work out why host validation on sqrt fails (float128)

[florianschanda]

The following program produces an off-by-one error:

#include <quadmath.h>
#include <stdio.h>
#include <stdint.h>

int main()
{
  union {
    __float128 f;
    uint32_t bv[2];
  } data;
  union {
    __float128 f;
    uint32_t bv[2];
  } result;
  char tmp[256];

  data.bv[3] = 0x000064B5;
  data.bv[2] = 0x59A3D7F7;
  data.bv[1] = 0x56924E4F;
  data.bv[0] = 0xE681141C;

  quadmath_snprintf(tmp, sizeof(tmp), "%Qa", data.f);
  printf("Input: %s\n", tmp);

  result.f = sqrtq(data.f);

  quadmath_snprintf(tmp, sizeof(tmp), "%Qa", result.f);
  printf("Output: %s\n", tmp);

  printf("As BV: 0x");
  for (int i=3; i>=0; --i) {
    printf("%08x", result.bv[i]);
  }
  printf("\n");
}

The result should be: 0x1FFF4121A5EFB86CD358C0F1BA4AE60B
However we're getting: 0x1FFF4121A5EFB86CD358C0F1BA4AE60C