The implementation of complex scalar * vector multiplication for Z14
makes some LAPACK tests fail because the numerical differences to the
reference implementation exceed the threshold (as can be seen by running
make lapack-test and replacing kernel/zarch/cscal.c with a generic
implementation for comparison).
The complex multiplication uses terms of the form a * b + c * d for both
real and imaginary parts. The assembly code (and compiler-emitted code
as well) uses fused multiply add operations for the second product and
sum. The results can be "surprising", for example when both terms in the
imaginary part nearly cancel each other out. In that case, the second
product contributes more digits to the sum than the first product that
has been rounded before.
One option is to use separate multiplications (which then round the same
way) and a distinct add. Change the code to pursue that path, by (1)
requesting the compiler not to contract the operations into FMAs and (2)
replacing the assembly kernel with corresponding vectorized C code
(where change 1 also applies).
Signed-off-by: Marius Hillenbrand <mhillen@linux.ibm.com>
Make clang slightly more cautious when contracting floating-point
operations (e.g., when applying fused multiply add) by setting
-ffp-contract=on (instead of fast).
Signed-off-by: Marius Hillenbrand <mhillen@linux.ibm.com>
An unrelated commit and merge inadvertently reverted our recent two
changes for simplifying DYNAMIC_ARCH on s390x. Simply reapply the
changes.
Simplify detection of which kernels we can compile on s390x. Instead of
decoding the gcc version in a complicated manner, just check if CC
supports a given -march=archXY flag. Together with the next patch, we
thereby gain support for builds with LLVM/clang with DYNAMIC_ARCH=1.
To enable builds with DYNAMIC_ARCH with older compiler releases, the
Makefile and drivers/other/dynamic_arch.c need a common view of the
architecture support built into the library.
We follow the notation from x86 when used with DYNAMIC_LIST, where
defines DYN_<ARCH NAME> denote support for a given generation to be
built in. Since there are far fewer architecture generations in OpenBLAS
for s390x, that does not bloat command lines too much.
Closes: #2842
Fixes: ba644378dc ("Copy BUILD_ options available to the compiler flags"
Signed-off-by: Marius Hillenbrand <mhillen@linux.ibm.com>
