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From: Uros Bizjak <ubizjak@gmail.com>
Date: Tue, 22 Jun 2021 12:17:02 +0200
Message-ID: <CAFULd4bXMF8UiqSthK0qxAYwNndgVdWeuvLG1eQzT15OKEtYJQ@mail.gmail.com>
Subject: Re: [PATCH][RFC] Add x86 subadd SLP pattern
To: Richard Biener <rguenther@suse.de>, Uros Bizjak <ubizjak@gmail.com>, 
 Hongtao Liu <hongtao.liu@intel.com>,
 "gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>, 
 Richard Sandiford <richard.sandiford@arm.com>
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On Tue, Jun 22, 2021 at 11:42 AM Richard Sandiford
<richard.sandiford@arm.com> wrote:

> >> Well, the pattern is called addsub in the x86 world because highpart
> >> does add and lowpart does sub. In left-to-right writing systems
> >> highpart comes before lowpart, so you have addsub.
> >
> > The other targets mentioned do not seem to agree but I can live
> > with that, thus I'll change back to addsub.
>
> FWIW, subadd sounds clearer to me too.  It seems surprising to put
> imaginary before real when interpreting something as complex, for example=
.
>
> Putting the highpart first feels especially odd on an LE system like x86=
=E2=80=A6

The XMM vector is documented left to right with MSB at the left (c.f.
most significant *DIGIT* of the number at the left)

xmm[MSB] .... xmm[LSB]

so, looking at x86 ADDSUBPD insn documentation:

xmm2[127:64] xmm2[63:0]
( + -)
xmm1[127:64] xmm1[63:0]
(=3D)
xmm1[127:64] holds ADD
xmm1[63:0] holds SUB

xmm1[127:64] xmm1 [63:0]
ADD SUB

Uros.