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Re: 01/03: gnu: Add QD.
From: |
Eric Bavier |
Subject: |
Re: 01/03: gnu: Add QD. |
Date: |
Tue, 6 Feb 2018 08:27:32 -0600 |
On Mon, 05 Feb 2018 20:24:45 -0500
Mark H Weaver <address@hidden> wrote:
> Hi Eric,
>
> Eric Bavier <address@hidden> writes:
>
> > On Sat, 03 Feb 2018 22:55:56 -0500
> > Mark H Weaver <address@hidden> wrote:
> >
> >> address@hidden (Eric Bavier) writes:
> >> > + `(#:configure-flags `("--disable-enable_fma" ;weird :/
> >>
> >> "weird :/" is not a very useful comment.
> >
> > Sorry. I thought it was obvious it was in reference to the
> > "disable-enable" bit.
>
> I thought that might be the case, but I was still curious why you were
> disabling FMA support, and the comment didn't help.
Sorry about the confusion. I'll revise it to something more
informative. I usually try to refrain from "witty" comments.
>
> >> Can you change it to explain
> >> why you added this flag? It seems unfortunate to disable fused
> >> multiply-add for this kind of library.
> >
> > The library does not support runtime ISA detection. I thought FMAs are
> > something we don't support in our baseline x86-64 builds?
>
> Indeed. I was surprised to learn that FMAs are only supported in very
> recent Intel processors. My Yeeloong with MIPS-based Loongson 2F
> processor from 2008 includes support for FMA instructions, and I guessed
> that Intel had them long before, but actually Intel didn't add them
> until several years later.
AMD also had a 4-argument FMA instruction much sooner.
Thank you for the attention to this matter,
`~Eric