[Bug middle-end/30789] complex folding inexact

[joseph at codesourcery dot com]

------- 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.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30789