(This is an old thread, but I've just seen it in the archive.) On 2005-01-18 13:23:58 -0800, James E Wilson wrote: > Phil Prentice wrote: > > We are having a few problems regarding floating point inaccuracies under > > Linux. Please see the code below. > > This is PR 323 in our bug database. One of our most commonly reported > problems. We call this the excess precision problem. > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323 > > This problem has been known for about 15 years now, and hasn't been > fixed yet. My suspicion is that it never will be. Well, this is not a bug. Both the IEEE-754 standard and the ISO C99 standard allow intermediate computations in an extended precision; however, this is a bad idea in practice. You can read http://www.vinc17.org/research/extended.en.html for the various reasons. But gcc has a real bug concerning the extended precision: it does not convert the result into double precision (when using the type "double") after a cast or assignment. Such a conversion is required by the ISO C standard (see 5.1.2.3#13, 6.3.1.5#2 and 6.3.1.8#2). There are no reasons not to fix it. For those who fear a performance loss, gcc could define a compiler switch (that could be included in -ffast-math). I know that there have been discussions about this bug, but is there an entry in the bug tracker? > Meanwhile, there are a few workarounds > 1) You can try changing the hardware FP rounding mode to double > precision. You lose long double support, but you now get correct > double precision FP. You still have excess precision problems for > float. This may break libraries (e.g. glibc math library) that > depend on long double support. Note that some non-x86 platforms don't have 80-bit arithmetic (i.e. long double = double, in general). So, such libraries would be non-portable. The glibc math library is a particular case; I don't think there's any problem with rounding to IEEE double-precision though (I'm not sure, and things may have changed). It isn't even affected by a change of the rounding mode on x86; changing the rounding mode to a directed rounding mode breaks the math library on PowerPC and Opteron, for instance. -- Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <http://www.vinc17.org/> 100% accessible validated (X)HTML - Blog: <http://www.vinc17.org/blog/> Work: CR INRIA - computer arithmetic / SPACES project at LORIA
