I have a question about the Aarch64 simd instructions and builtins.
I want to unpack a __Float32x4 (V4SF) variable into two __Float64x2
variables. I can get the upper part with:
__Float64x2_t a = __builtin_aarch64_vec_unpacks_hi_v4sf (x);
But I can't seem to find a builtin that would get me the lower half.
I assume this is due to the issue in aarch64-simd.md around the
vec_unpacks_lo_<mode> instruction:
;; ??? Note that the vectorizer usage of the vec_unpacks_[lo/hi] patterns
;; is inconsistent with vector ordering elsewhere in the compiler, in that
;; the meaning of HI and LO changes depending on the target endianness.
;; While elsewhere we map the higher numbered elements of a vector to
;; the lower architectural lanes of the vector, for these patterns we want
;; to always treat "hi" as referring to the higher architectural lanes.
;; Consequently, while the patterns below look inconsistent with our
;; other big-endian patterns their behavior is as required.
Does this mean we can't have a __builtin_aarch64_vec_unpacks_lo_v4sf
builtin that will work in big endian and little endian modes?
It seems like it should be possible but I don't really understand
the details of the implementation enough to follow the comment and
all its implications.
Right now, as a workaround, I use:
static inline __Float64x2_t __vec_unpacks_lo_v4sf (__Float32x4_t x)
{
__Float64x2_t result;
__asm__ ("fcvtl %0.2d,%1.2s" : "=w"(result) : "w"(x) : /* No clobbers */);
return result;
}
But a builtin would be cleaner.
Steve Ellcey
sell...@cavium.com
