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*From*: Joseph Myers <joseph at codesourcery dot com>*To*: Ondřej Bílka <neleai at seznam dot cz>*Cc*: <andrew dot n dot senkevich at gmail dot com>, <libc-alpha at sourceware dot org>*Date*: Wed, 12 Aug 2015 16:45:55 +0000*Subject*: Re: [RFC] Use simpler math functions when user selects -ffast-math*Authentication-results*: sourceware.org; auth=none*References*: <20150811223007 dot GA21338 at domone>

On Wed, 12 Aug 2015, Ondřej Bílka wrote: > Hi Andrew, > > When I checked isfinite performance I noticed that for most math > functions wrapper is unnecessary when -ffinite-math-only is set from > -ffast-math as they evaluate to constant. It's already the case that -ffinite-math-only causes __*_finite versions of many libm functions to be used, so bypassing the wrappers. Or do you mean something other than the math/w_*.c wrappers? > Adding new symbols would allow to fix bug that due accuracy math > functions need to be very slow on some inputs. As with -ffast-math you > don't have to worry much about accuracy we could finally fix these bugs. As far as I know, we have consensus on the documented accuracy goals for libm functions, which do *not* require pow, exp, log, sin, cos, tan, asin, acos, atan or atan2 to be correctly rounded. Thus, various dbl-64 performance issues (bugs 5781, 13932, 17211, at least) could probably be addressed by removing the very slow multiple-precision cases and allowing non-correctly-rounded results for some inputs. *But* simply removing the multiple-precision cases isn't appropriate without justification that it is safe. That is, each removal needs to include an error analysis of the existing non-multiple-precision code that shows why, even if you remove the multiple-precision case, the errors still won't be too big for any inputs. The error analysis might be fairly simple, but it does need to be there. -- Joseph S. Myers joseph@codesourcery.com

**Follow-Ups**:**Re: [RFC] Use simpler math functions when user selects -ffast-math***From:*OndÅej BÃlka

**References**:**[RFC] Use simpler math functions when user selects -ffast-math***From:*OndÅej BÃlka

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