与NEON内在的数据类型兼容性 [英] Data type compatibility with NEON intrinsics

问题描述

我对ARM的优化工作使用NEON内部函数，从C + + code。我了解和掌握大部分打字的问题，但我被困在这一个：

指令 vzip_u8 返回 uint8x8x2_t 值（其实两个数组 uint8x8_t ）。我想返回的值赋给一个普通的 uint16x8_t 。我没有看到相应的 vreinter preTQ 内在实现这一目标，和简单的强制类型转换将被拒绝。

解决方案

一些定义来回答清楚...

NEON 有32个寄存器，64位宽（双视图为16个寄存器，128位宽）。

  

    

NEON单元可以为查看相同的寄存器组：

    
    

    
• 16个128位四字寄存器Q0-Q15

    
• 32 64位双字寄存器D0-D31。

    

  

uint16x8_t 是一种需要128位存储因而它需要在四字注册。

ARM NEON内在函数具有矢量数组数据类型中的ARM®C语言扩展：

  

    

...在加载和存储操作使用，
    表查找操作，和的结果类型返回一对向量的操作。

  

VZIP 指令

  

    

...交织两个向量的元素

    
    

VZIP DD，DM

  

和有内在像

  uint8x8x2_t vzip_u8（uint8x8_t，uint8x8_t）
 

从这些我们可以得出这样的结论uint8x8x2_t实际上是两个随机编号的双寄存器的列表，因为VZIP说明不会对输入寄存器为了任何要求。

现在的答案是...

uint8x8x2_t 可以包含非连续两个dualword寄存器，而 uint16x8_t 是由两个连续的dualword寄存器的数据结构其中第一个具有偶数指数（D0-D31 - > Q0-Q15）。

由于这个你不能用两个双字寄存器投矢量数组数据类型来四字寄存器...很容易。

编译器可能会聪明地为您提供帮助，或者你可以强制转换，但是我会检查的正确性产生的装配以及性能。

I am working on ARM optimizations using the NEON intrinsics, from C++ code. I understand and master most of the typing issues, but I am stuck on this one:

The instruction vzip_u8 returns a uint8x8x2_t value (in fact an array of two uint8x8_t). I want to assign the returned value to a plain uint16x8_t. I see no appropriate vreinterpretq intrinsic to achieve that, and simple casts are rejected.

解决方案

Some definitions to answer clearly...

NEON has 32 registers, 64-bits wide (dual view as 16 registers, 128-bits wide).

The NEON unit can view the same register bank as:

• sixteen 128-bit quadword registers, Q0-Q15
• thirty-two 64-bit doubleword registers, D0-D31.

uint16x8_t is a type which requires 128-bit storage thus it needs to be in an quadword register.

ARM NEON Intrinsics has a definition called vector array data type in ARM® C Language Extensions:

... for use in load and store operations, in table-lookup operations, and as the result type of operations that return a pair of vectors.

vzip instruction

... interleaves the elements of two vectors.

vzip Dd, Dm

and has an intrinsic like

uint8x8x2_t vzip_u8 (uint8x8_t, uint8x8_t) 

from these we can conclude that uint8x8x2_t is actually a list of two random numbered doubleword registers, because vzip instructions doesn't have any requirement on order of input registers.

Now the answer is...

uint8x8x2_t can contain non-consecutive two dualword registers while uint16x8_t is a data structure consisting of two consecutive dualword registers which first one has an even index (D0-D31 -> Q0-Q15).

Because of this you can't cast vector array data type with two double word registers to a quadword register... easily.

Compiler may be smart enough to assist you, or you can just force conversion however I would check the resulting assembly for correctness as well as performance.

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