Re: 01/03: gnu: Add QD.

[Eric Bavier <ericbavier@centurylink.net>]

On Mon, 05 Feb 2018 20:24:45 -0500
Mark H Weaver <mhw@netris.org> wrote:

> Hi Eric,
> 
> Eric Bavier <ericbavier@centurylink.net> writes:
> 
> > On Sat, 03 Feb 2018 22:55:56 -0500
> > Mark H Weaver <mhw@netris.org> wrote:
> >  
> >> ericbavier@centurylink.net (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