On Fri, 6 Oct 2023, Robin Dapp wrote:
> > We might need a similar assert
> >
> > gcc_assert (HONOR_SIGNED_ZEROS (vectype_out)
> > && !HONOR_SIGN_DEPENDENT_ROUNDING (vectype_out));?
>
> erm, obviously not that exact assert but more something like
>
> if (HONOR_SIGNED_ZEROS && !HONOR_SIGN_DEPENDENT_ROUNDING...)
> {
> if (dump)
> ...
> return false;
> }
>
> or so.
Yeah, of course the whole point of a fold-left reduction is to
_not_ give up without -ffast-math which is why I added the above.
I obviously didn't fully verify what happens for an original
MINUS_EXPR. I think it's required to give up for -frounding-math,
but I think I might have put the code to do that in a generic
enough place.
For x86 you need --param vect-partial-vector-usage=2 and an
AVX512 enabled arch like -march=skylake-avx512 or -march=znver4.
I think tranforming - x to + (-x) works for signed zeros.
So if you think you got everything correct the patch is OK as-is,
I just wasn't sure - maybe the neutral_element change deserves
a comment as to how MINUS_EXPR is handled.
Richard.
