[Bug libgomp/58482] gomp4: user defined reduction produce wrong result

["jakub at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58482

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
It is well known that we don't vectorize this, right now we only handle
accesses to the SIMD lane privatized vars that cover the whole size of those
vars, while in your testcase the access size is half the size of the var.
The reason why -Ofast vectorizes it is likely that SRA manages to scalarize
those, but scalarizer can't do anything easily with the magic arrays indexed by
SIMD_LANE internal fn that we use to represent the privatized variables (and, I
couldn't find a better representation yet for those).
So, either the SRA pass would need to handle those (split the single array
with { float; float } pairs into two arrays with just float type), or the
vectorizer would need to do some ugly magic for selected cases (e.g. handle the
pair case by having two vector vars and use always either odd or even entries
from them).  I'm hoping that most people will actually use scalars or single
data member classes for reductions etc.