On 2010-03-18 15:32:04 +0100, Michael Matz wrote: > > So, pow(-0.0, 0.5) should return +0. But sqrt(-0.0) should return -0 > > according to the IEEE 754 standard (and F.9.4.5 from ISO C99). > > Yes, and I don't know why they specified it like that. After all > (-0)*(-0)==+0 (not ==-0), so the above definition is internally > insonsistent. Defining sqrt(-0) as +0 would be equally inconsistent, but > at least agree with the pow(-0, 0.5) result.
sqrt(-0) was defined first by the IEEE 754 standard in 1985. AFAIK, -0 was chosen to allow a hack for some convention in interval arithmetic (there may be other reasons). pow(-0, y) was defined by the C committee. > But unfortunately you are right, this expansion can only be done for > -fno-signed-zeros. (FWIW the general expandsion of pow(x,N/2) where > N!=1 is already guarded by unsafe_math, but for N==1 we do it > unconditionally). If GCC is able to track range of values, the transformation could be allowed when it can be proved that the value -0 is not possible (e.g. when x > 0). -- Vincent Lefèvre <vinc...@vinc17.net> - Web: <http://www.vinc17.net/> 100% accessible validated (X)HTML - Blog: <http://www.vinc17.net/blog/> Work: CR INRIA - computer arithmetic / Arénaire project (LIP, ENS-Lyon)
