Data type compatibility with NEON intrinsics
arm, gcc, intrinsics, neon
Solution
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.
Problem
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.