From: Szabolcs Nagy <nsz@port70.net>
To: musl@lists.openwall.com
Subject: Re: Using float_t and double_t in math functions
Date: Thu, 9 May 2013 15:21:57 +0200 [thread overview]
Message-ID: <20130509132157.GF12689@port70.net> (raw)
In-Reply-To: <20130509014327.GA6338@brightrain.aerifal.cx>
* Rich Felker <dalias@aerifal.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
at the same time all the STRICT_ASSIGN macros
can be removed (already a noop) which were
there to enforce store with the right precision
on i386 when musl is compiled without -ffloat-store,
but i dont think that should be supported
btw the other ugly macro that remains is
FORCE_EVAL to force evaluation of floating-point
expressions for their side-effect, which should
be eventually
#define FORCE_EVAL(expr) do{ \
_Pragma("STDC FENV_ACCESS ON") \
expr; \
} while(0)
but no compiler supports this that i know of
so now we have volatile hacks with unnecessary
stores
there are a few more 'volatile' in the code
but all should be possible to clean up
(fma and fmaf are probably exceptions similar
to FORCE_EVAL)
next prev parent reply other threads:[~2013-05-09 13:21 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-05-09 1:43 Rich Felker
2013-05-09 13:21 ` Szabolcs Nagy [this message]
2013-05-09 14:57 ` Rich Felker
2013-05-09 19:14 ` Szabolcs Nagy
2013-05-09 16:02 ` Rich Felker
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