arm_neon.h / vext_u64 (uint64x1_t __a, uint64x1_t __b, __const int __c)

[Jochen Barth via Gcc-help <gcc-help@xxxxxxxxxxx>]

Dear reader,

the definition of aarch64/arm_neon.h (gcc 10.2) is

__extension__ extern __inline uint64x1_t
__attribute__ ((__always_inline__, __gnu_inline__, __artificial__))
vext_u64 (uint64x1_t __a, uint64x1_t __b, __const int __c)
{
  __AARCH64_LANE_CHECK (__a, __c);
  /* The only possible index to the assembler instruction returns 
element 0.  */
  return __a;
}

So this function does essentially »return __a«.

If the function name »vext_...« has, as the name suggests, something to 
do with the »ext« neon simd instruction,

then I do not understand where the asm-equivalent »ext« neon instrinct 
is, because in the »Arm Architecture Reference Manual«, chapter C7.2.543 
states: »<index> Is the lowest numbered byte element to be 
extracted...«, ranging from 0..7 for Q=8 and 0..15 for Q=16 (extraction 
over the whole 128 bit register).

PS: gcc with vector expressions does not (?) use »ext« for y=(x<<(c*8)) 
| (x>>(64-c*8)); // for Q=8

Kind regards, Jochen