[Bug libquadmath/78415] New: sqrtq does not round correctly when round mode is upward

[walter.mascarenhas at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78415

            Bug ID: 78415
           Summary: sqrtq does not round correctly when round mode is
                    upward
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libquadmath
          Assignee: unassigned at gcc dot gnu.org
          Reporter: walter.mascarenhas at gmail dot com
  Target Milestone: ---

//
// The code below shows that sqrtq does not round
// correctly when the rounding mode is upwards.
//
#include <cassert>
#include <cmath>
#include <quadmath.h>
#include <cfenv>

int main()
{
  __float128 x = 1.0;
  x += 2.0 * FLT128_EPSILON;

  std::fesetround(FE_UPWARD);
  __float128 su = sqrtq(x);

  // the sqrt is rounded to 1 + 2 epsilon, whereas the least __float128
  // above the mathematical sqrt(1 + 2 epsilon) is 1 + epsilon:
  // The Taylor series for sqrt around 1 yields
  // sqrt(1 + 2 epsilon) ~ 1 + epsilon - epsilon^2/2 < 1 + epsilon.

  assert(su == x);
}