Am i doing something wrong........... ?!

P.S. Well for Matrices there are another question but right now lets talk only on vectors. These changes is pushing third party developers to create their own Math library and they had done it..... :)

Answer 1

This is actually explained in detail in the DirectXMath Programmer's Guide on MSDN:

The XMVECTOR and XMMATRIX types are the work horses for the DirectXMath Library. Every operation consumes or produces data of these types. Working with them is key to using the library. However, since DirectXMath makes use of the SIMD instruction sets, these data types are subject to a number of restrictions. It is critical that you understand these restrictions if you want to make good use of the DirectXMath functions.

You should think of XMVECTOR as a proxy for a SIMD hardware register, and XMMATRIX as a proxy for a logical grouping of four SIMD hardware registers. These types are annotated to indicate they require 16-byte alignment to work correctly. The compiler will automatically place them correctly on the stack when they are used as a local variable, or place them in the data segment when they are used as a global variable. With proper conventions, they can also be passed safely as parameters to a function (see Calling Conventions for details).

Allocations from the heap, however, are more complicated. As such, you need to be careful whenever you use either XMVECTOR or XMMATRIX as a member of a class or structure to be allocated from the heap. On Windows x64, all heap allocations are 16-byte aligned, but for Windows x86, they are only 8-byte aligned. There are options for allocating structures from the heap with 16-byte alignment (see Properly Align Allocations). For C++ programs, you can use operator new/delete/new[]/delete[] overloads (either globally or class-specific) to enforce optimal alignment if desired.

However, often it is easier and more compact to avoid using XMVECTOR or XMMATRIX directly in a class or structure. Instead, make use of the XMFLOAT3, XMFLOAT4, XMFLOAT4X3, XMFLOAT4X4, and so on, as members of your structure. Further, you can use the Vector Loading and Vector Storage functions to move the data efficiently into XMVECTOR or XMMATRIX local variables, perform computations, and store the results. There are also streaming functions (XMVector3TransformStream, XMVector4TransformStream, and so on) that efficiently operate directly on arrays of these data types.

By design, DirectXMath is encouraging you to write efficient, SIMD-friendly code. Loading or storing a vector is expensive, so you should try to work in a 'stream' model where you load data, work with it in-register a lot, then write the results.

That said, I totally get that the usage is a little complex for people new to SIMD math or DirectX in general, and is a bit verbose even for professional developers. That's why I also wrote the SimpleMath wrapper for DirectXMath which makes it work more like the classic math library you are looking for using XNA Game Studio like Vector2, Vector3, Matrix classes with 'C++ magic' covering up all the explicit loads & stores. SimpleMath types interop neatly with DirectXMath, so you can mix and match as you want.

See this blog post and GitHub as well.

DirectXMath is purposely an 'inline' library meaning in optimized code you shouldn't be passing variables much and instead just computing the value inside your larger function. The D3DXMath library in the deprecated D3DX9, D3DX10, D3DX11 library is more old-school which relies on function-pointer tables and is heavily performance bound by the calling-convention overhead.

These of course represent different engineering trade-offs. D3DXMath was able to do more substitution at runtime of specialized processor code paths, but pays for this flexibility with the calling-convention and indirection overhead. DirectXMath, on the other hand, assumes a SIMD baseline of SSE/SSE2 (or AVX on Xbox One) so you avoid the need for runtime detection or indirection and instead aggressively utilize inlining.