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Date: Fri, 6 Oct 2023 13:43:27 +0000 (UTC)
From: Richard Biener <rguenther@suse.de>
To: Robin Dapp <rdapp.gcc@gmail.com>
cc: Tamar Christina <Tamar.Christina@arm.com>, 
    gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH] ifcvt/vect: Emit COND_ADD for conditional scalar
 reduction.
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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.