Hi, On Thu, 18 Mar 2010, Vincent Lefevre wrote:
> On 2010-03-16 16:18:17 +0100, Richard Guenther wrote: > > pow (a, 0.5) is always expanded to sqrt(a). > > This violates the ISO C99 standard for -0.0. > > According to N1256, F.9.4.4: > > pow(±0, y) returns +0 for y > 0 and not an odd integer. > > 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. 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). Ciao, Michael.
