Re[2]: [lmi] Unit test for C99 round()

[Vadim Zeitlin]

On Wed, 04 Jun 2008 15:22:33 +0000 Greg Chicares <address@hidden> wrote:

GC> So I'd guess the problem must have something to do with whether
GC> detail::perform_rint() gets called in this block, because that's
GC> the only place it seems to be used:
GC> 
GC> // Choose the auxiliary rounding function indicated by the argument.
GC> template<typename RealType>
GC> typename round_to<RealType>::rounding_function_t
GC> round_to<RealType>::select_rounding_function(rounding_style const a_style) 
const
GC> {
GC> #ifndef LMI_LACKING_RINT_OR_EQUIVALENT
GC>     if
GC>         (  a_style == default_rounding_style()
GC>         && a_style != r_indeterminate
GC>         )
GC>         {
GC>         return detail::perform_rint;
GC>         }
GC> #endif // not defined LMI_LACKING_RINT_OR_EQUIVALENT
GC> 
GC> If you change that thus:
GC> -        return detail::perform_rint;
GC> +        // EXPERIMENTALLY SKIPPED return detail::perform_rint;
GC> to suppress that early exit (which should be a mere speed
GC> optimization), then does everything work?

 Yes, indeed, thanks for localizing the problem!

 However I still don't really know what's going on: the function code looks
to be correct:

Dump of assembler code for function _ZN6detail12perform_rintIfEET_S1_:
0x000000000040c7ee:      push   %rbp
0x000000000040c7ef:      mov    %rsp,%rbp
0x000000000040c7f2:      movss  %xmm0,0xfffffffffffffffc(%rbp)
0x000000000040c7f7:      flds   0xfffffffffffffffc(%rbp)
0x000000000040c7fa:      frndint
0x000000000040c7fc:      fstps  0xfffffffffffffffc(%rbp)
0x000000000040c7ff:      mov    0xfffffffffffffffc(%rbp),%eax
0x000000000040c802:      mov    %eax,0xfffffffffffffff4(%rbp)
0x000000000040c805:      movss  0xfffffffffffffff4(%rbp),%xmm0
0x000000000040c80a:      leaveq
0x000000000040c80b:      retq
End of assembler dump.

however it returns "-0" instead of the expected "-1e-15" when rounding
"-5.00000002e-16" to 15 decimals downwards (this is the first error).

 Does this give you any useful information or should I try to look into
this closer myself?

 Thanks,
VZ