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Date: Thu, 9 Jan 2020 12:00:02 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] math: move i386 sqrtf to C
On Thu, Jan 09, 2020 at 06:55:11PM +0300, Alexander Monakov wrote:
> I should have asked earlier, but - is everyone happy with this style of
> expressing removal of excess precision, or should I use eval_as_float?
>
> +{
> + long double t;
> [...]
> + return (float)t;
> +}
musl requires C99+ with conforming floating point behavior so I think
this is okay, and I'd prefer to avoid having dependency on libm.h
machinery that's not strictly needed in src/math/$(ARCH) since it
increases the cost of changing that machinery (requires looking at
arch-specific files). My recollection was that eval_as_float is mostly
an artifact of nsz's math functions supporting and being compatible
with somewhat non-conforming compilers.
If we want to ensure correct rounding (important for sqrt[f]) even on
broken compilers (some ppl use gcc 3.x, and pcc may be broken too?)
perhaps we should just do the store from asm too?
Note that eval_as_float only helps if -ffloat-store is used, which is
a nasty hack and also nonconforming, arguably worse than the behavior
without it, so we should probably drop use of that as a fallback, and
use fp_barrier[f] instead if needed.
Rich
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