Thanks a lot! Regards, Jochen Von meinem iPhone gesendet > Am 10.09.2020 um 10:35 schrieb Tamar Christina <Tamar.Christina@xxxxxxx>: > > Hi Jochen, > >> -----Original Message----- >> From: Jochen Barth <jpunktbarth@xxxxxxxxx> >> Sent: Thursday, September 10, 2020 9:14 AM >> To: Tamar Christina <Tamar.Christina@xxxxxxx> >> Cc: gcc-help <gcc-help@xxxxxxxxxxx> >> Subject: Re: arm_neon.h / vext_u64 (uint64x1_t __a, uint64x1_t __b, >> __const int __c) >> >> Dear Tamar, >> >> Sorry, I do no get the point: >> >>>>> EXT is a byte level extract, if you have a 64 bit vector and a >>>>> 64-bit type like uint64x1_t then the only possible index for n is 0. >> > > Because those intrinsics are not doing byte level extraction. They are convenience functions that > do not allow partial extraction of a type. For instance vext_s16 which takes an int16x4_t as input > restricts the values of n to 0 to 3 because when used with the EXT instruction it always > makes sure they're a multiple of 2 bytes since a int16 is two bytes. > > A uint64x1_t is a vector of 8 bytes which the intrinsic does as a group of 8 bytes since it > Always wants to extract whole numbers. As such the only possible index is 0. > > To get the behavior you have in your example you need to do the extraction on bytes using > vext_u8 which will allow you to corrupt the number. i.e. > > what you want is > > vreinterpret_u64_u8 (vext_u8 (vreinterpret_u8_u64 (a), vreinterpret_u8_u64 (b), <number>)) > > where your extraction happens on bytes. In this case n has the range 0-7. > > Instead of looking at the Arm ARM you should look at the definition of the intrinsics > https://developer.arm.com/architectures/instruction-sets/simd-isas/neon/intrinsics > > Regards, > Tamar > >> But my previous examples with n=c=1..7 showed that different (n=c)'s are >> possible, >> >> why is "the only possible index for n=0" ? >> >> Kind regards, Jochen
