[Bug target/25661] New: Wrong long double to float conversion

[amodra at bigpond dot net dot au]

Conversion from 128-bit long double to float goes via double.  This means the
conversions suffers from double rounding, and incorrect results may be
calculated for long double values near the midpoint of two adjacent float
values.

For example, f and g should have different values in the following.

#ifndef DO_DOUBLE
#define FP float
#define FP_MANT 24
#else
#define FP double
#define FP_MANT 53
#endif

volatile long double x;
volatile long double y;

int main (void)
{
  FP f, g;
  x = (1ull << 63) + (1ull << (63 - FP_MANT));
  y = 1.0;
  f = x + y;
  g = x - y;
  return f == g;
}


-- 
           Summary: Wrong long double to float conversion
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: amodra at bigpond dot net dot au
GCC target triplet: powerpc64-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25661