for skylake kernels. This is the same method as used in [sd]asum.
_mm_set1_epi64x was commented out for zasum, but has the advantage
of avoiding possible undefined behaviour (using an uninitialized
variable), optimized out by NVHPC and icx. The new code works
fine with those compilers.
For GCC 12.3 the generated code is identical; no matter what method
you use, the compiler optimizes the code into a compile-time
constant, there is no performance benefit using mm_cmpeq_epi8
since the corresponding instruction (VPCMPEQB) isn't actually
generated!
This kernel is only used on Skylake+ if the kernel with AVX512
intrinsics can't be used, but used the variable x1 incorrectly
in the tail end of the loop, as it is still at the initial
value instead of where x points to.
This caused 55 "other error"s in the LAPACK tests
(https://github.com/OpenMathLib/OpenBLAS/issues/4282)
This change makes casum.c as similar as possible as zasum.c,
because zasum.c does this correctly.
Unlike [dcz]scal, sscal still used the original GotoBLAS SSE code from scal_sse.S.
This code follows dscal as closely as possible, except for the inc_x > 1 code
for which a plain C loop is used much like the one in cscal.c, instead of an
adaptation of the SSE2 asm code of dscal.c (I tried but the performance wasn't
better than the plain C loop).
Using 256-bit registers in dscal makes this microkernel consistent with
cscal and zscal, and generally doubles performance if the vector fits in
L1 cache.
These are as similar to dscal_microk_skylakex-2.c as possible
for consistency.
Note that before this change SKYLAKEX+ uses generic C functions for
cscal/zscal via commit 2271c350 from #2610 (which is masked by
commit 086d87a30). However now #3799 disables FMAs (in turn enabled
by `-march=skylake-avx512`) in the plain C code which fixes excessive
LAPACK test failures more nicely.
If e.g. -march=haswell is set in CFLAGS, GCC generates FMAs by default, which
is inconsistent with the microkernels, none of which use FMAs. These
inconsistencies cause a few failures in the LAPACK testcases, where
eigenvalue results with/without eigenvectors are compared.
Moreover using FMAs for multiplication of complex numbers can give surprising
results, see 22aa81f for more information.
This uses the same syntax as used in 22aa81f for zarch (s390x).
