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Date: Thu, 9 May 2013 15:21:57 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: Using float_t and double_t in math functions
* 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
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)
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