------- Comment #4 from joseph at codesourcery dot com 2009-08-13 01:25 ------- Subject: Re: complex folding inexact
On Wed, 12 Aug 2009, ghazi at gcc dot gnu dot org wrote: > > > ------- Comment #3 from ghazi at gcc dot gnu dot org 2009-08-12 22:28 ------- > (In reply to comment #2) > > Joseph - Thanks for your reply and testvalues. > > > There are also cases for exact rounding where you'd expect MPC to produce > > the right results but would *not* expect operations executed at runtime to > > produce exactly those results. For example, (1.0 + DBL_EPSILON + 1.0i) * > > (1.0 - DBL_EPSILON + 1.0i), which would only work at runtime if the code > > happens to use exactly the right fused multiply-add operation. > > What is the "right result" for this case? GCC with MPC produces: > -4.93038065763132378382330353301741393545754021943139377981e-32 + 2.0i) > > Unpatched GCC as well as the runtime on my x86_64 box says: > 0.0 + 2.0i > > So the runtime here is not using the fused instruction? > > Is the MPC value correct? It looks correct. You expect real part -DBL_EPSILON*DBL_EPSILON, imaginary part 2.0. I believe existing x86_64 hardware does not have fused multiply-add instructions. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30789
