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Message-ID: <20130509160201.GN20323@brightrain.aerifal.cx>
Date: Thu, 9 May 2013 12:02:01 -0400
From: Rich Felker <dalias@...ifal.cx>
To: musl@...ts.openwall.com
Subject: Re: Using float_t and double_t in math functions
On Thu, May 09, 2013 at 03:21:57PM +0200, Szabolcs Nagy wrote:
> * Rich Felker <dalias@...ifal.cx> [2013-05-08 21:43:27 -0400]:
> > As far as I can tell, in most of the affected code, keeping excess
> > precision does not hurt the accuracy of the result, and it might even
> > improve the results. Thus, nsz and I discussed (on IRC) the
> > possibility of changing intermediate variables in functions that can
> > accept excess precision from float and double to float_t and double_t.
> > This would not affect the generated code at all on machines without
> > excess precision, but on x86 (without SSE) it eliminates all the
> > costly store/load pairs. As an example (on my test machine), it
>
> ie. it is only for i386 (without sse)
> (which is not a trendy platform nowadays)
> but there it improves performance and
> code size a bit so it is worth doing
By the way, part of the reason I think we should make the change where
it doesn't hurt (and probably helps) accuracy is so we're not telling
people:
"Yes, some math functions in musl are slower than glibc because we're
taking extra care to make sure they give you less-accurate results."
:-)
In practice it's very few that are slower. I think most will just go
from being 2-3 times as fast as glibc to 3-5 times as fast as glibc.
Rich
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