public inbox for
 help / color / mirror / Atom feed
From: Janne Blomqvist <>
To: Thomas Koenig <>
Cc: "" <>,
	gcc-patches <>
Subject: Re: [patch, libfortran] AMD-specific versions of library matmul
Date: Thu, 25 May 2017 17:58:00 -0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Thu, May 25, 2017 at 1:45 PM, Thomas Koenig <> wrote:
> Hello world,
> the attached patch speeds up the library version of matmul for AMD chips
> by selecting AVX128 instructions and, depending on which instructions
> are supported, either FMA3 (aka FMA) or FMA4.
> Jerry tested this on his AMD systems, and found a speedup vs. the
> current code of around 10%.
> I have been unable to test this on a Ryzen system (the new compile farm
> machines won't accept my login yet).  From the benchmarks I have read,
> this method should also work fairly well on a Ryzen.
> So, OK for trunk?

In some comments, you have -mprefer=avx128 whereas the option that gcc
understands is -mprefer-avx128. Also, have you verified that e.g.
contemporary Intel processors still use the avx256 codepath and don't
accidentally end up with avx128?

As for FMA4, are there sufficient numbers of processors supporting
FMA4 but not FMA3 around to justify bloating the library to support
them? I understood that this is only a single AMD CPU generation
("bulldozer" in 2011), the next one ("piledriver" in 2012) added FMA3
in addition to FMA4. And in the new Zen core (Ryzen, Epyc, etc.) AMD
has dropped support for FMA4 although there are reports that it will
still execute FMA4 for backward compatibility although it's no longer
advertised in CPUID, but in any case AMD seems to consider it a legacy
instruction that should not be used anymore (Intel never supported

Janne Blomqvist

  parent reply	other threads:[~2017-05-25 17:20 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
2017-05-25 11:19 ` Fwd: " Thomas Koenig
2017-05-25 14:11 ` Jerry DeLisle
2017-05-25 14:52   ` Thomas Koenig
2017-05-25 17:58 ` Janne Blomqvist [this message]
2017-05-25 20:31   ` Jerry DeLisle
2017-05-25 23:44     ` Thomas Koenig
2017-05-26  5:41       ` Jerry DeLisle
2017-05-26  6:21         ` Andrew Pinski
2017-05-26 15:33           ` Bill Seurer

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).