Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Terje Mathisen <terje.mathisen@tmsw.no>
Newsgroups: comp.arch
Subject: Re: Computer architects leaving Intel...
Date: Thu, 19 Sep 2024 08:34:19 +0200
Organization: A noiseless patient Spider
Lines: 52
Message-ID: <vcgglc$fivv$1@dont-email.me>
References: <vaqgtl$3526$1@dont-email.me>
 <p1cvdjpqjg65e6e3rtt4ua6hgm79cdfm2n@4ax.com>
 <2024Sep10.101932@mips.complang.tuwien.ac.at> <ygn8qvztf16.fsf@y.z>
 <2024Sep11.123824@mips.complang.tuwien.ac.at> <vbsoro$3ol1a$1@dont-email.me>
 <867cbhgozo.fsf@linuxsc.com> <20240912142948.00002757@yahoo.com>
 <vbuu5n$9tue$1@dont-email.me> <20240915001153.000029bf@yahoo.com>
 <vc6jbk$5v9f$1@paganini.bofh.team> <20240915154038.0000016e@yahoo.com>
 <vc70sl$285g2$4@dont-email.me> <vc73bl$28v0v$1@dont-email.me>
 <OvEFO.70694$EEm7.38286@fx16.iad>
 <32a15246310ea544570564a6ea100cab@www.novabbs.org>
 <vc7a6h$2afrl$2@dont-email.me>
 <50cd3ba7c0cbb587a55dd67ae46fc9ce@www.novabbs.org>
 <vc8qic$2od19$1@dont-email.me> <fCXFO.4617$9Rk4.4393@fx37.iad>
 <vcb730$3ci7o$1@dont-email.me> <7cBGO.169512$_o_3.43954@fx17.iad>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: quoted-printable
Injection-Date: Thu, 19 Sep 2024 08:34:20 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="9dcff06db3655d57972fd8e5503bf701";
	logging-data="510975"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/JiOWWXjf8TOQFQt/tGZ3BQln01yVVMF3FUq2uSbxknw=="
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
 Firefox/91.0 SeaMonkey/2.53.19
Cancel-Lock: sha1:WvYWgK2jFWQrgdj631/jB4CCSuI=
In-Reply-To: <7cBGO.169512$_o_3.43954@fx17.iad>
Bytes: 3293

EricP wrote:
> Terje Mathisen wrote:
>>
>> Very nice!
>>
>> This means that you can do integer IMAC(), right?
>>
>> (hi, lo) =3D imac(a, b, c); // =3D=3D a*b+c
>>
>> The only thing even nicer from the perspective of writing arbitrary=20
>> precision library code would be IMAA, i.e. a*b+c+d since that is the=20
>> largest combination which is guaranteed to never overflow the double=20
>> register target field.
>>
>=20
> I thought about IMAC but it was a bit too much.
> And unlike FMA there is no precision gain in IMAC, just convenience.
> IMAC requires 6 register specifiers, 2 dest and 4 source if you don't
> care about overflow/carry on the accumulate.
>  =C2=A0 2-wide =3D 2-wide + narrow * narrow

No, no! IMAC is three in, two out, so in your syntax:

  W =3D N*N+N

or

  (rhi, rlo) =3D imac(r0,r1,r2)

> It needs 7 registers, 3 dest and 4 source if you want overflow/carry
> on the accumulate.
>  =C2=A0 3-wide =3D 2-wide + narrow * narrow

Otoh, if you do have all the wide add forms you outlined below,=20
including the "full adder" with three inputs and a wirde/pair output,=20
then the carry propagations do become easier, and just doing

  (a,b) =3D muluw(e,f)
  (a,b) =3D addw1(a,b,g)

would do the same as my suggested

  (a,b) =3D imac(a,f,g)

Anyway, very nice!

Terje

--=20
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"