From: Jochen Barth <jpunktbarth@gmail.com>
To: gcc-help@gcc.gnu.org
Subject: arm_neon.h / vext_u64 (uint64x1_t __a, uint64x1_t __b, __const int __c)
Date: Wed, 2 Sep 2020 23:10:03 +0200 [thread overview]
Message-ID: <a2c81947-3c16-0ade-ac4a-60775a0d971a@gmail.com> (raw)
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
next reply other threads:[~2020-09-02 21:10 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-02 21:10 Jochen Barth [this message]
2020-09-02 21:11 ` Jochen Barth
[not found] ` <VI1PR08MB53253664D3E7644526965E27FF260@VI1PR08MB5325.eurprd08.prod.outlook.com>
[not found] ` <e06ec9ce-78ec-8eb2-83c4-4d3eda9e18f4@gmail.com>
2020-09-10 7:54 ` Tamar Christina
2020-09-10 8:14 ` Jochen Barth
2020-09-10 8:34 ` Tamar Christina
2020-09-10 10:35 ` Jochen Barth
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