https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89991
--- Comment #11 from Steve Kargl <sgk at troutmask dot apl.washington.edu> --- On Mon, Apr 08, 2019 at 02:32:38PM +0000, sgk at troutmask dot apl.washington.edu wrote: > > I don't have a copy of the C++ standard, so take this specualtion. > pow(z,0.5) is equivalent to sqrt(z). From the C standard, one has > > conj(csqrt(z)) = csqrt(conj(z)). > > g++ does not enforce this when the imaginary part is -0; > while gcc does. (code snipped) > % gcc8 -o z c.c -lm && ./z > z = CMPLX(-1.8425031517782417e-07, -0.0000000000000000e+00) > cpow(z, 0.5) = CMPLX( 2.6283607659835831e-20, -4.2924388775825818e-04) > csqrt(z) = CMPLX( 0.0000000000000000e+00, -4.2924388775825818e-04) > csqrt(conj(z)) = CMPLX( 0.0000000000000000e+00, 4.2924388775825818e-04) > conj(csqrt(z)) = CMPLX( 0.0000000000000000e+00, 4.2924388775825818e-04) (code snipped) > % g++8 -o z a.cpp -lm && ./z > z = (-1.84250315177824e-07,-0) > pow(z,0.5) = (2.62836076598358e-20,-0.000429243887758258) > sqrt(z) = (0,0.000429243887758258) > sqrt(conj(z)) = (0,0.000429243887758258) > conj(sqrt(z)) = (0,-0.000429243887758258) > > This looks wrong. It is wrong. From n4810.pdf, page 1102, template<class T> complex<T> sqrt(const complex<T>& x); Returns: The complex square root of x, in the range of the right half-plane. [Note: The semantics of this function are intended to be the same in C++ as they are for csqrt in C. -- end note] unless [Note: ...] is non-normative text.
