[Bug middle-end/30789] complex folding inexact

[joseph at codesourcery dot com]

------- Comment #8 from joseph at codesourcery dot com  2009-08-24 16:59 -------
Subject: Re:  complex folding inexact

On Mon, 24 Aug 2009, ghazi at gcc dot gnu dot org wrote:

> This brings up the question that it's possible for GCC to produce a
> compile-time result (via MPC) that is different from the runtime result (via
> libgcc2) where both are C99 compliant standard conforming values.  Which one
> the user receives would depend on the context (e.g. static init) or on whether
> optimizations allowed GCC to fold it at compile-time.
> 
> Now this happens all the time for finite values, MPFR and MPC are more exact
> that glibc's math library and often produce different results which are clearly
> better.  However having GCC be more accurate than glibc (where the C library is
> outside out control) is different IMHO than getting two entirely different
> results from two parts of GCC, i.e. compile-time folding vs libgcc2.  E.g. (Inf
> NaN) vs (0 -Inf) are both infinities per C99 Annex G.

There are certainly other cases where folding may produce different 
results from those produced at runtime.  If a processor may require kernel 
support for subnormals or other special values we fold following IEEE 
rather than having options to declare the exact state of kernel support.  
We do not necessarily always produce a NaN with the same significand as 
hardware does.  Out-of-range conversions from floating-point to integer 
(unspecified value in Annex F) do not necessarily produce consistent 
results between folding and runtime (or between software and hardware 
floating point).  If we implement IBM long double constant folding (bug 
26374) I expect that will produce the nearest representable value to the 
exact result rather than always producing the same value as at runtime 
which might sometimes be further away from the exact result.

> Should I continue to pursue having GCC fold the Annex G special cases via MPC
> if it leads to this kind of discrepancy?  Or should be seek to fold these

I think folding these cases via MPC is fine.


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